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-=-=-=-=-=-=-
==Phrack Inc.==
Volume 0x10, Issue 0x46, Phile #0x06 of 0x0f
|=-----------------------------------------------------------------------=|
|=--------=[ .NET Instrumentation via MSIL bytecode injection ]=---------=|
|=-----------------------------------------------------------------------=|
|=----------=[ by Antonio "s4tan" Parata <aparata@gmail.com>]=-----------=|
|=-----------------------------------------------------------------------=|
1 - Introduction
2 - CLR environment
2.1 - Basic concepts
2.1.1 - Metadata tables
2.1.2 - Metadata token
2.1.3 - MSIL bytecode
2.2 - Execution environment
3 - JIT compiler
3.1 - The compileMethod
3.2 - Hooking the compileMethod
4 - .NET Instrumentation
4.1 - MSIL injection strategy
4.2 - Resolving the method handle
4.3 - Implementing a trampoline via the calli instruction
4.4 - Crafting a dynamic method
4.5 - Invoking the user defined code
4.6 - Fixing the SEH table
5 - Real world examples
5.1 - Web application password stealer
5.2 - Malware inspection
6 - Conclusion
7 - References
8 - Source Code
--[ 1 - Introduction
In this article we will explore the internals of the .NET framework with
the purpose of providing an innovative method to instrument .NET programs
at runtime.
Actually, there are several libraries that allow to instrument .NET
programs; most of them install a hook in the code generated after compiling
a given method, or by modifying the Assembly and saving back the result of
the modification.
Microsoft also provides a profile API in order to instrument the execution
of a given program. However the API must be activated before executing the
program by setting specific environment variables.
Our goal is to instrument the program at runtime by leaving the Assembly
binary untouched; all this by using a high level .NET language. As we will
see, this is done by injecting additional MSIL code just before the target
method is compiled.
--[ 2 - CLR environment
Before describing in depth how to inject additional MSIL code in a method,
it is necessary to provide some basic concepts as on how the .NET framework
works and which are its basic components.
We will only describe the concepts that are relevant to our purpose.
---[ 2.1 - Basic concepts
A .NET binary is typically called Assembly (even if it doesn't contain any
assembly code). It is a self-describing structure, meaning that inside an
Assembly you will find all the necessary information to execute it (for
more information on this subject see [01]).
As we will shortly see, all this information can be accessed by using
reflection. Reflection allows us to have a full picture of which types and
methods are defined inside the Assembly. We can also have access to the
names and types of the parameters passed to a specific method. The only
missing information are the names of the local variables, but as we will
see this is not a problem at all.
----[ 2.1.1 - Metadata tables
All the above mentioned information is stored inside tables called
Metadata tables.
The following list taken from [02] shows the index and names of all the
existing tables:
00 - Module 01 - TypeRef 02 - TypeDef
04 - Field 06 - MethodDef 08 - Param
09 - InterfaceImpl 10 - MemberRef 11 - Constant
12 - CustomAttribute 13 - FieldMarshal 14 - DeclSecurity
15 - ClassLayout 16 - FieldLayout 17 - StandAloneSig
18 - EventMap 20 - Event 21 - PropertyMap
23 - Property 24 - MethodSemantics 25 - MethodImpl
26 - ModuleRef 27 - TypeSpec 28 - ImplMap
29 - FieldRVA 32 - Assembly 33 - AssemblyProcessor
34 - AssemblyOS 35 - AssemblyRef 36 - AssemblyRefProcessor
37 - AssemblyRefOS 38 - File 39 - ExportedType
40 - ManifestResource 41 - NestedClass 42 - GenericParam
44 - GenericParamConstraint
Each table is composed of a variable number of rows. The size of a row
depends on the kind of table and can contain a reference to other Metadata
tables.
Those tables are referenced by the Metadata token, a notion that is
described in the next paragraph.
----[ 2.1.2 - Metadata token
The Metadata token (or token for short) is a fundamental concept in the CLR
framework. A token allows you to reference a given table at a given index.
It is a 4-byte value, composed of two parts [08]: a table index and the
RID.
The table index is the topmost byte wich points to a table. A RID is a
3-byte record identifier pointing in the table, which starts at offset one.
As an example, let's consider the following Metadata token:
(06)00000F
0x06 is the number of the referenced table, which in this case is
MethodDef. The last three bytes are the RID, that in this case has a value
of 0x0F.
----[ 2.1.3 - MSIL bytecode
When we write a program in a .NET high level language, the compiler will
translate this code into an intermediate representation called MSIL or as
defined in the ECMA-335 [03] CIL, which stands for Common Intermediate
Language.
By installing Visual Studio you will also install a very handy utility
called ILDasm, that allows you to disassemble an Assembly by displaying the
MSIL code and other useful information.
