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AUSCERT External Security Bulletin Redistribution
ESB-1999.190 -- CERT Advisory CA-99-15
Buffer Overflows in SSH Daemon and RSAREF2 Library
14 December 1999
The CERT Coordination Centre has released the following advisory concerning
a combination of remotely exploitable buffer overflows in the RSAREF2
implementation and in the SSH daemon as discussed in AUSCERT ALERT
AL-1999.006. These vulnerabilities may allow remote users to execute
arbitrary code as the owner of the sshd process (typically root) and gain
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CERT Advisory CA-99-15 Buffer Overflows in SSH Daemon and RSAREF2 Library
Original release date: December 13, 1999
Last revised: --
A complete revision history is at the end of this file.
* Systems running some versions of sshd
* Systems using products that use RSAREF2 (e.g., some SSL-enabled
Some versions of sshd are vulnerable to a buffer overflow that can
allow an intruder to influence certain variables internal to the
program. This vulnerability alone does not allow an intruder to
However, a vulnerability in RSAREF2, which was discovered and
researched by Core SDI, can be used in conjunction with the
vulnerability in sshd to allow a remote intruder to execute arbitrary
Additional information about the RSAREF2 vulnerability can be found at
The RSAREF2 library was developed from a different code base than
other implementations of the RSA algorithm, including those from RSA
Security Inc. The vulnerability described in this advisory is specific
to the RSAREF2 library and does not imply any weakness in other
implementations of the RSA algorithm or the algorithm itself.
Also, only versions of SSH compiled with RSAREF support, via the
--with-rsaref option, are vulnerable to these issues.
The use of the RSAREF2 library in other products may present
additional vulnerabilities. RSAREF2 may be used in products such as
SSL-enabled web servers, ssh clients, or other cryptographically
enhanced products. Appendix A of this advisory will be updated with
new information as it becomes available regarding problems in other
products that use the RSAREF2 library.
Using the two vulnerabilities in conjunction allows an intruder to
execute arbitrary code with the privileges of the process running
sshd, typically root.
We are investigating whether vulnerabilities in other products may
expose the vulnerability in RSAREF2, and will update this advisory as
See Appendices A and B for more information that may affect the impact
of this vulnerability.
Apply patch(es) from your product vendor
Apply patch(es) to the RSAREF2 library. RSA Security Inc. holds a
patent on the RSA algorithm and a copyright on the RSAREF2
implementation. We encourage you to consult your legal counsel
regarding the legality of any fixes you are considering before
implementing those fixes. Please see RSA's vendor statement in
Exploiting the vulnerability in RSAREF2 requires an application
program to call the RSAREF2 library with malicious input. For products
that allow an intruder to influence the data provided to the RSAREF2
library, you may be able to protect against attacks by validating the
data they provide to RSAREF2.
Appendix A contains information provided by vendors for this advisory.
Appendix B contains information regarding test performed by the CERT
Coordination Center and other people, and advice based on those tests.
We will update the appendices as we receive or develop more
information. If you do not see your vendor's name in Appendix A, the
CERT/CC did not hear from that vendor. Please contact your vendor
Use a non-vulnerable implementation of the RSA algorithm
Sites not restricted by patent law may choose to use a non-vulnerable
implementation of RSA. Since RSA Security Inc. holds a patent on the
RSA algorithm, this option may not be legally available to you. Please
consult your legal counsel for guidance on this issue.
Appendix A. Vendor Information
Compaq Computer Corporation
(c) Copyright 1998, 1999 Compaq Computer Corporation. All rights
Compaq Computer Corporation
Software Security Response Team USA
Compaq's Tru64 UNIX is not vulnerable. Compaq does not ship ssl
Covalent Raven SSL module for Apache
The Raven SSL module is not vulnerable to this attack since the SSL
library used does not use the RSAREF library.
Data Fellows Inc.
F-Secure SSH versions prior 1.3.7 are vulnerable but F-Secure SSH 2.x
and above are not.
FreeBSD 3.3R and prior releases contain packages with this problem.
This problem was corrected December 2, 1999 in the ports tree.
