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Phrack Seventeen
07 April 1988
File 8 of 12 : Dialback Modem Security
In article <906@hoptoad.uucp> gnu@hoptoad.UUCP writes:
>Here are the two messages I have archived on the subject...
>[I believe the definitive article in that discussion was by Lauren Weinstein,
>vortex!lauren; perhaps he has a copy.
What follows is the original article that started the discussion. I
do not know whether it qualifies as the "definitive article" as I think I
remember Lauren and I both posted further comments.
- Dave
** ARTICLE FOLLOWS **
------------------------------------------------------------------------------
An increasingly popular technique for protecting dial-in ports from
the ravages of hackers and other more sinister system penetrators is dial back
operation wherein a legitimate user initiates a call to the system he desires
to connect with, types in his user ID and perhaps a password, disconnects and
waits for the system to call him back at a prearranged number. It is assumed
that a penetrator will not be able to specify the dial back number (which is
carefully protected), and so even if he is able to guess a user-name/password
pair he cannot penetrate the system because he cannot do anything meaningful
except type in a user-name and password when he is connected to the system. If
he has a correct pair it is assumed the worst that could happen is a spurious
call to some legitimate user which will do no harm and might even result in a
security investigation.
Many installations depend on dial-back operation of modems for their
principle protection against penetration via their dial up ports on the
incorrect presumption that there is no way a penetrator could get connected to
the modem on the call back call unless he was able to tap directly into the
line being called back. Alas, this assumption is not always true -
compromises in the design of modems and the telephone network unfortunately
make it all too possible for a clever penetrator to get connected to the call
back call and fool the modem into thinking that it had in fact dialed the
legitimate user.
The problem areas are as follows:
Caller control central offices
Many older telephone central office switches implement caller control
in which the release of the connection from a calling telephone to a called
telephone is exclusively controlled by the originating telephone. This means
that if the penetrator simply failed to hang up a call to a modem on such a
central office after he typed the legitimate user's user-name and password,
the modem would be unable to hang up the connection.
Almost all modems would simply go on-hook in this situation and not
notice that the connection had not been broken. If the same line was used to
dial out on as the call came in on, when the modem went to dial out to call
the legitimate user back the it might not notice (there is no standard way of
doing so electrically) that the penetrator was still connected on the line.
This means that the modem might attempt to dial and then wait for an
answerback tone from the far end modem. If the penetrator was kind enough to
supply the answerback tone from his modem after he heard the system modem
dial, he could make a connection and penetrate the system. Of course some
modems incorporate dial tone detectors and ringback detectors and in fact wait
for dial tone before dialing, and ringback after dialing but fooling those
with a recording of dial tone (or a dial tone generator chip) should pose
little problem.
Trying to call out on a ringing line
Some modems are dumb enough to pick up a ringing line and attempt to
make a call out on it. This fact could be used by a system penetrator to
break dial back security even on joint control or called party control central
offices. A penetrator would merely have to dial in on the dial-out line
(which would work even if it was a separate line as long as the penetrator was
able to obtain it's number), just as the modem was about to dial out. The
same technique of waiting for dialing to complete and then supplying
answerback tone could be used - and of course the same technique of supplying
dial tone to a modem which waited for it would work here too.
Calling the dial-out line would work especially well in cases where
the software controlling the modem either disabled auto-answer during the
period between dial-in and dial-back (and thus allowed the line to ring with
no action being taken) or allowed the modem to answer the line (auto-answer
enabled) and paid no attention to whether the line was already connected when
it tried to dial out on it.
The ring window
However, even carefully written software can be fooled by the ring
window problem. Many central offices actually will connect an incoming call
to a line if the line goes off hook just as the call comes in without first
having put the 20 hz. ringing voltage on the line to make it ring. The ring
voltage in many telephone central offices is supplied asynchronously every 6
seconds to every line on which there is an incoming call that has not been
answered, so if an incoming call reaches a line just an instant after the end
of the ring period and the line clairvoyantly responds by going off hook it
may never see any ring voltage.