As an example let try to compile the following C# source code:
------#------#------#------<START CODE>------#------#------#------
public class TestClass
{
private String _message;
public TestClass(String txt)
{
this._message = txt;
}
private String FormatMessage()
{
return "Hello " + this._message;
}
public void SayHello()
{
var message = this.FormatMessage();
Console.WriteLine(message);
}
}
------#------#------#------<END CODE>------#------#------#------
The result of the compilation is an Assembly with three methods:
.ctor : void(string), FormatMessage : string() and SayHello : void().
Let's try to display the MSIL code of the SayHello method:
------#------#------#------<START CODE>------#------#------#------
.method public hidebysig instance void SayHello() cil managed
// SIG: 20 00 01
{
// Method begins at RVA 0x21f8
// Code size 16 (0x10)
.maxstack 1
.locals init ([0] string message)
IL_0000: /* 00 | */ nop
IL_0001: /* 02 | */ ldarg.0
IL_0002: /* 28 | (06)00000F */
call instance string MockLibrary.TestClass::FormatMessage()
IL_0007: /* 0A | */ stloc.0
IL_0008: /* 06 | */ ldloc.0
IL_0009: /* 28 | (0A)000014 */
call void [mscorlib]System.Console::WriteLine(string)
IL_000e: /* 00 | */ nop
IL_000f: /* 2A | */ ret
} // end of method TestClass::SayHello
------#------#------#------<END CODE>------#------#------#------
For each instruction we can see the associated MSIL byte values. It is
interesting to see that the code doesn't contain any reference to unmanaged
memory but only to metadata tokens.
The two call instructions reference two different tables, due to the
FormatMessage method being implemented in the current Assembly and the
WriteLine method implemented in an external Assembly.
If we take a look at the list of tables presented in 2.1.1 we can see that
the Metadata token (0A)000014 references the table 0x0A which is the
MemberRef table, index 0x14 which is WriteLine. Instead the token
(06)00000F references the table 0x06 which is the MethodDef table, index
0x0F which is FormatMessage.
---[ 2.2 - Execution environment
The CLR execution environment is very strict and forbids any kind of
dangerous operation. If we compare it with the unmanaged world where we
were able to jump in the middle of an instruction to confuse the
disassembler, to create all kinds of opaque instructions or to jump to any
valid address, we will discover a sad truth: everything is forbidden.
The CLR is a stack based machine. This means that there is no concept of
registers and every parameter is pushed on the stack in order to be passed
to other functions. When we exit a method, the stack must be empty or at
least contain the value that should be returned.
As already said, everything is based on the definition of the Metadata
token. If we try to invoke a call with an invalid token we will receive a
fatal exception. This poses a serious problem for our goal, since we cannot
call methods that are not referenced by the original Assembly.
--[ 3 - JIT compiler
When a method is executed we have two different scenarios. The first one is
when the method is already compiled, in this case the code just jumps to
the compiled unmanaged code. The second scenario is when the method isn't
yet compiled, in this case the code jumps to a stub that will call the
exported method compileMethod, defined in corjit.h [04], in order to
compile and then execute the method.
---[ 3.1 - The compileMethod
Let's analyze this interesting method a bit more. The signature of
compileMethod is the following:
virtual CorJitResult __stdcall compileMethod (
ICorJitInfo *comp, /* IN */
struct CORINFO_METHOD_INFO *info, /* IN */
unsigned /* code:CorJitFlag */ flags, /* IN */
BYTE **nativeEntry, /* OUT */
ULONG *nativeSizeOfCode /* OUT */
) = 0;
The most interesting structure is the CORINFO_METHOD_INFO
which is defined in corinfo.h [05] and has the following format:
struct CORINFO_METHOD_INFO
{
CORINFO_METHOD_HANDLE ftn;
CORINFO_MODULE_HANDLE scope;
BYTE * ILCode;
unsigned ILCodeSize;
unsigned maxStack;
unsigned EHcount;
CorInfoOptions options;
CorInfoRegionKind regionKind;
CORINFO_SIG_INFO args;
CORINFO_SIG_INFO locals;
};
For our purpose the most important field is the ILCode byte pointer. It
points to a buffer which contains the MSIL bytecode. By modifying this
buffer we are able to alter the method execution flow.
As a side note, this method is also extensively used by .NET obfuscators.
In fact we can read the following comment in the source code:
Note: Obfuscators that are hacking the JIT depend on this method having
__stdcall calling convention
An obfuscator typically encrypts the MSIL bytecode of a method, then when
the method is bound to be executed they decrypt the bytecode and pass this
value as byte pointer instead of the encrypted one. This also explains why
if we open it in ILDasm or with a decompiler we receive back an error. How
can they know when a method is going to be called? This is pretty easy,
the code in charge for the replacement process is placed inside the type
constructor. This specific constructor is invoked only once: before a new
object of that specific type is created.
