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Category:   Application (Security)  >   SSH Vendors:   SSH Communications
(HP Issues Fix for Tru64) SSH Secure Shell RSA Signature Verification Flaw May Let Remote Users Forge Valid Signatures
SecurityTracker Alert ID:  1007576
SecurityTracker URL:  http://securitytracker.com/id/1007576
CVE Reference:   GENERIC-MAP-NOMATCH   (Links to External Site)
Date:  Aug 26 2003
Impact:   Host/resource access via network, User access via network
Fix Available:  Yes  Vendor Confirmed:  Yes  
Version(s): SSH Secure Shell 3.1.0 - 3.1.7 and 3.2.0 - 3.2.4; SSH IPSEC Express Toolkit 5.0.0
Description:   An authentication vulnerability was reported in the SSH Secure Shell software and in the SSH IPSEC Express Toolkit. A remote user may be able to forge RSA signatures.

SSH reports that certain RSA signatures may be incorrectly verified as valid when performing host or user authentication using digital certificates and RSA keys. The vulnerability affects the RSA PKCS v1.5 signature scheme implementation, according to the vendor.

The vendor reports that other user authentication methods are not affected (including password, RSA SecurID, and keyboard-interactive authentication methods).

The vulnerability is due to "incorrect error reporting in the code path."

A remote user may be able to forge a valid signature without having the corresponding RSA private key. However, the vendor reports that this may be difficult to exploit successfully.

Impact:   A remote user may be able to forge a valid RSA signature.
Solution:   The vendor has released a fix for HP Tru64 UNIX 5.1A, available at:

http://h30097.www3.hp.com/unix/ssh/index.html

For HP Tru64 UNIX 5.1B, a fix will be made available in the next aggregate patch kit (5.1B PK3), according to HP. As a workaround, HP recommends using the DSA algorithm as described below:

To choose the DSA algorithm rather than the RSA algorithm when generating host or client key pairs, either specify the "-t dsa" or, since DSA is the default, just don't specify the - -t option at all.

Example for explicitly specifying DSA:

# ssh-keygen2 -t dsa

Vendor URL:  www.ssh.com/company/newsroom/article/454/ (Links to External Site)
Cause:   Authentication error
Underlying OS:  UNIX (Tru64)
Underlying OS Comments:  5.1B PK2 (BL22), 5.1A (all Base Levels)

Message History:   This archive entry is a follow-up to the message listed below.
Jun 30 2003 SSH Secure Shell RSA Signature Verification Flaw May Let Remote Users Forge Valid Signatures



 Source Message Contents

Subject:  SSRT3588 (Tru64/ssh) Security Bulletin



-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

SECURITY BULLETIN

REVISION: 0

SSRT3588 - (Tru64) A Potential Security Vulnerability With ssh
- -----------------------------------------------------------
NOTICE: There are no restrictions for distribution of this
Bulletin provided that it remains complete and intact.

RELEASE DATE: 25 August 2003

SEVERITY:  1

SOURCE:  HEWLETT-PACKARD COMPANY
Software Security Response Team

REFERENCE:  None

PROBLEM SUMMARY
A potential security vulnerability may exist where HP Tru64
UNIX running ssh could allow local or remote users to gain
unauthorized privileged access because ssh (Secure Shell)
incorrectly handled RSA signatures when using digital
certificates and RSA keys.

VERSIONS IMPACTED
HP Tru64 UNIX 5.1B  PK2 (BL22)

HP Tru64 UNIX 5.1A (all Base Levels)

RESOLUTION
HP Tru64  UNIX 5.1A
An updated version of ssh containing the corrections for this
potential security vulnerability is available for download for
Tru64 UNIX 5.1A systems, from the following location:

http://h30097.www3.hp.com/unix/ssh/index.html

HP Tru64 UNIX 5.1B
The correction for Tru64 UNIX 5.1B will be available in the
next aggregate patch kit , 5.1B PK3. Until the next aggregate
patch kit for 5.1B is available, HP recommends using the DSA
algorithm workaround as outlined below.

HP recommends using the DSA algorithm rather than the RSA
algorithm until such time the vulnerability is corrected.

The  workaround for 5.1B is to use DSA, the default public key
algorithm, and other user authentication methods that are not
affected by the potential vulnerability.

To choose the DSA algorithm rather than the RSA algorithm when
generating host or client key pairs, either specify the
"-t dsa" or, since DSA is the default, just don't specify the
- -t option at all.

Example for explicitly specifying DSA:

# ssh-keygen2 -t dsa


SUPPORT: For further information, contact HP Services.

SUBSCRIBE: To subscribe to automatically receive future Security
Advisories from the Software Security Response Team via electronic
mail: http://www.support.compaq.com/patches/mail-list.shtml

REPORT: To report a potential security vulnerability with any HP
supported product, send email to: security-alert@hp.com

As always, HP urges you to periodically review your system
management and security procedures. HP will continue to review
and enhance the security features of its products and work with
our customers to maintain and improve the security and integrity
of their systems.

"HP is broadly distributing this Security Bulletin in order to
bring to the attention of users of the affected HP products the
important security information contained in this Bulletin.
HP recommends that all users determine the applicability of this
information to their individual situations and take appropriate
action. HP does not warrant that this information is necessarily
accurate or complete for all user situations and, consequently,
HP will not be responsible for any damages resulting from user's
use or disregard of the information provided in this Bulletin."

(c)Copyright 2001, 2003 Hewlett-Packard Development Company, L.P.
Hewlett-Packard Company shall not be liable for technical or
editorial errors or omissions contained herein. The information
in this document is subject to change without notice.
Hewlett-Packard Company and the names of Hewlett-Packard products
referenced herein are trademarks of Hewlett-Packard Company in
the United States and other countries. Other product and company
names mentioned herein may be trademarks of their respective owners.

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