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FreeBSD-SA-09:05 - telnetd Code Execution

FreeBSD-SA-09:05 - telnetd Code Execution
Posted Feb 17, 2009
Site security.freebsd.org

FreeBSD Security Advisory - In order to prevent environment variable based attacks, telnetd scrubs its environment; however, recent changes in FreeBSD's environment-handling code rendered telnetd's scrubbing inoperative, thereby allowing potentially harmful environment variables to be set. An attacker who can place a specially-constructed file onto a target system (either by legitimately logging into the system or by exploiting some other service on the system) can execute arbitrary code with the privileges of the user running the telnet daemon (usually root).

tags | advisory, arbitrary, root
systems | freebsd
SHA-256 | 8fd5f35be1f357357d7faa04aaf55fefca25b625f49ea0f157d81958e7d9b0a6

FreeBSD-SA-09:05 - telnetd Code Execution

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=============================================================================
FreeBSD-SA-09:05.telnetd Security Advisory
The FreeBSD Project

Topic: telnetd code execution vulnerability

Category: core
Module: contrib
Announced: 2009-02-16
Affects: FreeBSD 7.x
Corrected: 2009-02-16 21:56:17 UTC (RELENG_7, 7.1-STABLE)
2009-02-16 21:56:17 UTC (RELENG_7_1, 7.1-RELEASE-p10)
2009-02-16 21:56:17 UTC (RELENG_7_0, 7.0-RELEASE-p3)

For general information regarding FreeBSD Security Advisories,
including descriptions of the fields above, security branches, and the
following sections, please visit <URL:http://security.FreeBSD.org/>.

I. Background

The FreeBSD telnet daemon, telnetd(8), implements the server side of the
TELNET virtual terminal protocol. It has been disabled by default in
FreeBSD since August 2001, and due to the lack of cryptographic security
in the TELNET protocol, it is strongly recommended that the SSH protocol
be used instead. The FreeBSD telnet daemon can be enabled via the
/etc/inetd.conf configuration file and the inetd(8) daemon.

The TELNET protocol allows a connecting client to specify environment
variables which should be set in any created login session; this is used,
for example, to specify terminal settings.

II. Problem Description

In order to prevent environment variable based attacks, telnetd(8) "scrubs"
its environment; however, recent changes in FreeBSD's environment-handling
code rendered telnetd's scrubbing inoperative, thereby allowing potentially
harmful environment variables to be set.

III. Impact

An attacker who can place a specially-constructed file onto a target system
(either by legitimately logging into the system or by exploiting some other
service on the system) can execute arbitrary code with the privileges of
the user running the telnet daemon (usually root).

IV. Workaround

No workaround is available, but systems which are not running the telnet
daemon are not vulnerable.

V. Solution

Perform one of the following:

1) Upgrade your vulnerable system to 7-STABLE, or to the RELENG_7_1 or
RELENG_7_0 security branch dated after the correction date.

2) To patch your present system:

The following patches have been verified to apply to FreeBSD 7.0 and 7.1
systems.

a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.

# fetch http://security.FreeBSD.org/patches/SA-09:05/telnetd.patch
# fetch http://security.FreeBSD.org/patches/SA-09:05/telnetd.patch.asc

b) Execute the following commands as root:

# cd /usr/src
# patch < /path/to/patch
# cd /usr/src/lib/libtelnet
# make obj && make depend && make
# cd /usr/src/libexec/telnetd
# make obj && make depend && make && make install

VI. Correction details

The following list contains the revision numbers of each file that was
corrected in FreeBSD.

CVS:

Branch Revision
Path
- -------------------------------------------------------------------------
RELENG_7
src/contrib/telnet/telnetd/sys_term.c 1.18.22.1
RELENG_7_1
src/UPDATING 1.507.2.13.2.6
src/sys/conf/newvers.sh 1.72.2.9.2.7
src/contrib/telnet/telnetd/sys_term.c 1.18.30.2
RELENG_7_0
src/UPDATING 1.507.2.3.2.14
src/sys/conf/newvers.sh 1.72.2.5.2.14
src/contrib/telnet/telnetd/sys_term.c 1.18.26.1
- -------------------------------------------------------------------------

Subversion:

Branch/path Revision
- -------------------------------------------------------------------------
stable/7/ r188699
releng/7.1/ r188699
releng/7.0/ r188699
- -------------------------------------------------------------------------

VII. References

http://lists.grok.org.uk/pipermail/full-disclosure/2009-February/067954.html

The latest revision of this advisory is available at
http://security.FreeBSD.org/advisories/FreeBSD-SA-09:05.telnetd.asc
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