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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
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Phrack Seventeen
07 April 1988
File 9 of 12 : Data-Tapping Made Easy
--FEATURE ARTICLES AND REVIEWS-
TAPPING COMPUTER DATA IS EASY, AND CLEARER THAN PHONE CALLS !
BY RIC BLACKMON, SYSOP OF A FED BBS
Aquired by Elric of Imrryr & Lunatic Labs UnLtd