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  • CERT Advisory CA-1999-15 Buffer Overflows in SSH daemon and RSAREF2 Library

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Original release date: December 13, 1999
Last revised: March 3, 2000
Source: CERT/CC

A complete revision history is at the end of this file.

Systems Affected

  • Systems running some versions of sshd
  • Systems using products that use RSAREF2 (e.g., some SSL-enabled web servers)

I. Description

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 execute code.

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 code.

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.

II. Impact

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 appropriate.

See Appendices A and B for more information that may affect the impact of this vulnerability.

III. Solution

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 Appendix A.

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 tests 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 directly.

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 reserved.

Compaq Computer Corporation
Compaq Services
Software Security Response Team USA

Compaq's Tru64 UNIX is not vulnerable. Compaq does not ship ssl

Covalent Technologies

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, w3m-ssl, zope

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.


Fujitsu's UXP/V operating system does not support secure shell (SSH). Therefore, it is not vulnerable to this problem.

Hewlett-Packard Company

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 Corporation

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 Machines Corporation.


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 vulnerable.

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 situation permits.

In addition, the following list of software packages in the NetBSD "packages" system are also dependent on the RSAREF2 library:

  • archivers/hpack
  • security/openssl
  • security/pgp2
  • security/pgp5
  • www/ap-ssl

of those, the security/openssl package is itself a library, and the following packages depend on it:

  • net/ppp-mppe
  • net/speakfreely-crypto
  • www/ap-ssl

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.

Philip Zimmermann
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 to MAX_RSA_MODULUS_BITS.

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 advisory.

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.)

SSH Communications

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 CERT/CC.

This patch should be applied to the rsa.c source file that comes with the RSAREF distribution. Note that there is also an rsa.c source file that is part of the SSH distribution, and that this patch will not apply correctly to that file. When in the correct directory (the RSAREF source directory) the patch can be applied by issuing the command:

patch <rsa-patch.txt

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 the following:

% ssh badhost
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 expected.

The CERT Coordination Center would like to thank Alberto Solino <> and Gerardo Richarte <> of Core SDI S.A. Seguridad de la informacion, Buenos Aires, Argentina (, 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.

Copyright 1999, 2000 Carnegie Mellon University.

Revision History
December 13, 1999:  Initial release
March 3, 2000: Clarified how to apply RSAREF patch
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