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  • CERT Advisory CA-2001-21 Buffer Overflow in telnetd

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Original release date: July 24, 2001
Last revised: April 16, 2002
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

This vulnerability has been assigned the identifier CAN-2001-0554 by the Common Vulnerabilities and Exposures (CVE) group:

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 directly.

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 comments.

Apple Computer

(Apple Computer has released security updates for Mac OS X v10.1 to address this vulnerability. They are located at:

Berkeley Software Design, Inc. (BSDI)

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.

Caldera, Inc.

Caldera has determined that OpenServer, UnixWare 7 and OpenUnix 8 are vulnerable, and we are working on fixes. All of Caldera's Linux supported products are unaffected by this problem if all previously released security updates have been applied. If you're running either OpenLinux 2.3 or OpenLinux eServer 2.3, make sure you've updated your systems to netkit-telnet-0.16. This patch was released in March 2000, and are available from

OpenLinux 2.3:


OpenLinux eServer 2.3.1:


OpenLinux eDesktop 2.4, OpenLinux 3.1 Server, and OpenLinux 3.1 Workstation are not affected.

(Caldera has recently released CSSA-2001-030.0 - which updates the above information with other systems that are vulnerable.)

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

Update: Cisco has released Cisco Security Advisory: Cisco CatOS Telnet Buffer Vulnerability to address an occurrence of this vulnerability.

Compaq Computer Corporation

Hash: SHA1

SOURCE: Compaq Computer Corporation
        Compaq Services
        Software Security Response Team USA

Compaq case id SSRT0745U

ref:   potential telnetd option handling vulnerability

x-ref: TESO Security Advisory   06/2001
       CERT CA2001-21 Advisory  07/2001

 Compaq has evaluated this vulnerability to telnetd
 distributed for Compaq Tru64/UNIX and OpenVMS Operating
 Systems Software and has determined that telnetd is not
 vulnerable to unauthorized command execution or
 root compromise.

 Compaq appreciates your cooperation and patience.
 We regret any inconvenience applying this information
 may cause.

 As always, Compaq urges you to periodically review your system
 management and security procedures.  Compaq will continue to
 review and enhance the security features of its products and work
 with customers to maintain and improve the security and integrity
 of their systems.

 To subscribe to automatically receive future NEW Security
 Advisories from the Compaq's Software Security Response Team
 via electronic mail,

 Use your browser select the URL
  Select "Security and Individual Notices" for immediate dispatch
  notifications directly to your mailbox.

  To report new Security Vulnerabilities, send mail to:

(c) Copyright 2001 Compaq Computer Corporation. All rights reserved.

Version: PGP 7.0.1



(Conectiva has released advisory CLSA-2001:413, located at, to address this issue.)

Cray, Inc.

Cray, Inc. has found UNICOS and UNICOS/mk to be vulnerable. Please see Field Notice 5062 and spr 720789 for fix information. We are currently investigating the MTA for vulnerability.

FreeBSD, Inc.

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:
(FreeBSD has also released, a follow up advisory releated to third party implementations found in FreeBSD ports collection.)

Hewlett-Packard Company

...HP-UX 11.X is not vulnerable, HP_UX 10.X is vulnerable. Patches are in process, watch for the associated HP security Bulletin....

(Hewlett-Packard has release
Security Bulletin HPSBUX0110-172 Sec. Vulnerability in telnetd to address this issue.)

IBM Corporation

IBM's AIX operating system, versions 5.1L and under, is vulnerable to this exploit. IBM has these APAR assignments for this vulnerability: For AIX 4.3.3, the APAR number is IY22029. For AIX 5.1, the APAR number is IY22021.

An emergency fix (efix) is now available for downloading from the ftp site The efix package name to fix this vulnerability is "telnetd_efix.tar.Z". An advisory is included in the tarfile that gives installation instructions for the appropriate patched telnetd binary. Two patches are in the tarfile: one for AIX 4.3.3 (telnetd.433) and for AIX 5.1 (telnetd.510).

IBM is investigating the severity of the exploitation of this vulnerability.


All releases of NetBSD are affected. The issue was patched in NetBSD-current on July 19th. A Security Advisory including patches will be available shortly, at:

NetBSD releases since July 2000 have shipped with telnetd disabled by default. If it has been re-enabled on a system, it is highly recommended to disable it at least until patches are installed. Furthermore, NetBSD recommends the use of a Secure Shell instead of telnet for most applications."

Secure Computing Corporation

The telnetd vulnerability referenced is not applicable to Sidewinder as a result of disciplined security software design practices in combination with Secure Computing's patented Type Enforcement(tm) technology. Sidewinder's telnetd services are greatly restricted due to both known and theoretical vulnerabilities. This least privilege design renders the attack described in the CERT-2001-21 Advisory useless. In addition, Sidewinder's operating system, SecureOS(tm), built on Secure's Type Enforcement technology, has further defenses against this attack that would trigger multiple security violations.

Specifically, the attack first attempts to start a shell process. Sidewinder's embedded Type Enforcement security rules prevent telnetd from replicating itself and accessing the system shell programs. Even without this embedded, tamper proof rule in place, other Type Enforcement rules also defend against this attack. As an example, the new shell would need administrative privileges and those privileges are not available to the telnetd services.


SGI acknowledges the telnetd vulnerability reported by CERT and is currently investigating. Until SGI has more definitive information to provide, customers are encouraged to assume all security vulnerabilities as exploitable and take appropriate steps according to local site security policies and requirements.

As further information becomes available, additional advisories will be issued via the normal SGI security information distribution methods including the wiretap mailing list and

Sun Microsystems, Inc.

A buffer overflow has been discovered in in.telnetd which allows a local or a remote attacker to kill the in.telnetd daemon on the affected SunOS system. Sun does not believe that this issue can be exploited on SunOS systems to gain elevated privileges. As there was a buffer overflow, Sun has generated patches for this issue. The patches are described in the following SunAlert:


and are available from:


(SuSE has released a security announcement related to this vulnerability. It is located at

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 assistance.

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

Copyright 2001 Carnegie Mellon University.

Revision History

July 24, 2001:  Initial release
July 25, 2001:  Fixed HTML tags in vendor section
July 25, 2001:  Added vendor statements 
July 25, 2001:  Added CVE number CAN-2001-0554
July 26, 2001:  Added vendor statements
July 27, 2001:  Fixed vendor section HTML tags
July 31, 2001:  Revised IBM statement
July 31, 2001:  Added Secure Computing Corporation statement 
July 31, 2001:  Updated HP statement
August 10, 2001: Revised IBM statement
August 20, 2001: Updated Caldera statement
August 21, 2001: Updated FreeBSD statement
August 27, 2001: Added link to Conectiva advisory
October 4, 2001: Added Apple Computer Statement
October 11, 2001: Added SuSE Statement
October 16, 2001: Updated Hewlett-Packard Statement
November 19, 2001: Included Compaq Statement
February 1, 2002: Updated Cisco Statement
April 16, 2002: Updated Sun Statement
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