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AUSCERT External Security Bulletin Redistribution
ESB-2000.319 -- FreeBSD-SA-00:58
chpass family contains local root vulnerability
31 October 2000
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AusCERT Security Bulletin Summary
---------------------------------
Product: chfn chpass chsh ypchfn ypchpass ypchsh passwd
Vendor: FreeBSD
Operating System: FreeBSD
Impact: Root Compromise
Access Required: Local
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FreeBSD-SA-00:58 Security Advisory
FreeBSD, Inc.
Topic: chpass family contains local root vulnerability
Category: core
Module: chfn/chpass/chsh/ypchfn/ypchpass/ypchsh/passwd
Announced: 2000-10-30
Credits: Problem fixed during internal auditing.
Vulnerability pointed out by: caddis <caddis@DISSENSION.NET>
Affects: FreeBSD 3.x (all releases), FreeBSD 4.0-RELEASE,
FreeBSD 4.0-STABLE prior to the correction date
Corrected: 2000/07/20 (FreeBSD 4.0-STABLE)
2000/10/04 (FreeBSD 3.5.1-STABLE)
FreeBSD only: NO
I. Background
ch{fn,pass,sh} are utilities for changing user "finger" information,
passwords, and login shell, respectively. The yp* variants perform the
analogous changes on a NIS account.
II. Problem Description
A "format string vulnerability" was discovered in code used by the
vipw utility during an internal FreeBSD code audit in July 2000. The
vipw utility does not run with increased privileges and so it was
believed at the time that it did not represent a security
vulnerability. However it was not realised that this code is also
shared with other utilities -- namely chfn, chpass, chsh, ypchfn,
ypchpass, ypchsh and passwd -- which do in fact run setuid root.
Therefore, the problem may be exploited by unprivileged local users to
gain root access to the local machine.
All versions of FreeBSD prior to the correction date including 4.0 and
3.5.1 are vulnerable to this problem, but it was fixed in the 4.x
branch prior to the release of FreeBSD 4.1.
III. Impact
Local users can obtain root privileges on the local machine.
IV. Workaround
Remove the setuid bit on the following utilities. This has the
side-effect that non-root users cannot change their finger
information, passwords, or login shells.
# chflags noschg /usr/bin/chfn /usr/bin/chpass /usr/bin/chsh
# chmod u-s /usr/bin/chfn /usr/bin/chpass /usr/bin/chsh
# chflags noschg /usr/bin/ypchfn /usr/bin/ypchpass /usr/bin/ypchsh
# chmod u-s /usr/bin/ypchfn /usr/bin/ypchpass /usr/bin/ypchsh
# chflags noschg /usr/bin/passwd
# chmod u-s /usr/bin/passwd
V. Solution
One of the following:
1) Upgrade your vulnerable FreeBSD system to 4.1-RELEASE,
4.1.1-RELEASE, 4.1.1-STABLE or 3.5.1-STABLE after the respective
correction dates.
2) Apply the patch below and recompile the respective files:
Either save this advisory to a file, or download the patch and
detached PGP signature from the following locations, and verify the
signature using your PGP utility.
ftp://ftp.freebsd.org/pub/FreeBSD/CERT/patches/SA-00:58/vipw.patch
ftp://ftp.freebsd.org/pub/FreeBSD/CERT/patches/SA-00:58/vipw.patch.asc
Execute the following commands as root:
# cd /usr/src/usr.sbin/vipw
# patch -p < /path/to/patch_or_advisory
# make depend && make all install
# cd /usr/src/usr.bin/chpass/
# make depend && make all install
# cd /usr/src/usr.bin/passwd/
# make depend && make all install
Patch for vulnerable systems:
--- pw_util.c 1999/08/28 01:20:31 1.17
+++ pw_util.c 2000/07/12 00:49:40 1.18
@@ -250,7 +250,7 @@
extern int _use_yp;
#endif /* YP */
if (err)
- warn(name);
+ warn("%s", name);
#ifdef YP
if (_use_yp)
warnx("NIS information unchanged");
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