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CA-2003-24 Buffer Management Vulnerability in OpenSSH


CERT Advisory CA-2003-24 Buffer Management Vulnerability in OpenSSH

Original release date: September 16, 2003
Last revised: --
Source: CERT/CC

A complete revision history can be found at the end of this file.

Systems Affected

* Systems running versions of OpenSSH prior to 3.7
* Systems that use or derive code from vulnerable versions of


There is a remotely exploitable vulnerability in a general buffer
management function in versions of OpenSSH prior to 3.7. This may
allow a remote attacker to corrupt heap memory which could cause a
denial-of-service condition. It may also be possible for an attacker
to execute arbitrary code.

I. Description

A vulnerability exists in the buffer management code of OpenSSH. This
vulnerability affects versions prior to 3.7. The error occurs when a
buffer is allocated for a large packet. When the buffer is cleared, an
improperly sized chunk of memory is filled with zeros. This leads to
heap corruption, which could cause a denial-of-service condition. This
vulnerability may also allow an attacker to execute arbitrary code.
This vulnerability is described in an advisory from OpenSSH


and in FreeBSD-SA-03:12:


Other systems that use or derive code from OpenSSH may be affected.
This includes network equipment and embedded systems. We have
monitored incident reports that may be related to this vulnerability.

Vulnerability Note VU#333628 lists the vendors we contacted about this
vulnerability. The vulnerability note is available from


This vulnerability has been assigned the following Common
Vulnerabilities and Exposures (CVE) number:

II. Impact

While the full impact of this vulnerability is unclear, the most
likely result is heap corruption, which could lead to a denial of

If it is possible for an attacker to execute arbitrary code, then they
may be able to so with the privileges of the user running the sshd
process, typically root. This impact may be limited on systems using
the privilege separation (privsep) feature available in OpenSSH.

III. Solution

Upgrade to OpenSSH version 3.7

This vulnerability is resolved in OpenSSH version 3.7, which is
available from the OpenSSH web site at


Apply a patch from your vendor

A patch for this vulnerability is included in the OpenSSH advisory at


This patch may be manually applied to correct this vulnerability in
affected versions of OpenSSH. If your vendor has provided a patch or
upgrade, you may want to apply it rather than using the patch from
OpenSSH. Find information about vendor patches in Appendix A. We will
update this document as vendors provide additional information.

Use privilege separation to minimize impact

System administrators running OpenSSH versions 3.2 or higher may be
able to reduce the impact of this vulnerability by enabling the
"UsePrivilegeSeparation" configuration option in their sshd
configuration file. Typically, this is accomplished by creating a
privsep user, setting up a restricted (chroot) environment, and adding
the following line to /etc/ssh/sshd_config:

UsePrivilegeSeparation yes

This workaround does not prevent this vulnerability from being
exploited, however due to the privilege separation mechanism, the
intruder may be limited to a constrained chroot environment with
restricted privileges. This workaround will not prevent this
vulnerability from creating a denial-of-service condition. Not all
operating system vendors have implemented the privilege separation
code, and on some operating systems it may limit the functionality of
OpenSSH. System administrators are encouraged to carefully review the
implications of using the workaround in their environment and use a
more comprehensive solution if one is available. The use of privilege
separation to limit the impact of future vulnerabilities is

Appendix A. - Vendor Information

This appendix contains information provided by vendors for this
advisory. As vendors report new information to the CERT/CC, we will
update this section and note the changes in the revision history.
Additional vendors who have not provided direct statements, but who
have made public statements or informed us of their status are listed
in VU#333628. If a vendor is not listed below or in VU#333628, we have
not received their comments.


Our software shares no codebase with the OpenSSH implementation,
therefore we believe that, in our products, this problem does not

Cray, Inc.

Cray Inc. supports OpenSSH through its Cray Open Software (COS)
package. Cray is vulnerable to this buffer management error and is
in the process of compiling OpenSSH 3.7. The new version will be
made available in the next COS release.


A fix for the buffer management vulnerability is available for the
ssh package at

A fix for the ssh-krb5 (ssh with kerberos support) package is
available at

Mandrake Software

Mandrake Linux is affected and MDKSA-2003:090 will be released
today with patched versions of OpenSSH to resolve this issue.


PuTTY is not based on the OpenSSH code base, so it should not be
vulnerable to any OpenSSH-specific attacks.

The CERT/CC thanks Markus Friedl of the OpenSSH project for his
technical assistance in producing this advisory.

Authors: Jason A. Rafail and Art Manion

This document is available from:

CERT/CC Contact Information

Phone: +1 412-268-7090 (24-hour hotline)
Fax: +1 412-268-6989
Postal address:
CERT Coordination Center
Software Engineering Institute
Carnegie Mellon University
Pittsburgh PA 15213-3890

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Revision History

September 16, 2003: Initial release

Version: PGP 6.5.8


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