TUCoPS :: General Information :: dialback.txt

Dial back isn't always secure

Date:  Thu, 28-Feb-85
Subject:  Dial Back isn't always secure
From: [usenet via anonymous donor]

        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

        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 clairvointly responds by going off hook it may never see any ring

        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 so 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 tracable 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 ity of the Hackers controling satelites in geo-sync 
orbit are being moved out of there assigned orbits. Granted they 
did not move the bird,but did gain control of the rotation control 
for the satelite.
And  it was stated that the information needed to do such  things 
was  found  on an underground bulletin board.  Ok,that  might  be 
true,but information that is far more valuable to people on earth 
is being posted on the boards. And the information comes from the 
trash  can  or from insiders who have become disgruntled or  just 
from plain old research looking for publicly available sources. 
Some  of  these public sources are to include users manuals  and  
system documentation. Others are to include users groups and just 

Other  interesting facts about the boards is that they contain  a 
group   of   sub-sections  that  are  to  include forwarded to line B, to avoid callers when it is to dial out.
Line C is some phone in the attacker's control.  The attacker forwards
line C to line A, and then calls line C from yet another phone.  The
call is forwarded only from C to A, not from C to A to B.  --
                Joe Eykholt

[Opinions expressed by me are not necessarily held by any other entity.]


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