Vulnerability Note VU#315308

Weak CRC allows last block of IDEA-encrypted SSH packet to be changed without notice

Original Release date: 18 Jan 2001 | Last revised: 05 Mar 2002


There is an information integrity vulnerability in the SSH1 protocol that allows the last block of an IDEA-encrypted session to be modified without notice.



Session is encrypted using IDEA cipher.
Compression is disabled.

SSH clients configured to use the IDEA cipher do so in cipher feedback (CFB) mode, in which each block of ciphertext is used to encrypt the next block. Because of this feedback, if any block of the ciphertext is modified, the remaining blocks will be garbled upon decryption. However, the last block of the SSH stream is not protected against modification, which means that it is possible to modify it using the properties of CRC checksumming described in VU#25309.

Note that this vulnerability can only be used on the last block of an SSH stream.


Attackers can modify the last block of an SSH packet encrypted with IDEA.


Disable the IDEA cipher with SSH1.

Systems Affected (Learn More)

VendorStatusDate NotifiedDate Updated
SSH Communications SecurityAffected-06 Feb 2001
OpenSSHNot Affected-29 Oct 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



The CERT/CC thanks Antti Huima, Tuomas Aura, and Janne Salmi for their analysis and Tatu Ylonen for bringing this vulnerability to our attention.

This document was written by Jeffrey P. Lanza.

Other Information

  • CVE IDs: Unknown
  • Date Public: 18 Jan 2001
  • Date First Published: 18 Jan 2001
  • Date Last Updated: 05 Mar 2002
  • Severity Metric: 2.06
  • Document Revision: 25


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