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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.