AUSCERT External Security Bulletin Redistribution

                 ESB-2001.312 -- CERT Advisory CA-2001-21
                        Buffer Overflow in telnetd
                               25 July 2001


        AusCERT Security Bulletin Summary

Product:                telnetd
Operating System:       FreeBSD
Impact:                 Root Compromise
                        Denial of Service
Access Required:        Remote

Ref:                    ESB-2001.306

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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 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 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

    1. http://www.ietf.org/rfc/rfc0854.txt
    2. http://www.team-teso.net/advisories/teso-advisory-011.tar.gz
    3. http://www.kb.cert.org/vuls/id/745371
    4. ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/advisories/FreeBSD-SA-01:49.telnetd.asc

   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

   Email: cert@cert.org
          Phone: +1 412-268-7090 (24-hour hotline)
          Fax: +1 412-268-6989
          Postal address:
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   Copyright 2001 Carnegie Mellon University.

   Revision History
July 24, 2001:  Initial release

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