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Home » Security Advisories» FreeBSD » System stability compromise via mount_union program

System stability compromise via mount_union program

A bug was found in the union file system code which can allow an unprivileged local user to compromise system stability. This problem is present in all source code and binary distributions of FreeBSD version 2.x released before 1996-05-18.

  • Vendor: FreeBSD
  • Vendor ID: FreeBSD-SA-96:10
  • Date: May 22, 1996


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====================================================================
FreeBSD-SA-96:10 Security Advisory
Revised: Wed May 22 00:20:23 PDT 1996 FreeBSD, Inc.

Topic: system stability compromise via mount_union program

Category: core
Module: unionfs
Announced: 1996-05-17
Affects: FreeBSD 2.0, 2.0.5, 2.1, 2.1-stable, and 2.2-current
Corrected: (workaround) 2.1-stable and 2.2-current as of 1996-05-17
Source: 4.4BSD (lite)
FreeBSD only: no

Patches: ftp://freebsd.org/pub/CERT/patches/SA-96:10/

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

I. Background

A bug was found in the union file system code which can allow
an unprivileged local user to compromise system stability.
This problem is present in all source code and binary
distributions of FreeBSD version 2.x released before 1996-05-18.

All FreeBSD users are encouraged to use the workaround provided
until the FreeBSD Project distributes a full solution.


II. Problem Description

The union filesystem code had problems with certain mount ordering
problems. By executing a certain sequence of mount_union commands,
an unprivileged local user may cause a system reload.

NOTE: This is a different problem than the one discussed in
FreeBSD SA-96:09. The workaround for this vulnerability is
similar to the one discussed in 96:09, but the proper solution
for the unauthorized access problem in 96:09 does not address
this vulnerability.


III. Impact

The problem could allow local users to compromise system stability.

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


IV. Solution(s)

The FreeBSD project is currently developing a solution to this
problem, however the proper solution will not be available until
a future FreeBSD release. We do not anticipate releasing patches
for previous versions of FreeBSD due to the extensive nature of this
fix. This security advisory will be updated as new information is
made available.

V. Workaround

This vulnerability can quickly and easily be limited by removing
the setuid permission bit from the mount_union 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

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
-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 ----

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

Web Site: http://www.freebsd.org/
Confidential contacts: [email protected]
PGP Key: ftp://freebsd.org/pub/CERT/public_key.asc
Security notifications: [email protected]
Security public discussion: [email protected]

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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