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CERT Advisory CA-95:09 - Solaris ps Vulnerability

  • From: CERT Advisory
  • Date: Tue Aug 29 18:31:34 1995

=============================================================================
CA-95:09                       CERT Advisory
                              August 29, 1995
                          Solaris ps Vulnerability
-----------------------------------------------------------------------------

               The text of this advisory is taken primarily 
           from AUSCERT advisory AA-95.07, with their permission.  

A vulnerability exists in Solaris systems that allows a race condition to be
exploited to gain root access.  The essential problem is that the ps(1)
program maintains a data file in the /tmp directory, and the /tmp directory is
world-writable, allowing users to delete other users' files in /tmp. This
vulnerability affects Solaris 2.x (SunOS 5.x) systems. 

An exploit program for this vulnerability has been published. We urge
you to take the actions described in Section III as soon as possible.

As we receive additional information relating to this advisory, we
will place it in

        ftp://info.cert.org/pub/cert_advisories/CA-95:09.README

We encourage you to check our README files regularly for updates on
advisories that relate to your site.

------------------------------------------------------------------------------
I.   Description

     A race condition exists in at least one Solaris 2.x (SunOS 5.x) system
     program that can be exploited to gain root access if the user has access
     to the temporary files.  Access to temporary files may be obtained if the
     permissions on the /tmp and /var/tmp directories are set incorrectly. 
     The permissions on the /tmp directory are often reset incorrectly by
     the system if tmpfs (which is mounting swap as /tmp) is in use.

II.  Impact

     Users logged in to the system may gain unauthorized root privileges.
     
III. Solution
     
     A. Determine if your system is vulnerable

        To determine if you are running tmpfs, the following command can be
        used to verify if the filesystem for /tmp is swap:

          % /usr/sbin/df -k /tmp
          Filesystem          kbytes    used   avail capacity  Mounted on
          swap                 28348     12    28336     0%    /tmp

        or look in the file /etc/vfstab for the configuration line:

          #device     device   mount    FS      fsck    mount     mount
          #to mount   to fsck  point    type    pass    at boot   options
          swap          -      /tmp     tmpfs     -      yes         -

        If either of these two conditions exist, then you are running tmpfs
        and the system may automatically reset the permission bits of /tmp at
        the next reboot.

        To verify if your configuration is currently vulnerable, the following
        command may be used:

          % /usr/bin/ls -ld /tmp
          drwxrwxrwt   2 sys      sys        61 Aug 15 12:12 /tmp

        If the sticky bit (t) is not set (it will be an x), then the system is
        vulnerable.

     B. Perform the following workarounds

        These workarounds have been verified with Sun Microsystems. Sun
        Microsystems expects to release a patch in the near future.

        1. Immediate - fix /tmp permissions

           A workaround that takes effect immediately is to set the sticky bit
           on the /tmp directory using the following command as root:

             # /usr/bin/chmod 1777 /tmp

           Note that this command must be performed after each reboot if you
           are mounting swap as /tmp (using tmpfs).

           In addition, the ownership and group membership of the /tmp
           directory should be verified using /usr/bin/ls -ld /tmp, and if
           incorrect may be reset by: 

             # /usr/bin/chown sys /tmp
             # /usr/bin/chgrp sys /tmp

           The AUSCERT UNIX Security Checklist addresses this issue in
           Section 5.5.  This section is reproduced in the appendix of this
           advisory. The entire AUSCERT checklist may be obtained from these
           locations.
          
           Sites outside of Australia should use the info.cert.org FTP site.

       ftp://info.cert.org/pub/tech_tips/AUSCERT_checklist_1.0
       ftp://ftp.auscert.org.au/pub/auscert/papers/unix_security_checklist_1.0
    

        2. Permanent - make the above change to /tmp permissions permanent

           The change noted in item B.1 above will be lost upon reboot. To
           make the changes permanent, create the following script as
           /etc/init.d/tmpfsfix: 

-----------------------------cut here--8<-------------------------------------
#!/bin/sh

if [ -d /tmp ]
then
   /usr/bin/chmod 1777 /tmp
   /usr/bin/chgrp sys /tmp
   /usr/bin/chown sys /tmp
fi
------------------------------cut here---8<-----------------------------------

          A symbolic link should then be created called
          /etc/rc3.d/S79tmpfix which points to /etc/init.d/tmpfsfix by issuing
          the following command as root:

            # /usr/bin/ln -s /etc/init.d/tmpfsfix /etc/rc3.d/S79tmpfix

          If you have done item B.1 above, you can reboot at your leisure.
          Otherwise, reboot your system now. In either case, verify the 
          permissions of /tmp immediately after your next system reboot.