Packages built after this date with the rsaref updated should be
unaffected by this vulnerabilities. Some or all of the following ports
may be affected should be rebuilt:
p5-Penguin, p5-Penguin-Easy, jp-pgp, ja-w3m-ssl, ko-pgp, pgpsendmail,
pine4-ssl, premail, ParMetis, SSLtelnet, mpich, pipsecd, tund,
nntpcache, p5-Gateway, p5-News-Article, ru-pgp, bjorb, keynote,
OpenSSH, openssl, p5-PGP, p5-PGP-Sign, pgp, slush, ssh,
sslproxy, stunnel, apache+mod_ssl, apache+ssl, lynx-ssl,
Please see the FreeBSD Handbook for information on how to obtain a
current copy of the ports tree and how to rebuild those ports which
depend on rsaref.
HP does not supply SSH. HP has not conducted compatibility testing
with version 1.2.27 of SSH, when compiled with the option
--with-rsaref. Further, RSAREF2 has not been tested to date. As far
as the investigation to date, HP appears to be not vulnerable.
IBM AIX does not currently ship the secure shell (ssh) nor do the base
components of AIX ship or link with the RSAREF2 library.
IBM and AIX are registered trademarks of International Business
The Microsoft Security Response Team has investigated this issue, and
no Microsoft products are affected by the vulnerability.
NetBSD does not ship with ssh in either its US-only or International
variants at this time, so no default installation of NetBSD is
However, ssh is installed and widely used by many NetBSD
installations, and is available from our software package tree in
source form. The NetBSD ssh package can be compiled either with or
without RSAREF2, settable by the administrator at compile time
according to local copyright and license restrictions.
Installations which used RSAREF2 in compiling ssh are vulnerable, and
we recommend recompiling without RSAREF2 if their local legal
In addition, the following list of software packages in the NetBSD
"packages" system are also dependent on the RSAREF2 library:
of those, the security/openssl package is itself a library, and the
following packages depend on it:
We recommend recompiling and reinstalling these packages without
RSAREF2, if your local legal situation permits.
Network Associates, Inc.
After a technical review of the buffer overflow bug in RSAREF, we have
determined at Network Associates that PGP is not affected by this bug,
because of the careful way that PGP uses RSAREF.
This applies to all versions of PGP ever released by MIT, which are
the only versions of PGP that use RSAREF. All other versions of PGP,
such as the commercial versions and the international versions, avoid
the use of RSAREF entirely.
10 December 1999
[CERT/CC Note: A PGP signed copy of this information and additional
technical details are available as well.]
OpenSSL with RSAREF is not vulnerable.
OpenBSD / OpenSSH
More information is available from:
RSA Security Inc.
RSA Security Inc. recommends that developers implement the proposed or
similar patch to RSAREF version 2.0 or otherwise to ensure that the
length in bits of the modulus supplied to RSAREF is less than or equal
RSA Security Inc. is no longer distributing the RSAREF toolkit, which
it offered through RSA Laboratories in the mid-1990s as a free, source
implementation of modern cryptographic algorithms. Under the terms of
the RSAREF license, changes to the RSAREF code other than porting or
performance improvement require written consent. RSA Security hereby
gives its consent to implement a patch to RSAREF to address this
This advisory only applies to RSAREF, not RSA Security's current
toolkits and products, which were developed independently of RSAREF.
Although RSA Security is no longer distributing RSAREF, the toolkit is
still available in a number of "freeware" products such as SSH under
RSA Security's original RSAREF v2.0 software license ("license.txt",
March 25, 1994), which is distributed along with those products. As a
reminder, that license limits the use of RSAREF to noncommercial
purposes. RSAREF, RSAREF applications, and services based on RSAREF
applications may not be sold, licensed or otherwise transferred for
value. (There is a minor exception for small "shareware" deployments
as noted in the "info.txt" file, March 25, 1994.)
