Vulnerability Note VU#851340

OpenSSH contains a race condition vulnerability

Overview

A race condition vulnerability exists in the OpenSSH daemon. Successful exploitation of this vulnerability may result in a denial-of-service condition.

I. Description

OpenSSH is an open source client and server implementation of the Secure Shell (SSH) protocol.

The OpenSSH server includes the ability to authenticate via the Generic Security Services Application Programming Interface (GSSAPI). Versions of OpenSSH prior to 4.4 contain a race condition in a signal handler during a logging operation prior to user authentication.

From the OpenSSH 4.4 release notes:

The signal handler was vulnerable to a race condition that could be exploited to perform a pre-authentication denial of service. On portable OpenSSH, this vulnerability could theoretically lead to pre-authentication remote code execution if GSSAPI authentication is enabled, but the likelihood of successful exploitation appears remote.

II. Impact

A remote, unauthenticated attacker may be able to cause the OpenSSH server to crash, thereby creating a denial-of-service condition

III. Solution

Upgrade

See the systems affected section of this document for information about specific vendors. Users who compile OpenSSH from source are encouraged to update to the most recent version.

Restrict access
Restricting access to the SSH daemon may mitigate the affects of this vulnerability. Administrators can use application-level access controls or firewall rules to restrict access to the SSH server.

Disable the OpenSSH server
If the SSH server functionality is not required, disabling it will limit exposure to this vulnerability. Refer to system-specific documentation on how to disable the OpenSSH server.

Systems Affected

Vendor	Status	Date Updated
Apple Computer, Inc.	Vulnerable	13-Mar-2007
FreeBSD, Inc.	Vulnerable	4-Oct-2006
OpenSSH	Unknown	3-Oct-2006
Red Hat, Inc.	Vulnerable	3-Oct-2006
Ubuntu	Vulnerable	3-Oct-2006

References

http://rhn.redhat.com/errata/RHSA-2006-0697.html
http://secunia.com/advisories/22173/
http://openssh.org/txt/release-4.4
http://secunia.com/advisories/22208/
http://secunia.com/advisories/22236/
http://secunia.com/advisories/22183/
http://www.ubuntu.com/usn/usn-355-1
http://tools.ietf.org/html/rfc2743
http://support.avaya.com/elmodocs2/security/ASA-2006-216.htm
http://secunia.com/advisories/22362/
http://www.securityfocus.com/bid/20241
http://docs.info.apple.com/article.html?artnum=305214

Credit

OpenSSH credits Mark Dowd for reporting this vulnerability.

This document was written by Ryan Giobbi.

Other Information

Date Public	09/29/2006
Date First Published	10/04/2006 04:18:01 PM
Date Last Updated	03/13/2007
CERT Advisory
CVE-ID(s)	CVE-2006-5051
NVD-ID(s)	CVE-2006-5051
US-CERT Technical Alerts
Metric	1.66
Document Revision	34

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Produced 2006 by US-CERT, a government organization
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