       3. Check /var/tmp permissions

          We recommend that you also check and correct the /var/tmp directory.
          Note that this directory is not usually mounted as tmpfs, so
          it normally would not be subject to automatic resetting of its
          permission bits on reboot. 

            % /usr/bin/ls -ld /var/tmp
            drwxrwxrwt   2 sys      sys       512 Aug 15 11:35 /var/tmp

----------------------------------------------------------------------------
The CERT Coordination Center staff thanks AUSCERT, the Australian response
team, for their permission to reuse text from their advisory AA-95.07 and 
for their cooperation and assistance.
----------------------------------------------------------------------------

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 (FIRST).

If you wish to send sensitive incident or vulnerability information to
CERT staff by electronic mail, we strongly advise that the email be
encrypted.  The CERT Coordination Center can support a shared DES key, PGP
(public key available via anonymous FTP on info.cert.org), or PEM (contact
CERT staff for details).

Internet email: [email protected]
Telephone: +1 412-268-7090 (24-hour hotline)
           CERT personnel answer 8:30 a.m.-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
                 USA

CERT advisories and bulletins are posted on the USENET newsgroup
comp.security.announce. If you would like to have future advisories and
bulletins mailed to you or to a mail exploder at your site, please send mail
to [email protected]

Past CERT publications, information about FIRST representatives, and
other information related to computer security are available for anonymous
FTP from info.cert.org. 

This material may be reproduced and distributed without permission provided it
is used for noncommercial purposes, and the CERT Coordination Center and
AUSCERT are acknowledged.

CERT is a service mark of Carnegie Mellon University.

..............................................................................
Appendix: Excerpt from AUSCERT UNIX Security Checklist (Version 1.0)

 5.5  File Permissions

   *    CHECK that the permissions of /etc/utmp are set to 644.
   *    CHECK that the permissions of /etc/sm and /etc/sm.bak are set to 2755.
   *    CHECK that the permissions of /etc/state are set to 644.
   *    CHECK that the permissions of /etc/motd and /etc/mtab are set to 644.
   *    CHECK that the permissions of /etc/syslog.pid are set to 644.
   *    REMOVE setgid privileges on /usr/kvm/crash.
            A group of kmem allows users to read the virtual memory of a
            running system.
                # /bin/chmod g-s /usr/kvm/crash
   *    DO consider removing read access to files that users do not need to
        access.
   *    ENSURE that the kernel (eg. /vmunix) is owned by root, has group set
        to 0 (wheel on SunOS) and permissions set to 644.
   *    ENSURE that /etc, /bin, /usr/etc, /usr/bin and /tmp are owned by
        root [Note from CERT Coordination Center: "root" applies to SunOS; 
        "sys" applies to Solaris]  and that the sticky-bit is set on /tmp.
            ie. permissions on /tmp should be:  drwxrwxrwt.
            You should implement COPS or Tiger to check for this.
            Refer to section B.2 [of the AUSCERT UNIX Security Checklist] for 
            information where to obtain these.

   *    ENSURE that there are no unexpected world writable files or
        directories on your system.  The following commands find world
        writeable files and directories.

               # /bin/find / -type f -perm -22 -exec ls -l {} \;
               # /bin/find / -type d -perm -22 -exec ls -ld {} \;

   *    CHECK that files which have the SUID or SGID bit enabled, should have
        it enabled:
               # /bin/find / -type f \( -perm -004000 -o -perm -002000 \) \
                      -exec ls -l {} \;

   *    CHECK the umask value for each user and ensure it is set to
        something sensible like 027 or 077.  Refer to section E.1 [of the 
        AUSCERT UNIX Security Checklist] for a shell script to check this.