IT Baseline Protection Manual - The Information Warfare Site

Safeguard Catalogue - Communications Remarks
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S 5.49 Callback based on CLIP/COLP
Initiation responsibility: IT Security Management, Administrators
Implementation responsibility: Administrators
Many communication cards offer an automatic callback function. If this
function is active and the communication card receives a call, it waits for a
connection to be established successfully, then closes it down again
immediately, and calls a preset number back. This prevents unauthorised
callers from misusing a remote port as long as access is not possible via a
preset number. Callback should be used whenever a specific communications
partner needs to dial in automatically. It should be noted that automatic
callback also accepts the costs of data transfer.
ISDN offers a variant of callback to a specific subscriber number: Using
Calling Line Identification Presentation (CLIP), the addressed ISDN card
identifies the source of the call request and compares the forwarded subscriber
number with a table of subscriber numbers. If a valid subscriber number was
forwarded via CLIP, the corresponding number stored in the table is called
back.
An advantage here, compared with authentication exclusively via CLIP/COLP
(refer to S 5.48 Authentication via CLIP/COLP), is that even if an
unauthorised subscriber feigns an authorised call number, the call request is
refused because the unauthorised subscriber cannot be accessed via the
specified callback number.
Additional controls:
- Has payment of costs in the callback mode been clarified?
- When were preset call numbers last checked?
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IT-Baseline Protection Manual: Oktober 2000