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

Vulnerability Note VU#13877

Original Release Date: 2001-11-07 | Last Revised: 2003-05-20

Overview

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

Description

Preconditions:

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.

Impact

An attacker can modify arbitrary packets within an encrypted SSH session.

Solution

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.

Vendor Information

13877
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Cisco Systems Inc.

Updated:  June 27, 2002

Status

  Vulnerable

Vendor Statement

No statement is currently available from the vendor regarding this vulnerability.

Vendor Information

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

Addendum

Cisco has published an advisory regarding this issue; for more information, please visit


Please note that this vulnerability is seperate from the issue described in

OpenSSH

Updated:  November 06, 2001

Status

  Vulnerable

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.

SSH Communications Security

Updated:  November 06, 2001

Status

  Vulnerable

Vendor Statement

No statement is currently available from the vendor regarding this vulnerability.

Vendor Information

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

Addendum

This vulnerability first addressed by incorporating code written by CORE-SDI to detect and block CRC32 attacks. However, an implementation error in this code caused the vulnerability described in VU#945216, which was ultimately addressed in Secure Shell 1.2.32, available at


SSH Communications has released a public statment regarding


CVSS Metrics

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

References

Credit

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
Severity Metric: 6.84
Date Public: 1998-06-11
Date First Published: 2001-11-07
Date Last Updated: 2003-05-20 00:25 UTC
Document Revision: 20

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