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CA-2002-18 OpenSSH Vulnerabilities in Challenge Response Handling


CERT Advisory CA-2002-18 OpenSSH Vulnerabilities in Challenge Response

Original release date: June 26, 2002
Last revised: --
Source: CERT/CC

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

Systems Affected

* OpenSSH versions 2.3.1p1 through 3.3


There are two related vulnerabilities in the challenge response
handling code in OpenSSH versions 2.3.1p1 through 3.3. They may allow
a remote intruder to execute arbitrary code as the user running sshd
(often root). The first vulnerability affects OpenSSH versions 2.9.9
through 3.3 that have the challenge response option enabled and that
use SKEY or BSD_AUTH authentication. The second vulnerability affects
PAM modules using interactive keyboard authentication in OpenSSH
versions 2.3.1p1 through 3.3, regardless of the challenge response
option setting. Additionally, a number of other possible security
problems have been corrected in OpenSSH version 3.4.

I. Description

Two related vulnerabilities have been found in the handling of
challenge responses in OpenSSH.

The first vulnerability is an integer overflow in the handling of the
number of responses received during challenge response authentication.
If the challenge response configuration option is set to yes and the
system is using SKEY or BSD_AUTH authentication then a remote intruder
may be able to exploit the vulnerability to execute arbitrary code.
This vulnerability is present in versions of OpenSSH 2.9.9 through
3.3. An exploit for this vulnerability is reported to exist. This
vulnerability is partially described in a recent ISS security advisory
available at

The second vulnerability is a buffer overflow involving the number of
responses received during challenge response authentication.
Regardless of the setting of the challenge response configuration
option, systems using PAM modules that use interactive keyboard
authentication (PAMAuthenticationViaKbdInt), may be vulnerable to the
remote execution of code. At this time, it is not known if this
vulnerability is exploitable. Both vulnerabilities are corrected by
the patches in a recent OpenSSH security advisory available from

Both vulnerabilities exploit features present only in version 2 of the
SSH protocol.

Vulnerability Note VU#369347 lists the vendors we contacted about this
vulnerability. The vulnerability note is available from

II. Impact

A remote attacker can execute code with the privileges of the user
running the sshd (often root). These vulnerabilities may also be used
to cause a denial-of-service condition.

III. Solution

Upgrade to OpenSSH version 3.4

These vulnerabilities are eliminated by upgrading to OpenSSH version
3.4, which is available from the OpenSSH web site at

OpenSSH version 3.4 will correct several other software defects with
potential security implications not described in this advisory.

Apply a patch from your vendor

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

This patch may be manually installed with minor changes to correct
these vulnerabilities in all affected versions of OpenSSH. Please note
that applying the patches described in the OpenSSH advisory does not
correct the other software defects with potential security
implications not described in this advisory.

If your vendor has provided a patch to correct these vulnerabilities,
you may want to apply their patch rather than upgrading your version
of sshd. System administrators may want to confirm whether their
vendor's patch includes the other possible vulnerabilities corrected
in OpenSSH 3.4. More information about vendor-specific patches can be
found in the vendor section of this document. Because the publication
of this advisory was unexpectedly accelerated, statements from all of
the affected vendors were not available at publication time. We will
update this document as vendors provide additional information.

Disable SSH protocol version 2

Since both vulnerabilities are present only in protocol version 2
features, disabling version 2 of the protocol will prevent both
vulnerabilities from being exploited. Typically, this is accomplished
by adding the following line to /etc/ssh/sshd_config:

Protocol 1

This option may set to "2,1" by default. System administrators should
be aware that disabling protocol version 2 may prevent the sshd daemon
from accepting connections in certain configurations. Applying one or
both of the configuration changes described below may be a less
disruptive workaround for this problem.

Disable challenge response authentication

For OpenSSH versions greater than 2.9, system administrators can
disable the vulnerable portion of the code by setting the
"ChallengeResponseAuthentication" configuration option to "no" in
their sshd configuration file. Typically, this is accomplished by
adding the following line to /etc/ssh/sshd_config:

ChallengeResponseAuthentication no

This option may be enabled (set to "yes") by default. This workaround
should prevent the first vulnerability from being exploited if SKEY or
BSD_AUTH authentication is used. It will not prevent the possible
exploitation of the vulnerability via PAM interactive keyboard

Disable PAM authentication via interactive keyboard

For OpenSSH versions greater than 2.9, system administrators can
disable the vulnerable portion of the code affecting the PAM
authentication issue by setting the "PAMAuthenticationViaKbdInt"
configuration option to "no" in their sshd configuration file.
Typically, this is accomplished by adding the following line to

PAMAuthenticationViaKbdInt no

This option may be disabled (set to "no") by default. This workaround
should prevent the second vulnerability from being exploited if PAM
interactive keyboard authentication is used. It will not prevent the
possible exploitation of the vulnerability via SKEY or BSD_AUTH

Disable both options in older versions of OpenSSH

For OpenSSH versions between 2.3.1p1 and 2.9, system adminstrators
will instead need to set the following options in their ssh
configuration file:

KbdInteractiveAuthentication no
ChallengeResponseAuthentication no

Setting both of these options is believed to prevent the exploitation
of the vulnerabilities regardless of which authentication mechanisms
are used.

