Software Engineering Institute

Preconditions:

Attacker has a fragment of plaintext and its corresponding ciphertext.
Attacker must be able to actively intercept a connection attempt or hijack an existing SSH session.
Session is encrypted using a block cipher.
Compression is disabled.

SSH1 sessions that encrypt traffic with block ciphers in cipher feedback (CFB) mode are vulnerable to an attack similar to one described in VU#315308. However, if the attacker has additional information consisting of both a fragment of plaintext and its corresponding ciphertext, it is then possible to overcome the protection introduced by cipher feedback, thus allowing the attacker to modify any packet in the stream.

As in VU#315308, this vulnerability is caused by inherent weaknesses in CRC checksums. For more details on these weaknesses, please see VU#25309. Also, it is important to note that this vulnerability also affects SSH1 sessions that use block ciphers in cipher block chaining (CBC) mode.

An attacker can modify arbitrary packets within an encrypted SSH session.

Apply a patch from your vendor

In June 1998, CORE-SDI released code to detect and block attacks exploiting this vulnerability. This code was subsequently incorporated into several SSH implementations, but it contained a flaw that introduced a remote integer overflow. For vendor-specific information regarding this vulnerability, please see the Systems Affected section of this document. For more information regarding the vulnerability introduced by previous attempts to patch this vulnerability, please see VU#945216.