This means that a modem that picks up the line to dial out just as our
penetrator dials in may not see any ring voltage and may therefore have no way
of knowing that it is connected to an incoming call rather than the call
originating circuitry of the switch. And even if the switch always rings
before connecting an incoming call, most modems have a window just as they are
going off hook to originate a call when they will ignore transients (such as
ringing voltage) on the assumption that they originate from the going-off-hook
process. [The author is aware that some central offices reverse battery (the
polarity of the voltage on the line) in the answer condition to distinguish it
from the originate condition, but as this is by no means universal few if any
modems take advantage of the information supplied]
In Summary
It is thus impossible to say with any certainty that when a modem goes
off hook and tries to dial out on a line which can accept incoming calls it
really is connected to the switch and actually making an outgoing call. And
because it is relatively easy for a system penetrator to fool the tone
detecting circuitry in a modem into believing that it is seeing dial tone,
ringback and so forth until he supplies answerback tone and connects and
penetrates system security should not depend on this sort of dial-back.
Some Recommendations
Dial back using the same line used to dial in is not very secure and
cannot be made completely secure with conventional modems. Use of dithered
(random) time delays between dial in and dial back combined with allowing the
modem to answer during the wait period (with provisions made for recognizing
the fact that this wasn't the originated call - perhaps by checking to see if
the modem is in originate or answer mode) will substantially reduce this
window of vulnerability but nothing can completely eliminate it.
Obviously if one happens to be connected to an older caller control
switch, using the same line for dial in and dial out isn't secure at all. It
is easy to experimentally determine this, so it ought to be possible to avoid
such situations.
Dial back using a separate line (or line and modem) for dialing out is
much better, provided that either the dial out line is sterile (not readily
traceable by a penetrator to the target system) or that it is a one way line
that cannot accept incoming calls at all. Unfortunately the later technique
is far superior to the former in most organizations as concealing the
telephone number of dial out lines for long periods involves considerable
risk. The author has not tried to order a dial out only telephone line, so he
is unaware of what special charges might be made for this service or even if
it is available.
A final word of warning
In years past it was possible to access telephone company test and
verification trunks in some areas of the country by using mf tones from so
called "blue boxes". These test trunks connect to special ports on telephone
switches that allow a test connection to be made to a line that doesn't
disconnect when the line hangs up. These test connections could be used to
fool a dial out modem, even one on a dial out only line (since the telephone
company needs a way to test it, they usually supply test connections to it
even if the customer can't receive calls).
Access to verification and test ports and trunks has been tightened
(they are a kind of dial-a-wiretap so it ought to be pretty difficult) but in
any as in any system there is always the danger that someone, through
stupidity or ignorance if not mendacity will allow a system penetrator access
to one.
** Some more recent comments **
Since posting this I have had several people suggest use of PBX lines
that can dial out but not be dialed into or outward WATS lines that also
cannot be dialed. Several people have also suggested use of call forwarding
to forward incoming calls on the dial out line to the security office. [This
may not work too well in areas served by certain ESS's which ring the number
from which calls are being forwarded once anyway in case someone forgot to
cancel forwarding. Forwarding is also subject to being cancelled at random
times by central office software reboots]
And since posting this I actually tried making some measurements of
how wide the incoming call window is for the modems we use for dial in at
CRDS. It appears to be at least 2-3 seconds for US Robotics Courier 2400 baud
modems. I found I could defeat same-line-for-dial-out dialback quite handily
in a few dozen tries no matter what tricks I played with timing and watching
modem status in the dial back login software. I eventually concluded that
short of reprogramming the micro in the modem to be smarter about monitoring
line state, there was little I could do at the login (getty) level to provide
much security for same line dialback.
Since it usually took a few tries to break in, it is possible to
provide some slight security improvement by sharply limiting the number of
unsuccessful callbacks per user per day so that a hacker with only a couple of
passwords would have to try over a significant period of time.
Note that dialback on a dedicated dial-out only line is somewhat
secure.
David I. Emery Charles River Data Systems 617-626-1102
983 Concord St., Framingham, MA 01701.
uucp: decvax!frog!die
--
David I. Emery Charles River Data Systems
983 Concord St., Framingham, MA 01701 (617) 626-1102 uucp: decvax!frog!die