FreeBSD-SA-11:08.telnetd: telnetd code execution vulnerability

An attacker who can connect to the telnetd daemon can execute arbitrary code with the privileges of the daemon (which is usually the "root" superuser).

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FreeBSD-SA-11:08.telnetd                                    Security Advisory
                                                          The FreeBSD Project

Topic:          telnetd code execution vulnerability

Category:       core
Module:         contrib
Announced:      2011-12-23
Affects:        All supported versions of FreeBSD.
Corrected:      2011-12-23 15:00:37 UTC (RELENG_7, 7.4-STABLE)
                2011-12-23 15:00:37 UTC (RELENG_7_4, 7.4-RELEASE-p5)
                2011-12-23 15:00:37 UTC (RELENG_7_3, 7.3-RELEASE-p9)
                2011-12-23 15:00:37 UTC (RELENG_8, 8.2-STABLE)
                2011-12-23 15:00:37 UTC (RELENG_8_2, 8.2-RELEASE-p5)
                2011-12-23 15:00:37 UTC (RELENG_8_1, 8.1-RELEASE-p7)
                2011-12-23 15:00:37 UTC (RELENG_9, 9.0-STABLE)
                2011-12-23 15:00:37 UTC (RELENG_9_0, 9.0-RELEASE)
CVE Name:       CVE-2011-4862

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

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 has a mechanism for encryption of the data stream
(but it is not cryptographically strong and should not be relied upon
in any security-critical applications).

II.  Problem Description

When an encryption key is supplied via the TELNET protocol, its length
is not validated before the key is copied into a fixed-size buffer.

III. Impact

An attacker who can connect to the telnetd daemon can execute arbitrary
code with the privileges of the daemon (which is usually the "root"

IV.  Workaround

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

Note that the telnet daemon is usually run via inetd, and consequently
will not show up in a process listing unless a connection is currently
active; to determine if it is enabled, run

$ ps ax | grep telnetd | grep -v grep
$ grep telnetd /etc/inetd.conf | grep -vE '^#'

If any output is produced, your system may be vulnerable.

V.   Solution

Perform one of the following:

1) Upgrade your vulnerable system to 7-STABLE or 8-STABLE, or to the
RELENG_8_2, RELENG_8_1, RELENG_7_4, or RELENG_7_3 security branch dated
after the correction date.

2) To update your vulnerable system via a source code patch:

The following patches have been verified to apply to FreeBSD 7.4, 7.3,
8.2, and 8.1  systems.

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

# fetch
# fetch

b) Execute the following commands as root:

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

3) To update your vulnerable system via a binary patch:

Systems running 7.4-RELEASE, 7.3-RELEASE, 8.2-RELEASE, or 8.1-RELEASE on
the i386 or amd64 platforms can be updated via the freebsd-update(8)

# freebsd-update fetch
# freebsd-update install

VI.  Correction details

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


Branch                                                           Revision
- -------------------------------------------------------------------------
  src/UPDATING                                             1.507.
  src/UPDATING                                            1.507.
  src/UPDATING                                             1.632.
  src/UPDATING                                            1.632.
- -------------------------------------------------------------------------


Branch/path                                                      Revision
- -------------------------------------------------------------------------
stable/7/                                                         r228843
releng/7.4/                                                       r228843
releng/7.3/                                                       r228843
stable/8/                                                         r228843
releng/8.2/                                                       r228843
releng/8.1/                                                       r228843
stable/9/                                                         r228843
releng/9.0/                                                       r228843
- -------------------------------------------------------------------------

VII. References

The latest revision of this advisory is available at
Version: GnuPG v2.0.18 (FreeBSD)