-------- Original Message --------
Subject: CERT Advisory CA-2001-21
Date: Tue, 24 Jul 2001 21:50:53 -0400 (EDT)
From: CERT Advisory <cert-advisory at cert.org>
Organization: CERT(R) Coordination Center - +1 412-268-7090
To: cert-advisory at cert.org
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CERT Advisory CA-2001-21 Buffer Overflow in telnetd
Original release date: July 24, 2001
Last revised: --
A complete revision history can be found at the end of this file.
Systems running versions of telnetd derived from BSD source.
The telnetd program is a server for the Telnet remote virtual
protocol. There is a remotely exploitable buffer overflow in Telnet
daemons derived from BSD source code. This vulnerability can crash
server, or be leveraged to gain root access.
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
fixed-size buffer. It is assumed that the results are smaller than
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
An intruder can execute arbitrary code with the privileges of the
telnetd process, typically root.
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
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 http://www.bsdi.com/services/support/patches and
via ftp at ftp://ftp.bsdi.com/bsdi/support/patches as soon as testing
has been completed.
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,
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
vulnerability and a binary upgrade package suitable for use on
4.3-RELEASE systems. For more information, see the advisory at the
or use an FTP mirror site from the following URL:
[This issue is] actively under investigation to determine
Sun is currently investigating and have confirmed that one can make
the in.telnetd daemon dump core but Sun has not yet determined if
issue is potentially exploitable on Solaris.
Appendix B. - References
The CERT Coordination Center thanks TESO, who published an advisory
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
Email: cert at cert.org
Phone: +1 412-268-7090 (24-hour hotline)
Fax: +1 412-268-6989
CERT Coordination Center
Software Engineering Institute
Carnegie Mellon University
Pittsburgh PA 15213-3890
CERT personnel answer the hotline 08:00-17:00 EST(GMT-5) / EDT(GMT-4)
Monday through Friday; they are on call for emergencies during other
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Getting security information
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* "CERT" and "CERT Coordination Center" are registered in the U.S.
Patent and Trademark Office.
Any material furnished by Carnegie Mellon University and the Software
Engineering Institute is furnished on an "as is" basis. Carnegie
Mellon University makes no warranties of any kind, either expressed
implied as to any matter including, but not limited to, warranty of
fitness for a particular purpose or merchantability, exclusivity or
results obtained from use of the material. Carnegie Mellon University
does not make any warranty of any kind with respect to freedom from
patent, trademark, or copyright infringement.
Conditions for use, disclaimers, and sponsorship information
Copyright 2001 Carnegie Mellon University.
July 24, 2001: Initial release
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