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CERT* Advisory CA-98.07
Original issue date: June 26, 1998

Topic: Vulnerability in Some Usages of PKCS#1

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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.

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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

     A.  Obtain and install a patch for this problem.

         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.

     B.  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:

         -- 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 decrypt:s3_srvr.c:1259

         -- 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.

         -- Avoid using the same public/private key pair across multiple

         -- 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

        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 happens.

        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

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

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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.

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If you believe that your system has been compromised, contact the CERT
Coordination Center or your representative in the Forum of Incident Response
and Security Teams (see

CERT/CC Contact Information
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Email    This email address is being protected from spambots. You need JavaScript enabled to view it.

Phone    +1 412-268-7090 (24-hour hotline)
                CERT personnel answer 8:30-5:00 p.m. EST(GMT-5) / EDT(GMT-4)
                and are on call for emergencies during other hours.

Fax      +1 412-268-6989

Postal address
         CERT Coordination Center
         Software Engineering Institute
         Carnegie Mellon University
         Pittsburgh PA 15213-3890

Using encryption
   We strongly urge you to encrypt sensitive information sent by email. We can
   support a shared DES key or PGP. Contact the CERT/CC for more information.
   Location of CERT PGP key

Getting security information
   CERT publications and other security information are available from

   CERT advisories and bulletins are also posted on the USENET newsgroup

   To be added to our mailing list for advisories and bulletins, send
   email to
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   In the subject line, type
        SUBSCRIBE  your-email-address

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Copyright 1998 Carnegie Mellon University. Conditions for use, disclaimers,
and sponsorship information can be found in and .
If you do not have FTP or web access, send mail to This email address is being protected from spambots. You need JavaScript enabled to view it. with
"copyright" in the subject line.

*CERT is registered in the U.S. Patent and Trademark Office.

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