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

Vulnerability Note VU#315308

Original Release Date: 2001-01-18 | Last Revised: 2002-03-05

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.

Vendor Information

315308
 
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SSH Communications Security Affected

Updated:  February 06, 2001

Status

Affected

Vendor Statement

We have not received a statement from the vendor.

Vendor Information

The vendor has not provided us with any further information regarding this vulnerability.

Addendum

The CERT/CC has no additional comments at this time.

If you have feedback, comments, or additional information about this vulnerability, please send us email.

OpenSSH Not Affected

Updated:  October 29, 2001

Status

Not Affected

Vendor Statement

See http://www.openssh.com/security.html.

Vendor Information

The vendor has not provided us with any further information regarding this vulnerability.

Addendum

The CERT/CC has no additional comments at this time.

If you have feedback, comments, or additional information about this vulnerability, please send us email.


CVSS Metrics

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References

Acknowledgements

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: None
Severity Metric: 2.06
Date Public: 2001-01-18
Date First Published: 2001-01-18
Date Last Updated: 2002-03-05 20:23 UTC
Document Revision: 25

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