Security Management Guide (G06.29+, H06.08+, J06.03+)

Concerns for the System Administration Team
Security Management Guide — 522283-021
6 - 16
Automatic Terminal Authentication
4. The computer terminates the connection and then calls the user back at a
prearranged phone number.
5. The user (or the user’s modem) answers, reestablishes modem connections, and
then continues the logon sequence.
Because the list of phone numbers for any particular user is limited and prearranged,
the chances for intrusion are limited. A logon attempt is unsuccessful when initiated
from a telephone number not on the prearranged list.
In selecting a call-back product, consider these points:

Choose a product that uses a different phone line to call back. Products that use
the same phone line for incoming and outgoing phone calls can be subverted.

Also with separate incoming and outgoing phone lines, no call-back routine is
totally secure (because of protocol limitations between modems and phone
company equipment). Do not rely on a call-back routine as the only means of
authentication.

Have additional authentication take place after the phone connection is
reestablished. For example, if the user selects the wrong location code, the return
call goes to another site, where a waiting modem automatically connects with the
computer. If no additional authentication is required, any person near that terminal
can access the system.

Make special provisions for rovers. A rover is a dial-up user who has no permanent
location (computer service personnel, callers who dial out through PBX systems,
and so forth). To accommodate legitimate rovers, provide a standard dial-up port
that is heavily audited and alarmed, or have a method by which an operator can
connect to a requested phone number.
Automatic Terminal Authentication
Some terminals can be programmed to hold an answer-back string of characters. By
setting a terminal’s answer-back string to a value unknown to the user, you can create
an additional authentication method.
Upon request from the computer, the terminal transmits this string automatically. For
example, if all remote terminals are programmed with unique strings, the dial-up logon
sequence can query the terminal to verify that the terminal is correct. If the terminal
can be remotely programmed, the logon procedure can also update the terminal with a
newly selected value for the next logon, attempt providing a handshake, or initial
interchange, that an intruder would find hard to imitate.
Supplement such authentication with other mechanisms in a secure environment.
Screening Dial-Up Users
Give dial-up access to users who really need it and who will take extra care in
protecting your organization’s resources. Your policy and procedures regarding dial-up