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OpenSSH Remote Challenge Vulnerability
27 June 2002
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From: X-Force <xforce@iss.net>

Internet Security Systems Security Advisory
June 26, 2002

OpenSSH Remote Challenge Vulnerability


ISS X-Force has discovered a serious vulnerability in the default installation of OpenSSH on the OpenBSD operating system. OpenSSH is a free version of the SSH (Secure Shell) communications suite and is used as a secure replacement for protocols such as Telnet, Rlogin, Rsh, and Ftp. OpenSSH employs end-to-end encryption (including all passwords) and is resistant to network monitoring, eavesdropping, and connection hijacking attacks. X-Force is aware of active exploit development for this vulnerability.


OpenBSD, FreeBSD-Current, and other OpenSSH implementations may be vulnerable to a remote, superuser compromise.

Affected Versions:

OpenBSD 3.0
OpenBSD 3.1
OpenSSH 3.0-3.2.3

OpenSSH version 3.3 implements "privilege separation" which mitigates the risk of a superuser compromise. Prior to the release of this advisory, ISS and OpenBSD encouraged all OpenSSH users to upgrade to version 3.3. Versions of FreeBSD-Current built between March 18, 2002 and June 23, 2002 are vulnerable to remote superuser compromise. Privilege separation was implemented in FreeBSD-Current on June 23, 2002.

Note: OpenSSH is included in many operating system distributions, networking equipment, and security appliances. Refer to the following address for information about vendors that implement OpenSSH: http://www.openssh.com/users.html


A vulnerability exists within the "challenge-response" authentication mechanism in the OpenSSH daemon (sshd). This mechanism, part of the SSH2 protocol, verifies a user's identity by generating a challenge and forcing the user to supply a number of responses. It is possible for a remote attacker to send a specially-crafted reply that triggers an overflow. This can result in a remote denial of service attack on the OpenSSH daemon or a complete remote compromise. The OpenSSH daemon runs with superuser privilege, so remote attackers can gain superuser access by exploiting this vulnerability.

OpenSSH supports the SKEY and BSD_AUTH authentication options. These are compile-time options. At least one of these options must be enabled before the OpenSSH binaries are compiled for the vulnerable condition to be present. OpenBSD 3.0 and later is distributed with BSD_AUTH enabled. The SKEY and BSD_AUTH options are not enabled by default in many distributions. However, if these options are explicitly enabled, that build of OpenSSH may be vulnerable.


Internet Scanner X-Press Update 6.13 includes a check, OpenSshRunning, to detect potentially vulnerable installations of OpenSSH. XPU 6.13 is available from the ISS Download Center at: http://www.iss.net/download. For questions about downloading and installing this XPU, email support@iss.net.

ISS X-Force recommends that system administrators disable unused OpenSSH authentication mechanisms. Administrators can remove this vulnerability by disabling the Challenge-Response authentication parameter within the OpenSSH daemon configuration file. This filename and path is typically: /etc/ssh/sshd_config. To disable this parameter, locate the corresponding line and change it to the line below:

ChallengeResponseAuthentication no

The "sshd" process must be restarted for this change to take effect. This workaround will permanently remove the vulnerability. X-Force recommends that administrators upgrade to OpenSSH version 3.4 immediately. This version implements privilege separation, contains a patch to block this vulnerability, and contains many additional pro- active security fixes. Privilege separation was designed to limit exposure to known and unknown vulnerabilities. Visit http://www.openssh.com for more information.

Additional Information:

ISS X-Force and Black Hat consulting will host a presentation titled, "Professional Source Code Auditing" at Black Hat Briefings USA 2002. The presentation will explore advanced source code auditing techniques as well as secure development best-practices. Please refer to
http://www.blackhat.com and
http://www.blackhat.com/html/bh-usa-02/bh-usa-02-speakers.html#Dowd for more information.


The vulnerability described in this advisory was discovered and researched by Mark Dowd of the ISS X-Force. ISS would like to thank Theo de Raadt of the OpenBSD Project for his assistance with this advisory.


About Internet Security Systems (ISS)
Founded in 1994, Internet Security Systems (ISS) (Nasdaq: ISSX) is a pioneer and world leader in software and services that protect critical online resources from an ever-changing spectrum of threats and misuse. Internet Security Systems is headquartered in Atlanta, GA, with additional operations throughout the Americas, Asia, Australia, Europe and the Middle East.

Copyright (c) 2002 Internet Security Systems, Inc. All rights reserved worldwide.

Permission is hereby granted for the electronic redistribution of this document. It is not to be edited or altered in any way without the express written consent of the Internet Security Systems X-Force. If you wish to reprint the whole or any part of this document in any other medium excluding electronic media, please email xforce@iss.net for permission.

Disclaimer: The information within this paper may change without notice. Use of this information constitutes acceptance for use in an AS IS condition. There are NO warranties, implied or otherwise, with regard to this information or its use. Any use of this information is at the user's risk. In no event shall the author/distributor (Internet Security Systems X-Force) be held liable for any damages whatsoever arising out of or in connection with the use or spread of this information.

X-Force PGP Key available on MIT's PGP key server and PGP.com's key server, as well as at http://www.iss.net/security_center/sensitive.php

Please send suggestions, updates, and comments to: X-Force


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