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Original issue date: June 26, 1998
Last revised: August 24, 1998
Added vendor information for Silicon Graphics, Inc.

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

The CERT Coordination Center has received a report regarding a vulnerability in some implementations of products utilizing RSA Laboratories' Public-Key Cryptography Standard #1 (PKCS#1). Under some situations, a sophisticated intruder may be able to use the vulnerability in PKCS#1 to recover information from SSL-encrypted sessions.

The CERT/CC team recommends that sites install patches immediately as described in Appendix A. Appendix A also contains pointers to web pages containing additional information maintained by some vendors.

We will update this advisory as we receive additional information. Please check our advisory files regularly for updates that relate to your site.

I. Description

PKCS#1 is a standard for encrypting data using the RSA public-key cryptosystem. Its intended use is in the construction of digital signatures and digital envelopes.

One use for the digital envelopes constructed using PKCS#1 is to provide confidentiality during the session key negotiation of an SSL-encrypted session. The SSL protocol is widely used to encrypt traffic to and from web servers to protect the privacy of information such as personal data or a credit card number, as it traverses the internet. A sophisticated intruder may be able to use the vulnerability in PKCS#1 to recover information from an SSL-encrypted session.

Web pages employing SSL are accessed using the HTTPS protocol, rather than the HTTP protocol.

More information about PKCS#1 can be found at

Additional information regarding this vulnerability will be available at

This vulnerability does not affect all PKCS#1-enabled products. The attack is not effective against protocols in which there is not an interactive session setup, or where the error messages returned by the server do not distinguish among the types of failures. In particular, this vulnerability does not affect S/MIME or SET.

II. Impact

Under some circumstances, an intruder who is able to observe an SSL-encrypted session, and subsequently interrogate the server involved in the session, may be able to recover the session key used in that session, and then recover the encrypted data from that session.

The vulnerability can only be exploited if the intruder is able to make repeated session-establishment attempts to the same vulnerable web server which was involved in the original session. In addition, the server must return error messages that distinguish between several modes of failure. Although the number of session-establishment requests is large, it is significantly more efficient than a brute-force attack against the session key. Note that, although web servers comprise the majority of vulnerable servers, other PKCS#1-enabled servers may be vulnerable.

Note that the server's public and private key are not at risk from this vulnerability, and that an intruder is only able to recover data from a single session per attack. Compromising a single session does not give an intruder any additional ability to compromise subsequent sessions. Further, as mentioned above, this vulnerability does not affect all PKCS#1-enabled products.

III. Solution

  1. Obtain and install a patch for this problem.
  2. Appendix A contains input 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.

  3. Although applying vendor patches is the recommended course of action, you may wish to consider some of the following steps to reduce your exposure to this vulnerability:
    1. Examine your log files for repeated error messages indicating failed requests for session-establishment. For example, sites using C2Net's Stronghold server would see error messages of the form
      [Tue Jun 23 22:08:17 1998] SSL accept error
      1575:error:0407006B:rsa routines:RSA_padding_check_PKCS1_type_2:block type is not 02:rsa_pk1.c:207
      1575:error:04064072:rsa routines:RSA_EAY_PRIVATE_DECRYPT:padding check failed:rsa_eay.c:330
      1575:error:1408B076:SSL routines:SSL3_GET_CLIENT_KEY_EXCHANGE:bad rsa
    2. If you are unable to upgrade for an extended period of time, you may wish to consider obtaining a new public/private key pair for servers. Changing the key pair only protects those sessions which may have been previously recorded by an intruder. This does not prevent an intruder from launching attacks against newly-recorded sessions. This should only be considered in those cases where upgrading is infeasible. Again, note that the public/private key pair is not at risk from this vulnerability.
    3. Avoid using the same public/private key pair across multiple servers.

    4. A large increase in CPU utilization or network traffic may accompany an attack. If your web server does not provide sufficient detail in its logs to detect failures, you may wish to look for substantial deviation from established usage patterns, which may be indicative of an attack.

