Vulnerability Note VU#315308

Overview

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.

Description

Preconditions: 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.

Impact

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

Solution

Disable the IDEA cipher with SSH1.

Systems Affected (Learn More)

Vendor	Status	Date Notified	Date Updated
SSH Communications Security	Affected	-	06 Feb 2001
OpenSSH	Not 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

References

• VU#25309
• http://www.ssh.com/products/ssh/cert/
• http://www.kb.cert.org/vuls/id/25309
• http://xforce.iss.net/static/6472.php

Credit

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