(IBM Issues Fix for AIX) Re: Telnet Daemons May Give Remote Users Root Level Access Privileges
SecurityTracker Alert ID: 1002122|
SecurityTracker URL: http://securitytracker.com/id/1002122
(Links to External Site)
Date: Aug 1 2001
Execution of arbitrary code via network, Root access via network, User access via network|
Fix Available: Yes Vendor Confirmed: Yes |
TESO reported that many BSD-derived Telnet daemons (servers) contain a vulnerability that may allow a remote user to obtain root level access on the server.|
The vulnerability is reportedly due to a buffer overflow in the telnet option handling.
The following systems are reported to be vulnerable:
BSDI 4.x default, FreeBSD .x default, IRIX 6.5, Linux netkit-telnetd < 0.14, NetBSD 1.x default, OpenBSD 2.x, Solaris 2.x sparc, and "almost any other vendor's telnetd".
A remote user can send a specially formatted option string to the remote telnet server and overwrite sensitive memory, causing arbitrary code to be executed with the privileges of the telnet server (which is typically root level privileges).
Telnet options are reportedly processed by the 'telrcv' function. The results of the parsing, which are to be send back to the client, are stored in the 'netobuf' buffer. It is apparently assumed that the reply data is smaller than the buffer size, so no bounds checking is performed. By using a combination of options, especially the 'AYT' Are You There option, it is possible for a remote user to append data to the buffer. It is reported that the characters that can be written to the buffer are limited, which makes constructing a successful exploit difficult.
The report states that a working exploit has been developed for BSDI, NetBSD and FreeBSD. However, the exploit was not released.
A remote user can execute arbitrary code on the server with the privileges of the telnet server, which is typically root level privileges.|
IBM has released a fix for AIX 4.3.x and 5.1. See the Source Message for the IBM advisory.|
|Underlying OS: UNIX (AIX)|
|Underlying OS Comments: many Linux and Unix OSs are vulnerable, but not all - see the Alert text for more information|
This archive entry is a follow-up to the message listed below.|
Source Message Contents
Subject: IBM AIX 4.3.x and 5.1: Buffer overflow vulnerability in telnet daemon|
IBM Global Services
Managed Security Services
Outside Advisory Redistribution
1 AUG 2001 0:30 GMT MSS-OAR-E01-2001:298.1
The MSS Outside Advisory Redistribution is designed to provide customers of
IBM Managed Security Services with access to the security advisories
sent out by other computer security incident response teams, vendors, and
other groups concerned about security.
IBM makes no representations and assumes no responsibility for the contents
or accuracy of the advisories themselves.
IBM MSS is forwarding the following information from IBM.
Contact information for IBM is included in the forwarded text
below. Please contact them if you have any questions or need further
----------- Forwarded Information Starts Here.
-----BEGIN PGP SIGNED MESSAGE-----
IBM SECURITY ADVISORY
Fri Jul 27 13:17:01 CDT 2001
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
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.
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
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
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
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"
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 firstname.lastname@example.org 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
188.8.131.52, 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
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
or send email to "email@example.com" with the word "FixDist" in the
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 "firstname.lastname@example.org" with
the word "subscribe Security_APARs" in the "Subject:" line.
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 email@example.com
with a subject of "get key".
If you would like to subscribe to the AIX security newsletter, send a
note to firstname.lastname@example.org 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
IBM and AIX are a registered trademark of International Business
Machines Corporation. All other trademarks are property of their
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