      Implementors and researchers should consult RSA Laboratories Bulletin Number 7 for additional measures to reduce the effectiveness of this attack. This document will be available at

Appendix A - Vendor Information

Below is a list of the vendors who have provided information for this advisory. We will update this appendix as we receive additional information. If you do not see your vendor's name, the CERT/CC did not hear from that vendor. Please contact the vendor directly.

C2Net Software, Inc.

C2Net has developed a patch and is deploying new builds to combat this
problem. More information is available at

Microsoft Corporation

The Microsoft Product Security Response Team has produced an update for the
following affected Microsoft Internet server software:

        - Microsoft Internet Information Server 3.0 and 4.0
        - Microsoft Site Server 3.0, Commerce Edition
        - Microsoft Site Server, Enterprise Edition
        - Microsoft Exchange 5.0 and 5.5 (for SSL-enabled POP3 and SMTP)

Microsoft's Internet server software provides SSL 2.0, SSL 3.0, PCT 1.0, and
TLS 1.0 for securing Internet-based communications. These protocols are all
implemented in a single file called SCHANNEL.DLL, which is shared by the
Microsoft Internet server software listed above. Updating this single file
will resolve this vulnerability for these Microsoft server products.

No updates are required for Internet client software, such as Internet

This update is now available. Microsoft strongly recommends that customers
using secure SSL Internet services with any of the Microsoft products listed
above should update to the latest version of SCHANNEL.DLL.

Please visit the Microsoft Security Advisor web site for more information, or
link directly to our Microsoft security bulletin MS98-002 at

Netscape Communications Corporation

Netscape recommends that all customers running Netscape Enterprise Server
software, Netscape Proxy Server, Netscape Messaging Server and Netscape
Collabra Server download and install a simple patch before an attack ever

Product updates and full information about the countermeasures are available
immediately from the Netscape Internet site at:

Open Market, Inc.

Some of Open Market's products are affected by this vulnerability. Patches are
available. For more information, go to

RSA Data Security, Inc.

Information from RSA regarding this vulnerability is available at

Silicon Graphics, Inc.

See Silicon Graphics Inc. Security Advisory, "Vulnerability in Public-Key Cryptography Standard #1 (PKCS#1)," 19980606-01-A, issued June 26, 1998.

Currently, Silicon Graphics Inc. is investigating and is in communication with Netscape. No further information is available for public release at this time.

The SGI anonymous FTP site is ( or its mirror, Security information and patches can be found in the ~ftp/security and ~ftp/patches directories, respectfully.

For subscribing to the wiretap mailing list and other SGI security related information, please refer to the Silicon Graphics Security Headquarters website located at:


Information and SSLeay source patches related to this vulnerability are
available at

Terisa Systems, Inc. / Spyrus, Inc.

Terisa has determined that the SSL implementation in the Terisa SecureWeb
Toolkit is vulnerable to this attack. A patch to fix this vulnerability has
been developed for existing versions of the Toolkit. Further information may
be found at

This vulnerability was originally discovered by Daniel Bleichenbacher of the Secure Systems Research Department of Bell Labs, the research and development arm of Lucent Technologies.

The CERT Coordination Center thanks Scott Schnell of RSA and Jason Garms of Microsoft for reporting this problem to us and providing technical advice and other valuable input into the construction of this advisory. In addition, our thanks goes to Simona Nass, Douglas Barnes, and Tim Hudson of C2Net and David Wagner of the University of California at Berkeley for the example log files contained herein as well as additional technical advice and clarification during the production of this advisory.

Copyright 1998 Carnegie Mellon University.

Revision History
Aug. 24, 1998  Added vendor information for Silicon Graphics, Inc.
July 27, 1998  Added vendor information for Terisa Systems, Inc. / Spyrus, Inc.
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