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=============================================================================
FreeBSD-SA-96:09                                            Security Advisory
Revised: Wed May 22 00:20:09 PDT 1996                           FreeBSD, Inc.

Topic:          unauthorized access via mount_union / mount_msdos (vfsload)

Category:       core
Module:         libc
Announced:      1996-05-17
Affects:        FreeBSD 2.0, 2.0.5, 2.1, 2.1-stable, and 2.2-current
Corrected:      1996-05-17 2.1-stable and 2.2-current sources
Source:         FreeBSD native bug
FreeBSD only:   yes

Patches:        ftp://freebsd.org/pub/CERT/patches/SA-96:09/

=============================================================================

I.   Background    

     A bug was found in the vfsload(3) library call that affects all
     versions of FreeBSD from 2.0 through 2.2-CURRENT that caused a
     system vulnerability.  This problem is present in all source
     code and binary distributions of FreeBSD version 2.x released
     before 1996-05-18.

     The FreeBSD project is aware of active exploits of this
     vulnerability.

     All FreeBSD users are encouraged to use the workaround provided
     until they can update their operating system to a version with
     this vulnerability fixed.


II.  Problem Description

     The mount_union and mount_msdos programs invoke another system
     utility in an insecure fashion while setuid root.


III. Impact

     The problem could allow local users to gain unauthorized
     permissions.

     This vulnerability can only be exploited by users with a valid
     account on the local system.


IV. Solution(s)

     Update operating system sources and binaries to FreeBSD 2.1-stable
     or FreeBSD 2.2-current as distributed later than 1996-05-18 or
     if you are currently running 2.1 or later, you may apply the
     solution patches available at the URL listed at the top of this
     message.

     The OS updates fix the actual problem in the vfsload(3) library
     routine.  Once the vfsload() library routine is fixed, the
     workaround listed below is not necessary to solve this problem.
     However, an additional stability problem has come to light
     (ref. FreeBSD SA-96:10) so the FreeBSD project suggests
     using both the setuid workaround and the solution for best results.

V. Workaround

     This vulnerability can quickly and easily be limited by removing
     the setuid permission bit from the mount_union and mount_msdos
     program.  This workaround will work for all versions of FreeBSD
     affected by this problem.
     
     As root, execute the command:

          # chmod u-s /sbin/mount_union /sbin/mount_msdos

     then verify that the setuid permissions of the files have been
     removed.  The permissions array should read "-r-xr-xr-x" as
     shown here:

          # ls -l /sbin/mount_union /sbin/mount_msdos
          -r-xr-xr-x  1 root  bin  151552 Apr 26 04:41 /sbin/mount_msdos
          -r-xr-xr-x  1 root  bin   53248 Apr 26 04:40 /sbin/mount_union

     In addition to changing the permissions on the executable files,
     if you have the source code installed, we suggest patching the
     sources so that mount_union will not be installed with the
     setuid bit set:

    *** /usr/src/sbin/mount_union/Makefile      Sun Nov 20 14:47:52 1994
    --- /usr/src/sbin/mount_union/Makefile      Fri May 17 10:36:09 1996
    ***************
    *** 8,14 ****
      CFLAGS+= -I${.CURDIR}/../../sys -I${MOUNT}
      .PATH:    ${MOUNT}
      
    - BINOWN= root
    - BINMODE=4555
    - 
      .include 
    --- 8,11 ----
    *** /usr/src/sbin/i386/mount_msdos/Makefile Sun Dec  4 00:01:24 1994
    --- /usr/src/sbin/i386/mount_msdos/Makefile Fri May 17 11:31:57 1996
    ***************
    *** 6,14 ****
      SRCS=     mount_msdos.c getmntopts.c
      MAN8=     mount_msdos.8
      
    - BINOWN=   root
    - BINMODE= 4555
    - 
      MOUNT=    ${.CURDIR}/../../mount
      CFLAGS+= -I${MOUNT}
      .PATH:    ${MOUNT}
    --- 6,11 ----

=============================================================================
FreeBSD, Inc.

Web Site:                       http://www.freebsd.org/
Confidential contacts:          security-officer@freebsd.org
PGP Key:                        ftp://freebsd.org/pub/CERT/public_key.asc
Security notifications:         security-notifications@freebsd.org
Security public discussion:     security@freebsd.org

Notice: Any patches in this document may not apply cleanly due to
        modifications caused by digital signature or mailer software.
        Please reference the URL listed at the top of this document
        for original copies of all patches if necessary.
=============================================================================

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