Vulnerability Note VU#851340

OpenSSH contains a race condition vulnerability

Original Release date: 04 Oct 2006 | Last revised: 13 Mar 2007


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


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.


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



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 (Learn More)

VendorStatusDate NotifiedDate Updated
Apple Computer, Inc.Affected-13 Mar 2007
FreeBSD, Inc.Affected-04 Oct 2006
Red Hat, Inc.Affected-03 Oct 2006
UbuntuAffected-03 Oct 2006
OpenSSHUnknown-03 Oct 2006
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



OpenSSH credits Mark Dowd for reporting this vulnerability.

This document was written by Ryan Giobbi.

Other Information

  • CVE IDs: CVE-2006-5051
  • Date Public: 29 Sep 2006
  • Date First Published: 04 Oct 2006
  • Date Last Updated: 13 Mar 2007
  • Severity Metric: 1.66
  • Document Revision: 34


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