December 7, 2001

FreeBSD: Update - 'OpenSSH' Privilege escalation vulnerability

Author: JT Smith

FreeBSD: "[W]hen the OpenSSH server `sshd' is configured to use the system's login program (via the directive `UseLogin yes' in sshd_config), this environment is passed to login, which is invoked with superuser privileges. Because certain environmental variables such as LD_LIBRARY_PATH and LD_PRELOAD can be set using the previously described feature, the user may arrange for login to execute arbitrary code with superuser privileges."


=============================================================================
FreeBSD-SA-01:63                                           Security Advisory
                                                                FreeBSD, Inc.

Topic:          OpenSSH UseLogin directive permits privilege escalation
                [REVISED]

Category:       core/ports
Module:         openssh
Announced:      2001-12-02
Revised:        2001-12-06
Credits:        Markus Friedl markus@OpenBSD.org>Affects:        FreeBSD 4.3-RELEASE, 4.4-RELEASE
                FreeBSD 4.4-STABLE prior to the correction date
                Ports collection prior to the correction date
Corrected:      2001-12-03 00:53:28 UTC (RELENG_4)
                2001-12-03 00:54:18 UTC (RELENG_4_4)
                2001-12-03 00:54:54 UTC (RELENG_4_3)
                2001-12-02 06:52:40 UTC (openssh port)
FreeBSD only:   NO

0.   Revision History

v1.0  2001-12-02  Initial release
v1.1  2001-07-31  Corrected patch instructions

I.   Background

OpenSSH is an implementation of the SSH1 and SSH2 secure shell
protocols for providing encrypted and authenticated network access,
which is available free for unrestricted use. Versions of OpenSSH are
included in the FreeBSD ports collection and the FreeBSD base system.

II.  Problem Description

OpenSSH includes a feature by which a user can arrange for
environmental variables to be set depending upon the key used for
authentication.  These environmental variables are specified in the
`authorized_keys' (SSHv1) or `authorized_keys2' (SSHv2) files in the
user's home directory on the server.  This is normally safe, as this
environment is passed only to the user's shell, which is invoked with
user privileges.

However, when the OpenSSH server `sshd' is configured to use
the system's login program (via the directive `UseLogin yes' in
sshd_config), this environment is passed to login, which is invoked
with superuser privileges.  Because certain environmental variables
such as LD_LIBRARY_PATH and LD_PRELOAD can be set using the previously
described feature, the user may arrange for login to execute arbitrary
code with superuser privileges.

All versions of FreeBSD 4.x prior to the correction date including
FreeBSD 4.3 and 4.4 are potentially vulnerable to this problem.
However, the OpenSSH server is configured to not use the system login
program (`UseLogin no') by default, and is therefore not vulnerable
unless the system administrator has changed this setting.

In addition, there are two versions of OpenSSH included in the
ports collection.  One is ports/security/openssh, which is the
BSD-specific version of OpenSSH.  Versions of this port prior to
openssh-3.0.2 exhibit the problem described above.  The other is
ports/security/openssh-portable, which is not vulnerable, even if the
server is set to `UseLogin yes'.

III. Impact

Hostile but otherwise legitimate users that can successfully
authenticate using public key authentication may cause /usr/bin/login
to run arbitrary code as the superuser.

If you have not enabled the 'UseLogin' directive in the sshd
configuration file, you are not vulnerable to this problem.

IV.  Workaround

Doing one of the following will eliminate the vulnerability:

1) Configure sshd to not use the system login program.  Edit the
   server configuration file and change any `UseLogin' directives
   to `UseLogin no'.  This is the preferred workaround.

2) If for whatever reason, disabling `UseLogin' is not possible,
   then one can instead disable public key authentication.  Edit the
   server configuration file and change any `RSAAuthentication',
   `DSAAuthentication', or `PubKeyAuthentication' directives
   to `RSAAuthentication no', `DSAAuthentication no', and
   `PubKeyAuthentication no', respectively.

For sshd included in the base system (/usr/bin/sshd), the
server configuration file is `/etc/ssh/sshd_config'.  For sshd
from the ports collection, the server configuration file is
`/usr/local/etc/sshd_config'.

After modifying the sshd configuration file, the sshd daemon must be
restarted by executing the following command as root:

# kill -HUP `cat /var/run/sshd.pid`

V.   Solution

1) Upgrade the vulnerable system to 4.3-RELEASEp21, 4.4-RELEASEp1, or
4.4-STABLE after the correction date, or patch your current system
source code and rebuild.

2) FreeBSD 4.x systems prior to the correction date:

The following patch has been verified to apply to FreeBSD
4.3-RELEASE, 4.4-RELEASE, and 4.4-STABLE dated prior to the
correction date.  It may or may not apply to older, unsupported
versions of FreeBSD.

Download the patch and the detached PGP signature from the following
locations, and verify the signature using your PGP utility.

