Vulnerability Note VU#315308

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

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.

I. 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.

II. Impact

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

III. Solution

Disable the IDEA cipher with SSH1.

Systems Affected

Vendor	Status	Date Notified	Date Updated
OpenSSH	Not Vulnerable	29-Oct-2001
SSH Communications Security	Vulnerable	6-Feb-2001

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

Date Public:	2001-01-18
Date First Published:	2001-01-18
Date Last Updated:	2002-03-05
CERT Advisory:
CVE-ID(s):
NVD-ID(s):
US-CERT Technical Alerts:
Metric:	2.06
Document Revision:	25

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