Vulnerability Note VU#13877

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

Original Release date: 06 Nov 2001 | Last revised: 19 May 2003


There is an information integrity vulnerability in the SSH1 protocol that allows packets encrypted with a block cipher to be modified without notice.



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.

Systems Affected (Learn More)

VendorStatusDate NotifiedDate Updated
Cisco Systems Inc.Affected-27 Jun 2002
OpenSSHAffected-06 Nov 2001
SSH Communications SecurityAffected-06 Nov 2001
If you are a vendor and your product is affected, let us know.

CVSS Metrics (Learn More)

Group Score Vector
Base N/A N/A
Temporal N/A N/A
Environmental N/A N/A



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

This document was written by Jeffrey P. Lanza.

Other Information

  • CVE IDs: CVE-1999-1085
  • Date Public: 11 Jun 98
  • Date First Published: 06 Nov 2001
  • Date Last Updated: 19 May 2003
  • Severity Metric: 6.84
  • Document Revision: 20


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