# fetch  ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-01:63/sshd.patch
# fetch  ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-01:63/sshd.patch.asc

Execute the following commands as root:

# cd /usr/src/crypto/openssh
# patch security-officer@FreeBSD.org is requested so we can improve the
process for future advisories.

During the installation procedure, backup copies are made of the files
which are replaced by the package.  These backup copies will be
reinstalled if the package is removed, reverting the system to a
pre-patched state.  In addition, the package automatically restarts
the sshd daemon if it is running.

Three versions of the upgrade package are available, depending on
whether or not the system has the kerberosIV or kerberos5
distributions installed.

3a) For systems without kerberosIV or kerberos5 installed:

# fetch  ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/packages/SA-01:63/security-patch-sshd-01.63.tgz
# fetch  ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/packages/SA-01:63/security-patch-sshd-01.63.tgz.asc

Verify the detached PGP signature using your PGP utility.

# pkg_add security-patch-sshd-01.63.tgz

3b) For systems with kerberosIV only installed:

# fetch  
ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/packages/SA-01:63/security-patch-sshd-kerberosIV-01.63.tgz
# fetch  
ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/packages/SA-01:63/security-patch-sshd-kerberosIV-01.63.tgz.asc

Verify the detached PGP signature using your PGP utility.

# pkg_add security-patch-sshd-kerberosIV-01.63.tgz

3c) For systems with kerberos5 only installed:

# fetch  
ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/packages/SA-01:63/security-patch-sshd-kerberos5-01.63.tgz
# fetch  
ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/packages/SA-01:63/security-patch-sshd-kerberos5-01.63.tgz.asc

Verify the detached PGP signature using your PGP utility.

# pkg_add security-patch-sshd-kerberos5-01.63.tgz

3d) For systems with both kerberosIV and kerberos5 installed:

# fetch  
ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/packages/SA-01:63/security-patch-sshd-kerberosIV-kerberos5-01.63.tgz
# fetch  
ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/packages/SA-01:63/security-patch-sshd-kerberosIV-kerberos5-01.63.tgz.asc

Verify the detached PGP signature using your PGP utility.

# pkg_add security-patch-sshd-kerberosIV-kerberos5-01.63.tgz

[Ports collection]

One of the following:

1) Upgrade your entire ports collection and rebuild the OpenSSH port.

2) Deinstall the old package and install a new package dated after the
correction date, obtained from:

[i386] 
ftp://ftp.FreeBSD.org/pub/FreeBSD/ports/i386/packages-4-stable/security/openssh-3.0.2.tgzftp://ftp.FreeBSD.org/pub/FreeBSD/ports/i386/packages-5-current/security/openssh-3.0.2.tgz

[alpha]
Packages are not automatically generated for the alpha architecture at
this time due to lack of build resources.

NOTE: It may be several days before updated packages are available. Be
sure to check the file creation date on the package, because the
version number of the software has not changed.

3) Download a new port skeleton for the openssh port from:
 
http://www.freebsd.org/ports/

and use it to rebuild the port.

4) Use the portcheckout utility to automate option (3) above. The
portcheckout port is available in /usr/ports/devel/portcheckout or the
package can be obtained from:
 
ftp://ftp.FreeBSD.org/pub/FreeBSD/ports/i386/packages-4-stable/devel/portcheckout-2.0.tgzftp://ftp.FreeBSD.org/pub/FreeBSD/ports/i386/packages-5-current/devel/portcheckout-2.0.tgz

VI. Correction details

Path                                                             Revision
  Branch
-------------------------------------------------------------------------
src/crypto/openssh/session.c
  HEAD                                                               1.18
  RELENG_4                                                       1.4.2.11
  RELENG_4_4                                                  1.4.2.8.4.1
  RELENG_4_3                                                  1.4.2.8.2.1
src/crypto/openssh/version.h
  HEAD                                                                1.9
  RELENG_4                                                    1.1.1.1.2.7
  RELENG_4_4                                              1.1.1.1.2.5.2.1
  RELENG_4_3                                              1.1.1.1.2.4.2.1
ports/security/openssh/Makefile                                      1.79
-------------------------------------------------------------------------

For OpenSSH included in the base system, there is a version string
indicating which FreeBSD localizations are available.  The following
table lists the version strings for each branch which include this
security fix:

Branch                                                     Version string
-------------------------------------------------------------------------
HEAD                         OpenSSH_2.9 FreeBSD localisations 20011202
RELENG_4                     OpenSSH_2.9 FreeBSD localisations 20011202
RELENG_4_4                   OpenSSH_2.3.0 FreeBSD localisations 20011202
RELENG_4_3                   OpenSSH_2.3.0 green@FreeBSD.org 20011202
-------------------------------------------------------------------------

To view the version string of the OpenSSH server, execute the following
command:

  % /usr/sbin/sshd -\?

The version string is also displayed when a client connects to the
server.

VII. References

http://www.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/session.c#rev1.110>

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