---[ 3.2 - Hooking the compileMethod
Since the compileMethod is exported by the Clrjit.dll (or from mscorjit.dll
for older .NET versions), we can easily install a hook to intercept all the
requests for compilation. The following F# pseudo-code shows how to do
this:
------#------#------#------<START CODE>------#------#------#------
[<DllImport(
"Clrjit.dll",
CallingConvention = CallingConvention.StdCall, PreserveSig = true)
>]
extern IntPtr getJit()
[<DllImport("kernel32.dll", SetLastError = true)>]
extern Boolean VirtualProtect(
IntPtr lpAddress,
UInt32 dwSize,
Protection flNewProtect,
UInt32& lpflOldProtect)
let pVTable = getJit()
_pCompileMethod <- Marshal.ReadIntPtr(pVTable)
// make memory writable
let mutable oldProtection = uint32 0
if not <| VirtualProtect(
_pCompileMethod,
uint32 IntPtr.Size,
Protection.PAGE_EXECUTE_READWRITE,
&oldProtection)
then
Environment.Exit(-1)
let protection = Enum.Parse(
typeof<Protection>,
oldProtection.ToString()) :?> Protection
// save original compile method
_realCompileMethod <-
Some (Marshal.GetDelegateForFunctionPointer(
Marshal.ReadIntPtr(_pCompileMethod),
typeof<CompileMethodDeclaration>) :?> CompileMethodDeclaration
)
RuntimeHelpers.PrepareDelegate(_realCompileMethod.Value)
RuntimeHelpers.PrepareDelegate(_hookedCompileMethodDelegate)
// install compileMethod hook
Marshal.WriteIntPtr(
_pCompileMethod,
Marshal.GetFunctionPointerForDelegate(_hookedCompileMethodDelegate)
)
// repristinate memory protection flags
VirtualProtect(
_pCompileMethod,
uint32 IntPtr.Size,
protection,
&oldProtection
) |> ignore
------#------#------#------<END CODE>------#------#------#------
When we modify the MSIL code we must pay attention to the stack size. Our
framework needs some stack space in order to work and if the method that is
going to be compiled doesn't need any local variables, we will receive an
exception at runtime. In order to fix this problem it is enough to modify
the maxStack variable of CORINFO_METHOD_INFO structure before writing it
back.
--[ 4 - .NET Instrumentation
Now it is time to modify the MSIL buffer of our method of choice and
redirect the flow to our code. As we will see this is not a smooth process
and we need to take care of numerous aspects.
---[ 4.1 - MSIL injection strategy
In order to invoke our code the process that we will follow is composed of
the following steps:
1. Install a trampoline at the beginning of the code. This
trampoline will call a dynamically defined method.
2. Define a dynamic method that will have a specific method signature.
3. Construct an array of objects that will contain the parameters
passed to the method.
4. Invoke a dispatcher function which will load our Assembly
and will finally call our code by passing a handle to the original
method and an array of objects representing the method parameters.
In the end the structure that we are going to create will follow the path
defined in the following diagram:
| ... |
| ... | +---------------+
| Trampoline |----> | |
| Original MSIL | | Dynamic |
| ... | | Method |---------+
| ... | | | |
+---------------+ v
+---------------+
| |
| Framework |
| Dispatcher |
| |
+---------------+
|
+----------+
|
v
+---------------+
| |
| User |
| Code Monitor |
| |
+---------------+
---[ 4.2 - Resolving the method handle
As we will see in the next paragraph, it is necessary to resolve the handle
of the method that will be compiled in order to obtain the needed
information via reflection. I have found a method to resolve it, it is not
very elegant but it works :P.
The following F# pseudo-code will show you how to resolve a method handle
given the CorMethodInfo structure:
------#------#------#------<START CODE>------#------#------#------
let getMethodInfoFromModule(
methodInfo: CorMethodInfo,
assemblyModule: Module) =
let mutable info: FilteredMethod option = None
try
// dirty trick, is there a
// better way to know the module of the compiled method?
