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AUSCERT External Security Bulletin Redistribution
ESB-2000.358 -- FreeBSD-SA-00:76
tcsh/csh creates insecure temporary file
22 November 2000
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AusCERT Security Bulletin Summary
---------------------------------
Product: tcsh csh
Vendor: FreeBSD Ports Collection
Operating System: FreeBSD
BSD
Unix
Impact: Overwrite Arbitrary Files
Access Required: Local
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FreeBSD-SA-00:76 Security Advisory
FreeBSD, Inc.
Topic: tcsh/csh creates insecure temporary file
Category: core, ports
Module: tcsh, 44bsd-csh
Announced: 2000-11-20
Affects: FreeBSD 4.x, 3.x prior to the correction date.
Corrected: 2000-11-04 (FreeBSD 4.1.1-STABLE)
2000-11-05 (FreeBSD 3.5.1-STABLE)
2000-11-09 (44bsd-csh port)
2000-11-19 (tcsh port)
Credits: proton <proton@ENERGYMECH.NET>
FreeBSD only: NO
I. Background
tcsh is an updated version of the traditional BSD C Shell
(csh). Versions of csh and tcsh are included in the FreeBSD ports
collection (tcsh, 44bsd-csh) and the FreeBSD base system (csh, tcsh).
II. Problem Description
The csh and tcsh code creates temporary files when the '<<' operator
is used, however these are created insecurely and use a predictable
filename based on the process ID of the shell. An attacker can
exploit this vulnerability to overwrite an arbitrary file writable by
the user running the shell. The contents of the file are overwritten
with the text being entered using the '<<' operator, so it will
usually not be under the control of the attacker.
Therefore the likely impact of this vulnerability is a denial of
service since the attacker can cause critical files writable by the
user to be overwritten. It is unlikely, although possible depending
on the circumstances in which the '<<' operator is used, that the
attacker could exploit the vulnerability to gain privileges (this
typically requires that they have control over the contents the target
file is overwritten with).
All versions of FreeBSD prior to the correction date are vulnerable to
this problem: the /bin/csh shell included in the base system (which is
the same as /bin/tcsh in recent versions) as well as the tcsh
(versions prior to 6.09.03_1) and 44bsd-csh ports (versions prior to
44bsd-csh-20001106) in the ports collection. The problems with the
base system shells and the 44bsd-csh port were resolved prior to the
release of FreeBSD 4.2. The tcsh port was not fixed prior to the
release, but the port is disabled in FreeBSD 4.2 since the same
software exists in the base system.
III. Impact
Unprivileged local users can cause an arbitrary file writable by a
victim to be overwritten when the victim invokes the '<<' operator in
csh or tcsh (e.g. from within a shell script).
If you have not installed the tcsh or 44bsd-csh ports on your
4.1.1-STABLE system dated after the correction date, your system is
not vulnerable to this problem.
IV. Workaround
None practical.
V. Solution
Upgrade your vulnerable FreeBSD system to 4.1.1-STABLE after the
correction date, or patch your present system source code and
rebuild.
To patch your present system: download the relevant patch from the
below location, and execute the following commands as root:
[FreeBSD 4.x base system]
# fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-00:76/tcsh.patch
# fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-00:76/tcsh.patch.asc
Verify the detached PGP signature using your PGP utility.
cd /usr/src/contrib/tcsh
patch -p < /path/to/patch
cd /usr/src/bin/csh
make depend && make all install
[FreeBSD 3.x base system]
# fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-00:76/csh.patch
# fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-00:76/csh.patch.asc
Verify the detached PGP signature using your PGP utility.
cd /usr/src/bin/csh
patch -p < /path/to/patch
make depend && make all install
[Ports collection]
One of the following:
1) Upgrade your entire ports collection and rebuild the tcsh/44bsd-csh
port.
2) Deinstall the old package and install a new package dated after the
correction date, obtained from:
[tcsh]
ftp://ftp.FreeBSD.org/pub/FreeBSD/ports/i386/packages-3-stable/shells/tcsh-6.09.03_1.tgz
ftp://ftp.FreeBSD.org/pub/FreeBSD/ports/i386/packages-4-stable/shells/tcsh-6.09.03_1.tgz
ftp://ftp.FreeBSD.org/pub/FreeBSD/ports/alpha/packages-4-stable/shells/tcsh-6.09.03_1.tgz
ftp://ftp.FreeBSD.org/pub/FreeBSD/ports/i386/packages-5-current/shells/tcsh-6.09.03_1.tgz
ftp://ftp.FreeBSD.org/pub/FreeBSD/ports/alpha/packages-5-current/shells/tcsh-6.09.03_1.tgz
[44bsd-csh]
ftp://ftp.FreeBSD.org/pub/FreeBSD/ports/i386/packages-3-stable/shells/44bsd-csh-20001106.tgz
ftp://ftp.FreeBSD.org/pub/FreeBSD/ports/i386/packages-4-stable/shells/44bsd-csh-20001106.tgz
ftp://ftp.FreeBSD.org/pub/FreeBSD/ports/alpha/packages-4-stable/shells/44bsd-csh-20001106.tgz
ftp://ftp.FreeBSD.org/pub/FreeBSD/ports/i386/packages-5-current/shells/44bsd-csh-20001106.tgz
ftp://ftp.FreeBSD.org/pub/FreeBSD/ports/alpha/packages-5-current/shells/44bsd-csh-20001106.tgz
3) download a new port skeleton for the tcsh/44bsd-csh port from:
http://www.freebsd.org/ports/
and use it to rebuild the port.
4) Use the portcheckout utility to automate option (3) above. The
portcheckout port is available in /usr/ports/devel/portcheckout or the
package can be obtained from:
ftp://ftp.FreeBSD.org/pub/FreeBSD/ports/i386/packages-3-stable/devel/portcheckout-2.0.tgz
ftp://ftp.FreeBSD.org/pub/FreeBSD/ports/i386/packages-4-stable/devel/portcheckout-2.0.tgz
ftp://ftp.FreeBSD.org/pub/FreeBSD/ports/alpha/packages-4-stable/devel/portcheckout-2.0.tgz
ftp://ftp.FreeBSD.org/pub/FreeBSD/ports/i386/packages-5-current/devel/portcheckout-2.0.tgz
ftp://ftp.FreeBSD.org/pub/FreeBSD/ports/alpha/packages-5-current/devel/portcheckout-2.0.tgz
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