Vulnerability Note VU#13877

Weak CRC allows packet injection into SSH sessions encrypted with block ciphers

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.

II. Impact

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.

Systems Affected

VU#25309
http://www.cisco.com/warp/public/707/SSH-multiple-pub.html
http://www.corest.com/files/files/11/CRC32.pdf
http://www1.corest.com/common/showdoc.php?idx=131&idxseccion=10
http://www.kb.cert.org/vuls/id/25309
http://www.kb.cert.org/vuls/id/13877
http://www.kb.cert.org/vuls/id/945216
http://www.ssh.com/products/ssh/cert/

Credit

This vulnerability was first published by CORE-SDI on June 11, 1998.

This document was written by Jeffrey P. Lanza.

Other Information

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