let mPtr =
assemblyModule.ModuleHandle.GetType()
.GetField("m_ptr",
BindingFlags.NonPublic ||| BindingFlags.Instance)
let mPtrValue = mPtr.GetValue(assemblyModule.ModuleHandle)
let mpData =
mPtrValue.GetType()
.GetField("m_pData",
BindingFlags.NonPublic ||| BindingFlags.Instance)
if mpData <> null then
let mpDataValue = mpData.GetValue(mPtrValue) :?> IntPtr
if mpDataValue = methodInfo.ModuleHandle then
// module found, get method name
let tokenNum =
Marshal.ReadInt16(nativeint(methodInfo.MethodHandle))
let token = (0x06000000 + int32 tokenNum)
let methodBase = assemblyModule.ResolveMethod(token)
if methodBase.DeclaringType <> null &&
isMonitoredMethod(methodBase) then
let mutable numOfParameters =
methodBase.GetParameters() |> Seq.length
if not methodBase.IsStatic then
// take into account the this parameter
numOfParameters <- numOfParameters + 1
// compose the result info
info <- Some {
TokenNum = tokenNum
NumOfArgumentsToPushInTheStack = numOfParameters
Method = methodBase
IsConstructor = methodBase :? ConstructorInfo
Filter = this
}
with _ -> ()
info
------#------#------#------<END CODE>------#------#------#------
This method must be invoked for each module of all loaded Assemblies.
Now that we have a MethodBase object, we can use it to extract the needed
information, like the number of accepted parameters and their types.
---[ 4.3 - Implementing a trampoline via the calli instruction
Our first obstacle is to create a MSIL bytecode that can invoke an
arbitrary function. Among all the available OpCodes, the one of interest
for us is the calli instruction [06] (beware of its usage, as it makes our
code unverifiable).
From the MSDN page we can read that:
"The method entry pointer is assumed to be a specific pointer to native
code (of the target machine) that can be legitimately called with the
arguments described by the calling convention (a metadata token for a
stand-alone signature). Such a pointer can be created using the Ldftn or
Ldvirtftn instructions, or passed in from native code."
Nice, we can specify an arbitrary pointer to native code. The only
difficulty is that we cannot use the Ldftn or Ldvirtftn since they need a
metadata token, and we cannot specify this value. Not too bad, since from
the Ldftn documentation we can read that [07]:
"Pushes an unmanaged pointer (type native int) to the native code
implementing a specific method onto the evaluation stack."
So, if we have an unmanaged pointer we can simulate the Ldftn with a simple
Ldc_I4 instruction (supposing that we are operating on a 32 bit
environment) [09].
Unfortunately now we have another, even bigger, problem. The calli
instruction needs a callSiteDescr. From [08] we can read that:
"<token> - referred to callSiteDescr - must be a valid StandAloneSig".
The StandAloneSig is the table number 17. As I have already said we cannot
specify this Metadata token (since it probably doesn't exist in the table).
I have played a bit with the calli instruction in order to see if it
accepts also other kinds of Metadata tokens. In the end I discovered that
it also accepts a token from one of the following tables: TypeSpec, Field
and MethodDef.
For our purpose, the MethodDef table is the most interesting one, since we
can fake a valid MethodDef token by creating a DynamicMethod (more on this
later). We can now close the circle by using the calli instruction and
modifying the metadata token in order to specify a MethodDef.
We will use the MethodBase object that we obtained in the previous step in
order to know how many parameters the method accepts and push them in the
stack before invoking calli.
The following F# pseudo-code shows how to build the calli instruction:
------#------#------#------<START CODE>------#------#------#------
// load all arguments on the stack
for i=0 to filteredMethod.NumOfArgumentsToPushInTheStack-1 do
ilGenerator.Emit(OpCodes.Ldarg, i)
// emit calli instruction with a pointer to the dynamic method,
// the token used by the calli is not important as I'll modify it soon
ilGenerator.Emit(OpCodes.Ldc_I4, functionAddress)
ilGenerator.EmitCalli(
OpCodes.Calli,
CallingConvention.StdCall,
dispatcherMethod.ReturnType,
dispatcherArgs)
// this index allow to modify the right byte
let patchOffset = ilGenerator.ILOffset - 4
ilGenerator.Emit(OpCodes.Nop)
// check if I have to pop the return value
match filteredMethod.Method with
| :? MethodInfo as mi ->
if mi.ReturnType <> typeof<System.Void> then
ilGenerator.Emit(OpCodes.Pop)
| _ -> ()
// end method
ilGenerator.Emit(OpCodes.Ret)
------#------#------#------<END CODE>------#------#------#------
The functionAddress variable contains the native pointer of our dynamic
method. One last step is to patch the calli Metadata token with a MethodDef
token whose value we know to be correct. As value we will use the token of
the method that it is being compiled.
The following F# pseudo-code show how to modify the MSIL bytecode at the
right offset:
------#------#------#------<START CODE>------#------#------#------
// craft MethodDef metadata token index
let b1 = (filteredMethod.TokenNum &&& int16 0xFF00) >>> 8
let b2 = filteredMethod.TokenNum &&& int16 0xFF
// calli instruction accept 0x11 as table index (StandAloneSig),
// but seems that also other tables are allowed.
