__________________________________________________________
The U.S. Department of Energy
Computer Incident Advisory Center
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__________________________________________________________
INFORMATION BULLETIN
IBM AIX telnetd Buffer Overflow
August 10, 2001 22:00 GMT Number L-131
______________________________________________________________________________
PROBLEM: A remote buffer overflow exists in the telnet daemon in use on
many platforms.
PLATFORM: IBM AIX 4.3.x and 5.1
DAMAGE: A remote attacker can cause the telnetd process to overflow the
buffer and crash, or execute arbitrary code as the user running
telnetd, usually root.
SOLUTION: Check with your vendor for specific information on the
operating systems in use at your site. For IBM AIX systems,
follow the workaround or solutions as described below.
______________________________________________________________________________
VULNERABILITY The risk is HIGH. A valid user account and password are not
ASSESSMENT: required to exploit this vulnerability, only the ability to
connect to a telnetd server. The exploit code has been
published on the Internet.
______________________________________________________________________________
LINKS:
CIAC BULLETIN: http://www.ciac.org/ciac/bulletins/l-131.shtml
______________________________________________________________________________
[****** Start IBM Security Advisory ******]
IBM SECURITY ADVISORY
Fri Jul 27 13:17:01 CDT 2001
Revised: Thu Aug 09 11:31:34 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
There is no practical workaround. To protect against an exploit
before the efix or APAR is applied, the telnet daemon can be
disabled to prevent the use of telnet.
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:
APAR number for AIX 4.3.3: IY22029
APAR number for AIX 5.1: IY22021
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:
ftp://aix.software.ibm.com/aix/efixes/security/telnetd_efix.tar.Z
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 security-alert@austin.ibm.com 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
4.3.3.50, 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/telnet.xyz 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
http://techsupport.services.ibm.com/rs6k/fixes.html
or send email to "aixserv@austin.ibm.com" 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 "aixserv@austin.ibm.com" 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:
security-alert@austin.ibm.com
To request the PGP public key that can be used to encrypt new AIX
security vulnerabilities, send email to security-alert@austin.ibm.com
with a subject of "get key".
If you would like to subscribe to the AIX security newsletter, send a
note to aixserv@austin.ibm.com 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.
[****** End IBM Security Advisory ******]
_______________________________________________________________________________
CIAC wishes to acknowledge the contributions of IBM Corporation for the
information contained in this bulletin.
_______________________________________________________________________________
CIAC, the Computer Incident Advisory Center, is the computer
security incident response team for the U.S. Department of Energy
(DOE) and the emergency backup response team for the National
Institutes of Health (NIH). CIAC is located at the Lawrence Livermore
National Laboratory in Livermore, California. CIAC is also a founding
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This document was prepared as an account of work sponsored by an
agency of the United States Government. Neither the United States
Government nor the University of California nor any of their
employees, makes any warranty, express or implied, or assumes any
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