The bug only affects ssh when it is compiled with RSAREF (i.e., only
when --with-rsaref is explicitly supplied on the command line). Any
version compiled without --with-rsaref is not affected. The problem
should not affect users of the commercial versions (who are licensed
to use the built-in RSA) or users outside the United States (who are
presumably not using RSAREF and can use the built-in RSA without
needing a license). I.e., only those non-commercial users who actually
compile with a separately obtained RSAREF should be affected.
The bug is present in all versions of SSH1, up to and including
1.2.27. It will be fixed in ssh-1-2.28 (expected to go out in a few
days to fix this problem). It does not affect SSH2. (Please note that
ssh1 is no longer maintained, except for security fixes, due to
certain rather fundamental problems that have been fixed in ssh2.)
Any implementation compiled without an explicitly specified
--with-rsaref is not affected by this problem.
A patch provided by SSH Communications is available from the CERT/CC
web site. This version of the patch has been signed by the CERT/CC.
Stronghold does not use RSAREF and is unaffected.
Appendix B. CERT/CC and Other Third-Party Tests
RSAREF Patch from Core SDI and the CERT/CC
With the assistance of Core SDI, the CERT Coordination Center tested
sshd version 1.2.27 running on an Intel-based RedHat Linux system and
found that configuration to be vulnerable. Tests conducted by Core SDI
indicate that sshd 1.2.27 running on OpenBSD and FreeBSD on Intel is
also vulnerable, and it is likely that other configurations are
vulnerable as well.
CERT/CC has developed a patch for the RSAREF2 vulnerability based in
part on work done by Core SDI. This patch is available at
You can verify this patch with a detached PGP signature from the
We believe the patch originally provided by Core SDI in their advisory
may not be a complete fix to this particular problem. We have worked
with them to develop an updated patch and gratefully acknowledge their
contribution to the fix provided here. Neither the CERT/CC, the
Software Engineering Institute, nor Carnegie Mellon University
provides any warranties regarding this patch. Please see our
disclaimer at the end of this advisory.
Possible vulnerability of ssh clients
The possible vulnerability of ssh clients is of particular concern. As
we learn more regarding the vulnerability of ssh clients, we will
update this advisory. One possible way to attack an ssh client would
be to construct a malicious ssh server and lure or trick victims into
connecting to the server. The ssh client will warn users when it
connects to a site that presents a key that does not match one
previously associated with the server. The dialog may be similar to
% ssh badhost
@ WARNING: HOST IDENTIFICATION HAS CHANGED! @
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that the host key has just been changed.
Please contact your system administrator.
Add correct host key in /etc/.ssh/known_hosts to get rid of this message.
Are you sure you want to continue connecting (yes/no)? no
If you see this warning, you should answer "no" to the prompt and
investigate why the key you received does not match the key you
The CERT Coordination Center would like to thank Alberto Solino
<Alberto_Solino@core-sdi.com> and Gerardo Richarte
<Gerardo_Richarte@core-sdi.com> of Core SDI S.A. Seguridad de la
informacion, Buenos Aires, Argentina (http://www.core-sdi.com), who
discovered the problem in RSAREF2 and provided valuable technical
assistance. We would also like to thank Andrew Cormack of JANET CERT,
who provided technical assistance; Theo de Raadt of the OpenBSD
project, who provided valuable feedback used in the construction of
this advisory; Burt Kaliski of RSA Security Inc.; and Tatu Ylonen of
SSH Communications Security.
This document is available from:
CERT/CC Contact Information
Phone: +1 412-268-7090 (24-hour hotline)
Fax: +1 412-268-6989
CERT Coordination Center
Software Engineering Institute
Carnegie Mellon University
Pittsburgh PA 15213-3890
CERT personnel answer the hotline 08:00-20:00 EST(GMT-5) / EDT(GMT-4)
Monday through Friday; they are on call for emergencies during other
hours, on U.S. holidays, and on weekends.
We strongly urge you to encrypt sensitive information sent by email.
Our public PGP key is available from
If you prefer to use DES, please call the CERT hotline for more
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Copyright 1999 Carnegie Mellon University.
Conditions for use, disclaimers, and sponsorship information can be
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patent, trademark, or copyright infringement.
December 13, 1999: Initial release
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