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CA-2001-21 Buffer Overflow in telnetd


CERT Advisory CA-2001-21 Buffer Overflow in telnetd

Original release date: July 24, 2001
Last revised: --
Source: CERT/CC

A complete revision history can be found at the end of this file.

Systems Affected

Systems running versions of telnetd derived from BSD source.


The telnetd program is a server for the Telnet remote virtual terminal
protocol. There is a remotely exploitable buffer overflow in Telnet
daemons derived from BSD source code. This vulnerability can crash the
server, or be leveraged to gain root access.

I. Description

There is a remotely exploitable buffer overflow in Telnet daemons
derived from BSD source code. During the processing of the Telnet
protocol options, the results of the "telrcv" function are stored in a
fixed-size buffer. It is assumed that the results are smaller than the
buffer and no bounds checking is performed.

The vulnerability was discovered by TESO. An exploit for this
vulnerability has been publicly released; internal testing at CERT/CC
confirms this exploit works against at least one target system. For
more information, see

II. Impact

An intruder can execute arbitrary code with the privileges of the
telnetd process, typically root.

III. Solution

Apply a patch

Appendix A contains information from vendors who have provided
information for this advisory. We will update the appendix as we
receive more information. If you do not see your vendor's name, the
CERT/CC did not hear from that vendor. Please contact your vendor

Restrict access to the Telnet service (typically port 23/tcp) using a
firewall or packet-filtering technology.

Until a patch can be applied, you may wish to block access to the
Telnet service from outside your network perimeter. This will limit
your exposure to attacks. However, blocking port 23/tcp at a network
perimeter would still allow attackers within the perimeter of your
network to exploit the vulnerability. It is important to understand
your network's configuration and service requirements before deciding
what changes are appropriate.

Appendix A. - Vendor Information

This appendix contains information provided by vendors for this
advisory. When vendors report new information to the CERT/CC, we
update this section and note the changes in our revision history. If a
particular vendor is not listed below, we have not received their


All current versions of BSD/OS are vulnerable. Patches are available
via our web site at and
via ftp at as soon as testing
has been completed.

Cisco Systems

Cisco IOS does not appear to be vulnerable. Certain non-IOS products
are supplied on other operating system platforms which themselves may
be vulnerable as described elsewhere in this CERT Advisory. The Cisco
PSIRT is continuing to investigate the vulnerability to be certain
and, if necessary, will provide updates to the CERT and publish an
advisory. Cisco Security Advisories are on-line at


All released versions of FreeBSD are vulnerable to this problem, which
was fixed in FreeBSD 4.3-STABLE and FreeBSD 3.5.1-STABLE on July 23,
2001. An advisory has been released, along with a patch to correct the
vulnerability and a binary upgrade package suitable for use on FreeBSD
4.3-RELEASE systems. For more information, see the advisory at the
following location:

or use an FTP mirror site from the following URL:


[This issue is] actively under investigation to determine
vulnerability ramifications.

Sun Microsystems

Sun is currently investigating and have confirmed that one can make
the in.telnetd daemon dump core but Sun has not yet determined if this
issue is potentially exploitable on Solaris.

Appendix B. - References


The CERT Coordination Center thanks TESO, who published an advisory on
this issue. We would also like to thank Jeff Polk for technical

Authors: Jason A. Rafail, Ian A. Finlay, and Shawn V. Hernan.

This document is available from:

CERT/CC Contact Information

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Fax: +1 412-268-6989
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Copyright 2001 Carnegie Mellon University.

Revision History
July 24, 2001: Initial release

Version: PGPfreeware 5.0i for non-commercial use
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