K-058: OpenSSH UseLogin Vulnerability

PROBLEM:       If the UseLogin option is enabled, then the remote user's
               commands will be executed with an incorrect user id.
PLATFORM:      All platforms running FreeBSD 4.0-RELEASE, FreeBSD 4.0-STABLE,
               and FreeBSD 5.0-CURRENT prior to the correction date.
DAMAGE:        This vulnerability allows remote users to run commands with
               elevated (usually root) privileges.
SOLUTION:      Upgrade or apply the patch.
VULNERABILITY  The risk is HIGH if the UseLogin option is enabled. The
ASSESSMENT:    vulnerability has been discussed in public forums.


[ Start FreeBSD Advisory ]

=============================================================================
FreeBSD-SA-00:30                                           Security Advisory
                                                                FreeBSD, Inc.

Topic:          OpenSSH UseLogin directive permits remote root access

Category:       core
Module:         openssh
Announced:      2000-07-05
Credits:        Markus Friedl <markus@OpenBSD.org>
Affects:        FreeBSD 4.0-RELEASE, FreeBSD 4.0-STABLE and 5.0-CURRENT
                prior to the correction date
Corrected:      2000-06-11
Vendor status:  Disclosed vulnerability.
FreeBSD only:   NO

I.   Background

OpenSSH is an implementation of the SSH1 (and SSH2 in later versions)
secure shell protocols for providing encrypted and authenticated
network access, which is available free for unrestricted use.

II.  Problem Description

The sshd server is typically invoked as root so it can manage general
user logins. OpenSSH has a configuration option, not enabled by
default ("UseLogin") which specifies that user logins should be done
via the /usr/bin/login command instead of handled internally.

OpenSSH also has a facility to enable remote users to execute commands
on the server non-interactively. In this case, the UseLogin directive
fails to correctly drop root privileges before executing the command,
meaning that remote users without root access can execute commands on
the local system as root.

Note that with the default configuration, OpenSSH is not vulnerable to
this problem, and this option is not needed for the vast majority of
systems.

OpenSSH is installed if you chose to install the 'crypto' distribution
at install-time or when compiling from source, and you either have the
international RSA libraries or installed the RSAREF port.

III. Impact

If your sshd configuration was modified to enable the 'UseLogin'
directive then remote users with SSH access to the local machine can
execute arbitrary commands as root.

IV.  Workaround

Set 'UseLogin No' in your /etc/ssh/sshd_config file and restart the
SSH server by issuing the following command as root:

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

This will cause the parent process to respawn and reread its
configuration file, and should not interfere with existing SSH sessions.

Note that a bug in sshd (discovered during preparation of this
advisory, fixed in FreeBSD 5.0-CURRENT and 4.0-STABLE as of
2000-07-03) means that it will fail to restart correctly unless it was
originally invoked with an absolute path (i.e. "/usr/sbin/sshd"
instead of "sshd"). Therefore you should verify that the server is
still running after you deliver the HUP signal:

# ps -p `cat /var/run/sshd.pid`
  PID  TT  STAT      TIME COMMAND
 2110  ??  Ss     0:00.97 /usr/sbin/sshd

If the server is no longer running, restart it by issuing the
following command as root:

# /usr/sbin/sshd

V.   Solution

One of the following:

1) Upgrade to FreeBSD 4.0-STABLE or 5.0-CURRENT after the correction
date. Note that these versions of FreeBSD contain a newer version of
OpenSSH than was in 4.0-RELEASE, version 2.1, which provides enhanced
functionality including support for the SSH2 protocol and DSA keys.

2) Save this advisory as a file and extract the relevant patch for
your version of FreeBSD, or download the relevant patch and detached
PGP signature from the following location:

# fetch ftp://ftp.freebsd.org/pub/FreeBSD/CERT/patches/SA-00:30/sshd.patch
# fetch ftp://ftp.freebsd.org/pub/FreeBSD/CERT/patches/SA-00:30/sshd.patch.asc

Verify the detached signature using your PGP utility.

Issue the following commands as root:

# cd /usr/src/crypto/openssh
# patch -p < /path/to/patch/or/advisory
# cd /usr/src/secure/lib/libssh
# make all
# cd /usr/src/secure/usr.sbin/sshd
# make all install
# kill -HUP `cat /var/run/sshd.pid`

See the note in the "Workarounds" section about verifying that the
sshd server is still running.

VI.   Patch

   Index: sshd.c
   ===================================================================
   RCS file: /home/ncvs/src/crypto/openssh/sshd.c,v
   retrieving revision 1.6
   diff -u -r1.6 sshd.c
   --- sshd.c   2000/03/09 14:52:31     1.6
   +++ sshd.c   2000/07/04 03:40:46
   @@ -2564,7 +2564,13 @@
        char *argv[10];
    #ifdef LOGIN_CAP
        login_cap_t *lc;
   +#endif

   +    /* login(1) is only called if we execute the login shell */
   +    if (options.use_login && command != NULL)
   +            options.use_login = 0;
   +
   +#ifdef LOGIN_CAP
        lc = login_getpwclass(pw);
        if (lc == NULL)
                lc = login_getclassbyname(NULL, pw);

[ End FreeBSD Advisory ]

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