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TUCoPS :: Unix :: General :: ca-9803.txt

CERT Advisory 98-03 SSH-Agent Vulnerability

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CERT* Advisory CA-98.03
Original issue date: Jan. 22, 1998
Last revised: March 2, 1998  Updates section - described two cases in which
                             the vulnerability is present.

              A complete revision history is at the end of this file.

Topic:  Vulnerability in ssh-agent

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   The text of this advisory was originally released on January 20, 1998,
   as SNI-23, developed by Secure Networks, Inc. (SNI). To more widely
   broadcast this information, we are reprinting the SNI advisory here with
   their permission. Some technical details in the original advisory are
   not included in this reprint, and these are indicated thus:


   We have also removed SNI's PGP public key block and added our contact

   The original advisory is available from

   We will update this advisory as we receive additional information.
   Look for it in an "Updates" section at the end of the advisory.


This advisory details a vulnerabily in the SSH cryptographic login
program.  The vulnerability enables users to use RSA credentials
belonging to other users who use the ssh-agent program.  This
vulnerability may allow an attacker on the same local host to login
to a remote server as the user utilizing SSH.

Problem Description:

In order to avoid forcing users of RSA based authentication to go
through the trouble of retyping their pass phrase every time they wish
to use ssh, slogin, or scp, the SSH package includes a program called
ssh-agent, which manages RSA keys for the SSH program.  The ssh-agent
program creates a mode 700 directory in /tmp, and then creates an
AF_UNIX socket in that directory.  Later, the user runs the ssh-add
program, which adds his private key to the set of keys managed by the
ssh-agent program.  When the user wishes to access a service which
permits him to log in using only his RSA key, the SSH client connects
to the AF_UNIX socket, and asks the ssh-agent program for the key.

Unfortunately, when connecting to the AF_UNIX socket, the SSH client is
running as super-user, and performs insufficient permissions checking.
This makes it possible for users to trick their SSH clients into using
credentials belonging to other users.  The end result is that any user
who utilizes RSA authentication AND uses ssh-agent, is vulnerable.
Attackers can utilize this vulnerability to access remote accounts
belonging to the ssh-agent user.


Vulnerable Systems:

This vulnerability effects the Unix versions of SSH ONLY.

SSH for unix versions 1.2.17 through 1.2.21 are vulnerable if installed
with default permissions.  Versions of SSH prior to 1.2.17 are subject to
a similar (but different) attack.

F-Secure SSH for Unix systems prior to release 1.3.3 ARE vulnerable.

You can determine the version of SSH you are running by issuing the case
sensitive command:

% ssh -V

Version 1.1 of the windows-based SSH client sold by Data Fellows Inc.
under the F-Secure brand name is NOT vulnerable to this attack.

Versions 1.0 and 1.0a of Mac SSH are NOT vulnerable to this attack.

Fix Resolution:

Non-commercial users:

If using the free non-commercial SSH distribution for Unix, administrators
are urged to upgrade to SSH 1.2.22 or later.  Updated versions of the free
unix SSH can be found at

Commercial users:

F-Secure SSH version 1.3.3 fixes this security problem.  If you are using
the commercial Data Fellows SSH package and you have a support contract,
you can obtain SSH version 1.3.3 from your local retailer.

Users without a support contract can obtain a diff file which fixes
this problem.  This file can be obtained from:


As a temporary workaround, administrators may remove the setuid bit from
the SSH binary.  This will prevent the attack from working, but will
disable a form of authentication documented as rhosts-RSA.  For example,
if your SSH binary is in the /usr/local/bin directory, the following
command will remove the setuid bit from the SSH binary:

# chmod u-s /usr/local/bin/ssh

Additional Information

SSH is a cryptographic rsh, rlogin, and rcp replacement.  SSH was
written by Tatu Ylonen <>.  For more information about the
noncommercial unix version of SSH, please see

Commercial versions of ssh are marketed by Data Fellows Inc.  For
information about the F-secure ssh derivatives sold by Data Fellows Inc,
please see

This vulnerability was discovered by David Sacerdote <>.


Copyright Notice
The contents of this advisory are Copyright (C) 1997 Secure Networks
Inc, and may be distributed freely provided that no fee is charged for
distribution, and that proper credit is given.

 You can find Secure Networks papers at
 and advisories at

 You can browse our web site at

 You can subscribe to our security advisory mailing list by sending mail
 to with the line "subscribe sni-advisories"


- ----------------------------------------------------------------------------
The CERT Coordination Center thanks Secure Networks, Inc. for permission to
reproduce technical content from their advisory SNI-23, which is copyrighted
1997 Secure Networks, Inc.
- ----------------------------------------------------------------------------

If you believe that your system has been compromised, contact the CERT
Coordination Center or your representative in the Forum of Incident Response
and Security Teams (see

CERT/CC Contact Information
- ----------------------------

Phone    +1 412-268-7090 (24-hour hotline)
                CERT personnel answer 8:30-5:00 p.m. EST(GMT-5) / EDT(GMT-4)
                and are on call for emergencies during other hours.

Fax      +1 412-268-6989

Postal address
         CERT Coordination Center
         Software Engineering Institute
         Carnegie Mellon University
         Pittsburgh PA 15213-3890

Using encryption
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Getting security information
   CERT publications and other security information are available from

   CERT advisories and bulletins are also posted on the USENET newsgroup

   To be added to our mailing list for advisories and bulletins, send
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   In the subject line, type
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- ---------------------------------------------------------------------------
Copyright 1998 Carnegie Mellon University. Conditions for use, disclaimers,
and sponsorship information can be found in and .
If you do not have FTP or web access, send mail to with
"copyright" in the subject line.

*CERT is registered in the U.S. Patent and Trademark Office.
- ---------------------------------------------------------------------------

This file:
               click on "CERT Advisories"


(added March 2, 1998)
Readers should note that the vulnerability is present in two distinct cases:

	1. On machines where the ssh-agent is running (the ssh client), as
	   described above. The solution is described in the body of this

        2. On the remote machine that the user logs into using ssh. This case
           arises when the user logs into the sshd server via ssh with agent
           forwarding enabled on the client machine and when the server is
           using a version of SSH earlier than 1.2.22. Until remote sites have
           upgraded to SSH 1.2.22 or later, we strongly encourage users to
           ensure that they have the following line in their ssh configuration
           file on the client machine:

		ForwardAgent no

Revision history

Mar 02, 1998  Updates section - described two cases in which the
              vulnerability is present.

Version: PGP for Personal Privacy 5.0
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