// In particular the following ones seem to be accepted as
// valid: TypeSpec, Field and Method (most important)
trampolineMsil.[patchOffset] <- byte b2
trampolineMsil.[patchOffset+1] <- byte b1
trampolineMsil.[patchOffset + 3] <- 6uy // 6(0x6): MethodDef Table
------#------#------#------<END CODE>------#------#------#------
Since this step is a bit complex let's try to summarize our actions:
1. We use the calli instruction to invoke an arbitrary method by specifying
a native address pointer.
2. We modify the calli metadata token by specifying a MethodDef token and
not a StandAloneSig token.
3. We pass as Metadata token value the token of the method currently
compiled. This kind of token describes the method that must be called.
Our next step is to be sure that the method invoked by calli satisfies the
information contained in the referenced Metadata token.
---[ 4.4 - Crafting a dynamic method
We now have to create the dynamic method that satisfies the information
provided by the token passed to the calli instruction. From [10] we can
read that:
"The method descriptor is a metadata token that indicates the method to
call and the number, type, and order of the arguments that have been placed
on the stack to be passed to that method as well as the calling convention
to be used."
So, in order to create a method that satisfies the signature of the method
referenced by the token we will use a very powerful .NET capability, which
allows us to define dynamic method. This step allows the following:
1. Create a method that has the same signature of the method that will be
compiled. This will guarantee that the information carried by the
metadata token is legit.
2. We are now in a situation where we can specify a valid metadata
token, since the new dynamic type is created in the current execution
environment.
This dynamic method will call another method (a dispatcher) that accepts
two arguments: a string representing the location of the Assembly to load
(more on this later) and an array of objects which contains the arguments
passed to the method.
In creating this method you have to pay attention when creating the objects
array, since in .NET not everything is an object.
The following F# pseudo-code creates the dynamic method with the right
signature:
------#------#------#------<START CODE>------#------#------#------
let argumentTypes = [|
if not filteredMethod.Method.IsStatic then
yield typeof<Object>
yield!
filteredMethod.Method.GetParameters()
|> Array.map(fun p -> p.ParameterType)
|]
let dynamicType =
_dynamicModule.DefineType(
filteredMethod.Method.Name + "_Type" + string(!_index))
let dynamicMethod =
dynamicType.DefineMethod(
dynamicMethodName,
MethodAttributes.Static |||
MethodAttributes.HideBySig |||
MethodAttributes.Public,
CallingConventions.Standard,
typeof<System.Void>,
argumentTypes
)
------#------#------#------<END CODE>------#------#------#------
We can now proceed with the creation of the method body. We need to pay
attention to two facts: ValueType parameters must be boxed, and Enum
parameters must be converted to another form (after some trials and errors
I found that Int32 is a good compromise).
------#------#------#------<START CODE>------#------#------#------
// push the location of the Assembly to load containing the monitors
let assemblyLocation =
if filteredMethod.Filter.Invoker <> null
then filteredMethod.Filter.Invoker.Assembly.Location
else String.Empty
ilGenerator.Emit(OpCodes.Ldstr, assemblyLocation)
// get the parameter types
let parameters =
filteredMethod.Method.GetParameters()
|> Seq.map(fun pi -> pi.ParameterType)
|> Seq.toList
// create argv array
ilGenerator.Emit(OpCodes.Ldc_I4,
filteredMethod.NumOfArgumentsToPushInTheStack)
ilGenerator.Emit(OpCodes.Newarr, typeof<Object>)
// fill the argv array
for i=0 to filteredMethod.NumOfArgumentsToPushInTheStack-1 do
ilGenerator.Emit(OpCodes.Dup)
ilGenerator.Emit(OpCodes.Ldc_I4, i)
ilGenerator.Emit(OpCodes.Ldarg, i)
// check if I have to box the value
if filteredMethod.Method.IsStatic || i > 0 then
// this check is necessary becasue the
// GetParameters method doesn't consider the 'this' pointer
let paramIndex = if filteredMethod.Method.IsStatic then i else i - 1
if parameters.[paramIndex].IsEnum then
// consider all enum as Int32 type to avoid access problems
ilGenerator.Emit(OpCodes.Box, typeof<Int32>)
elif parameters.[paramIndex].IsValueType then
// all value types must be boxed
ilGenerator.Emit(OpCodes.Box, parameters.[paramIndex])
// store the element in the array
ilGenerator.Emit(OpCodes.Stelem_Ref)
// emit call to dispatchCallback
let dispatchCallbackMethod =
Type.GetType("ES.Anathema.Runtime.Dispatcher")
.GetMethod("dispatchCallback", BindingFlags.Static ||| BindingFlags.Public)
ilGenerator.EmitCall(OpCodes.Call, dispatchCallbackMethod, null)
ilGenerator.Emit(OpCodes.Ret)
------#------#------#------<END CODE>------#------#------#------
The call will end up invoking a framework method that is in charge for the
dispatch of the call to the user defined code.
