IT Baseline Protection Manual - The Information Warfare Site

Safeguard Catalogue - Communications Remarks
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communication partners and forwarding forged public keys. If the
communication partners are unable to check the public keys, the attacker can
read and manipulate all of the traffic by decrypting the data himself, then
reading it and modifying it before finally encrypting it with another key and
forwarding it to its destination. This can be prevented with the aid of a suitable
key/certificate management structure. When Secure Shell is in practical
operation, however, a compromise solution is often used, which allows the use
of ssh without any additional infrastructure. When a connection is set up to a
host whose public key is not yet known, the public key is sent via the nonsecure
network and stored in a local database. For all subsequent connections
with that host, the public key can then be checked using the local database. It
must be clarified within the framework of the security concept whether this
approach, which offers a lower level of security against man-in-the-middle
attacks, is adequate for the application concerned.
The Internet drafts contain definitions of cryptographic procedures which have
to be made available by the Secure Shell implementations. There is also the
option, however, of implementing additional cryptographic algorithms. The
procedures that are actually used are negotiated during the establishment of a
connection. Suitable client and server programs must be chosen and an
appropriate configuration put in place in order to ensure that the ssh client and
ssh server agree on eligible cryptographic algorithms which satisfy the
security requirements.
Whenever ssh is used, if possible all other protocols whose functionality is
covered by Secure Shell, i.e. for example the r-services and telnet, should be
entirely deactivated so that the safeguards cannot be circumvented. This
assumes, however, that all communication partners have suitable
implementations at their disposal.
There are known to have been program errors relevant to security in older
implementations of ssh. A version should therefore be used in which any such
deficiencies have been eliminated. Compatibility between implementations
with widely differing program versions may be a problem in some
circumstances. Mixed operation should therefore be avoided if possible.
It should be noted that when ssh is used across firewalls, it is no longer
possible to have content-sensitive control of the data flow.
Additional controls:
- Are r-services or similar protocols used via non-secure communication
channels?
- How is the verification of public host keys regulated when using Secure
Shell (organisational measures, for example)?
- Have all of the known security deficiencies been corrected in the version of
Secure Shell that is used?
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IT-Baseline Protection Manual: Oktober 2000