Use privilege separation to minimize impact

System administrators running OpenSSH versions 3.2 or 3.3 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 adding the
following line to /etc/ssh/sshd_config:

UsePrivilegeSeparation yes

This workaround does not prevent these vulnerabilities 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 these
vulnerabilities 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 our revision history. If a
particular vendor is not listed below, we have not received their

Compaq Computer Corporation

SOURCE: Compaq Computer Corporation, a wholly-owned subsidiary of
Hewlett-Packard Company and Hewlett-Packard Company HP Services.
Software Security Response Team


At the time of writing this document, Compaq is currently
investigating the potential impact to HP Tru64 UNIX, commercial
version of SSH for V5.1a.

As further information becomes available notice will be provided of
the completion/availability of any necessary patches through standard
product and security bulletin announcements and be available from your
normal HP Services support channel.


Caldera OpenLinux OpenSSH has neither the S/KEY nor BSD Auth features
compiled in, so it is not vulnerable to the Challenge/Response
vulnerability. We do have the ChallengeResponseAuthentication option
on by default, however, so to be safe, we recommend that the option be
disabled in the sshd_config file.

In addition, the sshd_config PAMAuthenticationViaKbdInt option is off
by default, so OpenLinux is not vulnerable to the other alleged
vulnerability in a default configuration, either. However, Caldera
recommends that this option be disabled if it has been enabled by the
system administrator.

Cray, Inc.

Cray, Inc. has found the OpenSSH released in Cray Open Software 3.0 to
be vulnerable. Please see Field Notice 5105 and spr 722588 for fix


Debian 2.2 (the current stable release) is not affected by these
problems. The current versions of our "testing" distribution, to
become Debian 3.0, and our "unstable" distribution, are both affected
by default.

We recommend that users be certain that both:

ChallengeResponseAuthentication no


PAMAuthenticationViaKbdInt no

are present and uncommented in /etc/ssh/sshd_config (and that the
server is restarted). Also, we recommend the use of version 3.3p1, now
available from (DSA-134). Stable users do not need
to upgrade and may wish to wait until the packages have received
better testing.

We intend to provide 3.4p1 packages in the near future.


Guardian Digital ships OpenSSH in all versions of EnGarde Secure
Linux. Version 3.3p1 was introduced by ESA-20020625-015 on June 25,
2002. This update introduces privilege separation. All users are
strongly urged to upgrade to this version as soon as possible.

An upgrade to version 3.4p1 (which properly fixes the bugs) will be
made available sometime in the next few days.

Hewlett-Packard Company

Hewlett-Packard provides a version of SSH: HP-UX Secure Shell
(T1471AA) for HP-UX versions 11.00 and 11i. We are investigating to
determine whether this product is vulnerable.

IBM Corporation

IBM's AIX operating system does not ship with OpenSSH; however,
OpenSSH is available for installation on AIX via the Linux Affinity
Toolkit. The version included on the CD containing the Toolkit is
vulnerable to the latest discovered vulnerability discussed here as is
the version of OpenSSH available for downloading from the IBM Linux
Affinity website. Anyone running this version is advised to follow the
recommendations above to limit their vulnerability.

We working with the changes for version 3.4 and will have a new
package availble for download as soon as possible. When available the
new packages can be downloaded from:

This site contains Linux Affinity applications containing
cryptographic algorithms, and new users of this site are asked to
register first.


Lotus products are not vulnerable to this problem.

Mandrake Software

MandrakeSoft released OpenSSH 3.3p1 in updates Monday night to
mitigate this vulnerability. Updates to OpenSSH 3.4p1 will be
available for download later this week.

Microsoft Corporation

Microsoft products are not affected by the issues detailed in this

Network Appliance

NetApp systems are not vulnerable to this problem.





Process Software

MultiNet, TCPware, and SSH for OpenVMS are not affected by the
problems outlined in this advisory.

RedHat Inc.

Red Hat Linux versions 7, 7.1, 7.2 and 7.3 as well as Red Hat Linux
Advanced Server version 2.1 ship with OpenSSH. The Red Hat Linux
OpenSSH packages were not compiled with either BSD_AUTH or SKEY
enabled, therefore in order to be vulnerable to this issue a user
would need to have enabled the configuration option
"PAMAuthenticationViaKbdInt" in their sshd configuration file (the
default is disabled).

We are continuing to investigate this vulnerability and will release
updated packages where appropriate.


At this time, SGI does not ship OpenSSH as a part of IRIX.

The OpenSSH privilege separation code mostly works with IRIX, but it
uses a flag to mmap that isn't in IRIX (MAP_ANON) for compression so
you can't have both on at the same time. IRIX doesn't ship with PAM so
a lot of the PAM issues aren't issues for us.

The CERT/CC thanks Theo de Raadt and Markus Friedl of the OpenSSH
project for their technical assistance in producing this advisory.

Author: Cory F. Cohen

This document is available from:

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Revision History
June 26, 2002: Initial release

Version: PGP 6.5.8


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