---[ 4.5 - Invoking the user defined code
In order to make the code easy to extend, we can implement a mechanism that
will load a user defined Assembly and invoke a specific method. In this way
we have an architecture that resembles that of a plugin-based architecture.
We call these plugins: monitors. Each monitor can be configured in order to
intercept a specific method.
In order to locate the monitors we will use the software design paradigm
"convention over configuration", which implies that all classes whose name
ends in "Monitor" are loaded.
This last method is very simple, it just retrieves the MethodBase object
from the stack in order to pass it to the monitor and finally invoke it.
The assemblyLocation parameter is the one that specifies where the user
defined Assembly is located.
------#------#------#------<START CODE>------#------#------#------
let dispatchCallback(assemblyLocation: String, argv: Object array) =
if File.Exists(assemblyLocation) then
let callingMethod =
try
// retrieve the calling method from the stack trace
let stackTrace = new StackTrace()
let frames = stackTrace.GetFrames()
frames.[2].GetMethod()
with _ -> null
// invoke all the monitors, we use "convention over configuration"
let bytes = File.ReadAllBytes(assemblyLocation)
for t in Assembly.Load(bytes).GetTypes() do
try
if t.Name.EndsWith("Monitor") && not t.IsAbstract then
let monitorConstructor =
t.GetConstructor([|
typeof<MethodBase>;
typeof<Object array>|])
if monitorConstructor <> null then
monitorConstructor.Invoke([|callingMethod; argv|]) |> ignore
with _ -> ()
------#------#------#------<END CODE>------#------#------#------
---[ 4.6 - Fixing the SEH table
We are near the end, we have modified the MSIL bytecode, we have created a
dynamic method and a trampoline. The final step is to write back the
CORINFO_METHOD_INFO structure and call the real compileMethod.
Unfortunately by doing so you will soon receive a runtime error when you
try to instrument a method that uses a try/catch clause.
This is due to the fact that the creation of the trampoline has made the
SEH table invalid. This table contains information on the portions of code
that are inside try/catch clauses. From [11] we can see that by adding
additional MSIL code, the properties TryOffset and HandlerOffset will
assume an invalid value.
This table is located after the IL Code, as shown in the following
diagram:
+--------------------+
| |
| Fat Header |
| |
+--------------------+
| |
| |
| IL Code |
| |
| |
+--------------------+
| |
| SEH Table |
| |
+--------------------+
We also have a confirmation from the source code, in fact in corhlpr.cpp
([12]) we can see that the SEH table is added to the outBuff variable after
that was already filled with the IL code.
So, to get the address of the SEH table it is enough to add to the IlCode
pointer, located in the CorMethodInfo structure, the length of the MSIL
code.
Before showing the code that does that we have to take into account that
the SEH Table can be of two different types: FAT or SMALL. What changes is
only the dimensions of its fields. So fixing this table it is just a matter
of locating it and enumerating each clause to fix their values.
The following F# pseudo-code does exactly this:
------#------#------#------<START CODE>------#------#------#------
let fixEHClausesIfNecessary(
methodInfo: CorMethodInfo,
methodBase: MethodBase,
additionalCodeLength: Int32) =
let clauses = methodBase.GetMethodBody().ExceptionHandlingClauses
if clauses.Count > 0 then
// locate SEH table
let codeSizeAligned =
if (int32 methodInfo.IlCodeSize) % 4 = 0 then 0
else 4 - (int32 methodInfo.IlCodeSize) % 4
let mutable startEHClauses =
methodInfo.IlCode +
new IntPtr(int32 methodInfo.IlCodeSize + codeSizeAligned)
let kind = Marshal.ReadByte(startEHClauses)
// try to identify FAT header
let isFat = (int32 kind &&& 0x40) <> 0
// it is always plus 3 because even if it is small it is
// padded with two bytes. See: Expert .NET 2.0 IL Assembler p. 296
startEHClauses <- startEHClauses + new IntPtr(4)
for i=0 to clauses.Count-1 do
if isFat then
let ehFatClausePointer =
box(startEHClauses.ToPointer())
:?> nativeptr<CorILMethodSectEhFat>
let mutable ehFatClause = NativePtr.read(ehFatClausePointer)
// modify the offset value
ehFatClause.HandlerOffset <-
ehFatClause.HandlerOffset + uint32 additionalCodeLength
ehFatClause.TryOffset <-
ehFatClause.TryOffset + uint32 additionalCodeLength
// write back the result
let mutable oldProtection = uint32 0
let memSize = Marshal.SizeOf(typeof<CorILMethodSectEhFat>)
if not <| VirtualProtect(
startEHClauses,
uint32 memSize,
Protection.PAGE_READWRITE,
&oldProtection) then
Environment.Exit(-1)
let protection = Enum.Parse(
typeof<Protection>,
oldProtection.ToString()) :?> Protection
NativePtr.write ehFatClausePointer ehFatClause
// repristinate memory protection flags
VirtualProtect(
startEHClauses,
uint32 memSize,
protection,
&oldProtection) |> ignore
// go to next clause
startEHClauses <- startEHClauses + new IntPtr(memSize)
else
//... do same as above but for small size table
------#------#------#------<END CODE>------#------#------#------
Once we have fixed this table we can finally invoke the real compileMethod.
