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AUSCERT External Security Bulletin Redistribution
ESB-2001.287 -- FreeBSD-SA-01:42.signal
signal handling during exec may allow local root compromise
12 July 2001
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AusCERT Security Bulletin Summary
---------------------------------
Product: kernel
Vendor: FreeBSD
Operating System: FreeBSD 4.X
Impact: Root Compromise
Access Required: Existing Account
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FreeBSD-SA-01:42 Security Advisory
FreeBSD, Inc.
Topic: signal handling during exec may allow local root
compromise
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
binary.
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
system.
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 ]
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