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IBM AIX 4.3.x and 5.1: Buffer overflow vulnerability in telnet daemon

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Date: Tue, 31 Jul 2001 20:24:13 -0400
From: IBM MSS Advisory Service <>
Subject: IBM AIX 4.3.x and 5.1: Buffer overflow vulnerability in telnet daemon

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1 AUG 2001  0:30 GMT                              MSS-OAR-E01-2001:298.1

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----------- Forwarded Information Starts Here. -----BEGIN PGP SIGNED MESSAGE----- IBM SECURITY ADVISORY Fri Jul 27 13:17:01 CDT 2001
VULNERABILITY SUMMARY VULNERABILITY: Buffer overflow vulnerability in telnet daemon PLATFORMS: IBM AIX 4.3.x and 5.1 SOLUTION: Apply the emergency-fixes described below, or employ the workaround, also described below. THREAT: Malicious user could obtain root privileges or could force a system crash. CERT Advisory: CA-2001-21
DETAILED INFORMATION I. Description AIX ships with a version of the "telnet" daemon, derived from the original BSD version. This daemon is shipped SUID, or "set user ID", and is executable by an ordinary user. In the AIX version of "telnetd", as well as most other versions of "telnetd" derived from the BSD telnet daemon, there exists a buffer overflow vulnerability in telrcv(), the function that processes various options under telnet. There is an output buffer in the function that holds the information gathered during the parsing of the option request and the daemon's internal state. This buffer is not bounds checked, allowing for the possibility of forcing an overflow condition in the stack when the buffer returns its data to the telnet client. II. Impact A malicious local or remote user can use a well-crafted exploit code to gain root privileges on the attacked system, compromising the integrity of the system and its attached local network. IBM believes this is a difficult vulnerability to exploit with the goal of obtaining enhanced system privileges, but it is not very difficult to force a core dump, and possibly a system crash. Exploits already exist in the wild, and are being maliciously used. An exploit obtained by the AIX Security Team has been shown to produce a core dump, though AIX remained stable. Other exploits, though, may cause more serious harm. Customers are urged to take measures to close this vulnerability. III. Solutions A. WORKAROUND If you do not wish to install the efix for this vulnerability but instead wait for the APAR that fixes it to be made available, you can also negate this vulnerability by making the telnet daemon to be non-SUID. You must be "root" to do this. However, ordinary users will not be able to use the daemon if the SUID bit is removed. Customers may wish to consider replacing telnet with a version of Secure Shell (SSH), available from a variety of providers, as a security enhancement over telnet. B. Official fix IBM is working on the following fixes which will be available soon: AIX 4.3.x and 5.1: APAR assignment pending. NOTE: Fix will not be provided for versions prior to 4.3 as these are no longer supported by IBM. Affected customers are urged to upgrade to 4.3.3 at the latest maintenance level, or to 5.1. C. How to minimize the vulnerability Temporary fixes for AIX 4.3.x and 5.1 systems are available. The temporary fixes can be downloaded via ftp from: The efix compressed tarball consists of two fixes: one for AIX 4.3.3 and one for AIX 5.1. It also includes this Advisory. The two fix files are "telnetd.433" for 4.3.3 and "telnetd.510" for 5.1. These temporary fixes have not been fully regression tested; thus, IBM does not warrant the fully correct functioning of the efix. Customers install the efix and operate the modified version of AIX at their own risk. To proceed with efix installation: First, verify the MD5 cryptographic hash sums of each efix file you obtain from unpacking the tarball with those given below. These should match exactly; if they do not, contact the AIX Security Team at and describe the discrepancy. Filename sum md5
telnetd.433 47297 408 c7b16982f7f2011560c1b726eeae5c64 telnetd.510 33124 383 7fa323119fa312c2c62dc7cd539d58ec Efix Installation Instructions: ------------------------------- IMPORTANT NOTICE: If you are running AIX 4.3.3 you must install the version of libc (in package bos.rte.libc) that is at the level of, or higher, before proceeding with the efix installation for AIX 4.3.3. 1. Become root, if not already done. 2. Change to the /usr/sbin directory. Make a backup copy of the existing telnet binary, giving it a distinctive, meaningful name, such as "telnetd.original" or "telnetd.backup". This is IMPORTANT to do, so you can recover the orginal telnetd binary if something goes wrong during the installation of the efix! Do the above by executing "mv telnetd telnetd.original". 3. In the tmp ("/tmp") directory, download, uncompress, and untar the efix. a. uncompress telnetd_efix.tar b. tar -xvf telnetd_efix.tar 4. You will have two files: "telnetd.433" and "telnetd.510". Keep the tarfile appropriate for your version of AIX (i.e., "433" for 4.3.3; "510" for 5.1); You may remove the unneeded version. 5. Now change back to the directory /usr/sbin. Doublecheck that you have made a backup of your original telnetd. 6. Execute "cp /tmp/ telnetd", where "xyz" is either "433" or "510", as appropriate. 7. Execute "chmod 4554 telnetd". 8. Execute "chown root:system telnetd". 9. Execute "sync; sync; sync;" 10. Execute "refresh -s inetd". IV. Obtaining Fixes IBM AIX APARs may be ordered using Electronic Fix Distribution (via the FixDist program), or from the IBM Support Center. For more information on FixDist, and to obtain fixes via the Internet, please reference or send email to "" with the word "FixDist" in the "Subject:" line. To facilitate ease of ordering all security related APARs for each AIX release, security fixes are periodically bundled into a cumulative APAR. For more information on these cumulative APARs including last update and list of individual fixes, send email to "" with the word "subscribe Security_APARs" in the "Subject:" line. V. Acknowledgements Many thanks to the TESO group in Germany and to "Sebastian", a poster to the BUGTRAQ mailing list, for finding & bringing this vulnerability to our attention. VI. Contact Information Comments regarding the content of this announcement can be directed to: To request the PGP public key that can be used to encrypt new AIX security vulnerabilities, send email to with a subject of "get key". If you would like to subscribe to the AIX security newsletter, send a note to with a subject of "subscribe Security". To cancel your subscription, use a subject of "unsubscribe Security". To see a list of other available subscriptions, use a subject of "help". IBM and AIX are a registered trademark of International Business Machines Corporation. All other trademarks are property of their respective holders. -----BEGIN PGP SIGNATURE----- Version: PGP 6.5.2 iQCVAwUBO2HFfwsPbaL1YgqvAQEJfgQAqHSYb7+ir5P3WfPO9aVlh1EzpD7nD9pA LnXY9cf5MQCpm1N+3I5WxbELhyZi87I8WKcGuEROdlN1gPEuhImIPLIkT3866Ifc zJUHf75Dv90RMDJWnAxihTxsYJo8nT8vd+O5mQ9zqFgYXs1eaSoFuHsBPGr1yk5i 3r1Y8H+QGt4= =ehBa -----END PGP SIGNATURE----- ----------- Forwarded Information Ends Here.
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