--[ 5 - Real world examples
The code presented is part of a project called Anathema that will allow you
to easily instrument .NET programs. Let's try to use the framework by
instrumenting a web application in order to steal the user passwords and to
instrument a real world malware in order to log all method calls.
---[ 5.1 - Web application password stealer
Let's see how we can use this instrumentation method in order to implement
a password stealer for a web application. For our demo we will use a very
popular .NET web server called Suave ([13]). We will write the web
application in F# and the password stealer as a C# console application, in
this way we can instrument the interesting method before it is compiled. In
the other case we have to force the .NET runtime to recompile the method in
order to apply the instrumentation (see [14] for a possible approach).
The web application is very simple and contains only a form; its HTML code
is shown below:
------#------#------#------<START CODE>------#------#------#------
<h1>-= Secure Web Shop Login =-</h1>
<form method="POST" action="/login">
<table>
<tr>
<td>Username:</td>
<td><input type="text" name="username"></td>
</tr>
<tr>
<td>Password:</td>
<td><input type="password" name="password"></td>
</tr>
<tr>
<td></td>
<td><input type="submit" name="Login"></td>
</tr>
</table>
</form>
------#------#------#------<END CODE>------#------#------#------
The F# code in charge for the authentication is the following:
------#------#------#------<START CODE>------#------#------#------
let private _accounts = [
("admin", BCrypt.HashPassword("admin"))
("guest", BCrypt.HashPassword("guest"))
]
let private authenticate(username: String, password: String) =
_accounts
|> List.exists(fun (user, hash) ->
let usernameMatch = user.Equals(username, StringComparison.Ordinal)
let passwordMatch = BCrypt.Verify(password, hash)
usernameMatch && passwordMatch
)
let private doLogin(ctx: HttpContext) =
match (tryGetParameter(ctx, "username"), tryGetParameter(ctx, "password")) with
| (Some username, Some password) when authenticate(username, password) ->
OK "Authentication successfully executed!" ctx
| _ -> OK "Wrong username/password combination" ctx
------#------#------#------<END CODE>------#------#------#------
So, the best way to intercept passwords is the 'authenticate' method. We
will start by creating a class in charge of printing the received password,
this is done by creating the following simple class:
------#------#------#------<START CODE>------#------#------#------
class PasswordStealerMonitor
{
public PasswordStealerMonitor(MethodBase m, object[] args)
{
Console.WriteLine(
"[!] Username: '{0}', Password: '{1}'",
args[0],
args[1]);
}
}
------#------#------#------<END CODE>------#------#------#------
Now, the final step is to instrument the application, this is done using
the following code:
------#------#------#------<START CODE>------#------#------#------
// create runtime
var runtime = new RuntimeDispatcher();
var hook = new Hook(runtime.CompileMethod);
var authenticateMethod = GetAuthenticateMethod();
runtime.AddFilter(
typeof(PasswordStealerMonitor),
"SecureWebShop.Program.authenticate");
// apply hook
var jitHook = new JitHook();
jitHook.InstallHook(hook);
jitHook.Start();
// start the real web application
SecureWebShop.Program.main(new String[] { });
------#------#------#------<END CODE>------#------#------#------
Once the web application is run and we try to login, we will see the
following output in the console:
-= Secure Web Shop =-
Start web server on 127.0.0.1:8080
[14:45:49 INF] Smooth! Suave listener started in 631.728 with binding 127.0.0.1:8080
[!] Username: 's4tan', Password: 'wrong_password'
[!] Username: 'admin', Password: 'admin'
---[ 5.2 - Malware inspection
Let's consider a sample of the Hawkeye malware, written in .NET, with the
following MD5 hash: 130efba199b389ab71a374bf95be2304.
The sample contains two levels of packing. We could trace the packers but
let's focus on the main payload (MD5: 97d74c20f5d148ed68e45dad0122d3b5).
