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FreeBSD Security Advisory: FreeBSD-SA-00:30.openssh

Jul 06, 2000, 19:11 (0 Talkback[s])

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Date: Wed, 5 Jul 2000 16:12:36 -0700
From: FreeBSD Security Advisories
To: BUGTRAQ@SECURITYFOCUS.COM
Subject: FreeBSD Security Advisory: FreeBSD-SA-00:30.openssh

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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 
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);

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