CERT Advisory CA-2001-21 Buffer Overflow in telnetd
Original release date: July 24, 2001
Last revised: Fri Aug 10 08:36:50 EDT 2001
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.
Overview
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
http://www.team-teso.net/advisories/teso-advisory-011.tar.gz.
This vulnerability has been assigned the identifier CAN-2001-0554 by
the Common Vulnerabilities and Exposures (CVE) group:
http://cve.mitre.org/cgi-bin/cvename.cgi?name=CAN-2001-0554
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.
Berkeley Software Design, Inc. (BSDI)
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.
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 ftp://ftp.caldera.com
OpenLinux 2.3:
/pub/openlinux/updates/2.3/022/RPMS/netkit-telnet-0.16-1.i386.rpm
OpenLinux eServer 2.3.1:
/pub/eServer/2.3/updates/2.3/007/RPMS/netkit-telnet-0.16-1.i386.rpm
OpenLinux eDesktop 2.4, OpenLinux 3.1 Server, and OpenLinux 3.1
Workstation are not affected.
(Caldera has recently released CSSA-2001-030.0 -
http://www.caldera.com/support/security/advisories/CSSA-2001-030.0.txt
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
http://www.cisco.com/go/psirt/.
Conectiva
(Conectiva has released advisory CLSA-2001:413, located at
http://distro.conectiva.com.br/atualizacoes/?id=a&anuncio=000413, 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:
ftp://ftp.freebsd.org/pub/FreeBSD/CERT/advisories/FreeBSD-SA-01
:49.telnetd.asc
or use an FTP mirror site from the following URL:
http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/mirro
rs-ftp.html
(FreeBSD has also released
ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/advisories/FreeBSD-SA-01%3A54.p
orts-telnetd.asc, 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....
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 ftp://aix.software.ibm.com/aix/efixes/security. 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.
NetBSD
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:
ftp://ftp.netbsd.org/pub/NetBSD/security/advisories/NetBSD-SA2001-012.
txt.asc
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
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
http://www.sgi.com/support/security/
Sun Microsystems, Inc.
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
assistance.
_________________________________________________________________
Authors: Jason A. Rafail, Ian Finlay, and Shawn Hernan.
______________________________________________________________________
This document is available from:
http://www.cert.org/advisories/CA-2001-21.html
______________________________________________________________________
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Fax: +1 412-268-6989
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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
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