When the main payload is launched the following method calls are logged:
c:\>MLogger.exe malware.exe
[+] Debugger.My.MyApplication.Main(Args: System.String[]) : System.Void
[+] Debugger.My.MyProject..cctor()
[...]
[+] Debugger.My.MyProject.get_Application() : Debugger.My.MyApplication
[+] Debugger.My.MyProject+ThreadSafeObjectProvider`1.get_GetInstance() : T
[+] Debugger.My.MyApplication..ctor()
[+] Debugger.My.MyProject+ThreadSafeObjectProvider`1.get_GetInstance() : T
[+] Debugger.My.MyProject+MyForms..ctor()
[+] Debugger.Debugger..ctor()
[+] Debugger.Clipboard..ctor()
[+] Debugger.Clipboard.add_Changed(obj: Debugger.Clipboard+ChangedEventHandler)
: System.Void
[+] Debugger.My.Resources.Resources.get_CMemoryExecute() : System.Byte[]
[+] Debugger.My.Resources.Resources.get_ResourceManager() :
System.Resources.ResourceManager
[+] Debugger.Debugger.InitializeComponent() : System.Void
[+] Debugger.Debugger.Decrypt(
encryptedBytes: System.String, secretKey: System.String) : System.String
[+] Debugger.Debugger.getAlgorithm(secretKey: System.String) :
System.Security.Cryptography.RijndaelManaged
[+] Debugger.Debugger.Decrypt(
encryptedBytes: System.String, secretKey: System.String) : System.String
[+] Debugger.Debugger.getAlgorithm(secretKey: System.String) :
System.Security.Cryptography.RijndaelManaged
[+] Debugger.Debugger.Decrypt(
encryptedBytes: System.String, secretKey: System.String) : System.String
[+] Debugger.Debugger.getAlgorithm(secretKey: System.String) :
System.Security.Cryptography.RijndaelManaged
[...]
[+] Debugger.Debugger.IsConnectedToInternet() : System.Boolean
[+] Debugger.Debugger.GetInternalIP() : System.String
[+] Debugger.Debugger.GetExternalIP() : System.String
[+] Debugger.Debugger.GetBetween(
Source: System.String, Before: System.String, After: System.String) : System.String
[+] Debugger.Debugger.GetAntiVirus() : System.String
[+] Debugger.Debugger.GetFirewall() : System.String
[+] Debugger.Debugger.unHide() : System.Void
[+] Debugger.My.MyProject+ThreadSafeObjectProvider`1.get_GetInstance() : T
[+] Debugger.My.MyComputer..ctor()
[+] Debugger.Debugger.unhidden(path: System.String) : System.Void
[...]
[+] Debugger.My.Resources.Resources.get_mailpv() : System.Byte[]
[+] Debugger.My.Resources.Resources.get_ResourceManager() :
System.Resources.ResourceManager
[+] Debugger.Debugger.HookKeyboard() : System.Void
[+] Debugger.Clipboard.Install() : System.Void
[+] Debugger.My.MyProject+ThreadSafeObjectProvider`1.get_GetInstance() : T
[+] Debugger.My.MyComputer..ctor()
[+] Debugger.Debugger.IsConnectedToInternet() : System.Boolean
[+] Debugger.Debugger.IsConnectedToInternet() : System.Boolean
[+] Debugger.My.MyProject.get_Computer() : Debugger.My.MyComputer
[...]
--[ 6 - Conclusion
Instrumenting a .NET program via MSIL bytecode injection is a pretty useful
technique that allows you to have full control of method invocation by
using a high level .NET language.
As we have seen, doing so requires a lot of attention and knowledge of the
internal workings of the CLR, but in the end the outcome is worth the
trouble.
--[ 7 - References
[01] Metadata and Self-Describing Components - https://goo.gl/bbSG7p
[02] The .NET File Format - http://www.ntcore.com/files/dotnetformat.htm
[03] Standard ECMA-335 - https://goo.gl/J9kko6
[04] corjit.h - https://goo.gl/J68Poi
[05] corinfo.h - https://goo.gl/G31KHP
[06] OpCodes.Calli Field - https://goo.gl/D7ug93
[07] OpCodes.Ldftn Field - https://goo.gl/sHzz1S
[08] Expert .NET 2.0 IL Assembler - https://goo.gl/3LKLSW
[09] OpCodes.Ldc_I4 Field - https://goo.gl/qEW2Lx
[10] OpCodes.Call Field - https://goo.gl/29rqZk
[11] ExceptionHandlingClause Class - https://goo.gl/bjLqSv
[12] corhlpr.cpp - https://goo.gl/DDVKgH
[13] Suave web server - https://suave.io/
[14] .NET CLR Injection - https://goo.gl/nryxYB
--[ 8 - Source Code
begin 766 Anathema.zip
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