TUCoPS :: BSD :: ciacl111.txt

FreeBSD Signal Handling Flaw


                       The U.S. Department of Energy
                     Computer Incident Advisory Center
                           ___  __ __    _     ___
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                             INFORMATION BULLETIN

                          FreeBSD Signal Handling Flaw
                    [FreeBSD, Inc. Security Advisory 01:42]

July 10, 2001 22:00 GMT                                           Number L-111
PROBLEM:       A flaw exists in FreeBSD signal handling clearing that allows 
               some signals handlers to remain in effect in child processes 
PLATFORM:      All versions of 4.x prior to correction date including 
DAMAGE:        An attacker can execute arbitrary code 
SOLUTION:      Apply the appropriate patch or upgrade system as described 
VULNERABILITY  The risk is MEDIUM: A local attacker could gain root access by 
ASSESSMENT:    executing code in a setuid context 

[******  Start FreeBSD Bulletin ******]

FreeBSD-SA-01:42                  Security Advisory
                                     FreeBSD, Inc.

Topic:          signal handling during exec may allow local root

Category:       core
Module:         kernel
Announced:      2001-07-10
Credits:        Georgi Guninski <guninski@guninski.com>
Affects:        All released versions of FreeBSD 4.x,
                FreeBSD 4.3-STABLE prior to the correction date.
Corrected:      2001-07-09
FreeBSD only:   Yes

I.   Background

When a process forks, it inherits the parent's signals.  When the
process execs, the kernel clears the signal handlers because they are
not valid in the new address space.

II.  Problem Description

A flaw exists in FreeBSD signal handler clearing that would allow for
some signal handlers to remain in effect after the exec.  Most of the
signals were cleared, but some signal hanlders were not.  This allowed
an attacker to execute arbitrary code in the context of a setuid

All versions of 4.x prior to the correction date including and
4.3-RELEASE are vulnerable to this problem.  The problem has been
corrected by copying the inherited signal handlers and resetting the
signals instead of sharing the signal handlers.

III. Impact

Local users may be able to gain increased privileges on the local

IV.  Workaround

Do not allow untrusted users to gain access to the local system.

V.   Solution

One of the following:

1) Upgrade your vulnerable FreeBSD system to 4.3-STABLE after the
correction date.

2) To patch your present system: download the relevant patch from the
below location, and execute the following commands as root:

[FreeBSD 4.1, 4.2, and 4.3 base systems]

This patch has been verified to apply to FreeBSD 4.1, 4.2, and 4.3 only.
It may or may not apply to older releases.

# fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-01:42/signal-4.3.patch
# fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-01:42/signal-4.3.patch.asc

Verify the detached PGP signature using your PGP utility.

# cd /usr/src/sys/kern
# patch -p < /path/to/patch

[ Recompile your kernel as described in
http://www.freebsd.org/handbook/kernelconfig.html and reboot the
system ] 

[******  End FreeBSD Bulletin ******]

CIAC wishes to acknowledge the contributions of FreeBSD, Inc for the information contained in this bulletin.

CIAC, the Computer Incident Advisory Center, is the computer
security incident response team for the U.S. Department of Energy
(DOE) and the emergency backup response team for the National
Institutes of Health (NIH). CIAC is located at the Lawrence Livermore
National Laboratory in Livermore, California. CIAC is also a founding
member of FIRST, the Forum of Incident Response and Security Teams, a
global organization established to foster cooperation and coordination
among computer security teams worldwide.

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This document was prepared as an account of work sponsored by an
agency of the United States Government. Neither the United States
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employees, makes any warranty, express or implied, or assumes any
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