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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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