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CERT Advisory CA-2000-18 PGP May Encrypt Data With Unauthorized ADKs



   Original release date: August 24, 2000

   Last revised: --

   Source: CERT/CC

   

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

   

Systems Affected



     * PGP versions 5.5.x through 6.5.3, domestic and international

       

Overview



   Additional Decryption Keys (ADKs) is a feature introduced into PGP

   (Pretty Good Privacy) versions 5.5.x through 6.5.3 that allows

   authorized extra decryption keys to be added to a user's public key

   certificate. However, an implementation flaw in PGP allows unsigned

   ADKs which have been maliciously added to a certificate to be used for

   encryption.

   

   Data encrypted with PGP 5.5.x through 6.5.3 using a modified

   certificate will generate ciphertext encrypted with the ADK subject to

   the conditions list in the impact section. The attacker who modified

   the certificate can obtain the plaintext from this ciphertext.

   

   PGP does not correctly detect this form of certificate modification,

   because it fails to check if the ADK is stored in the signed (hashed)

   portion of the public certificate. As a result normal methods for

   evaluating the legitimacy of a public certificate (fingerprint

   verification) are not sufficient for users of vulnerable versions of

   PGP.

   

I. Description



   A serious problem in the handling of certificates when encrypting with

   PGP versions 5.5.x through 6.5.3 has recently been discovered by Ralf

   Senderek. A detailed description of his research and conclusions can

   be found at

   

   [25]http://senderek.de/security/key-experiments.html

          

   This advisory refers to "PGP certificates", which most users would

   refer to as a "PGP keys". PGP certificates are the files used to store

   and exchange keys. A certificate contains one or more keys, as well as

   other information such as the creation time, signatures by other keys,

   and "additional decryption keys".

   

   An Additional Decryption Key (ADK) is a mechanism by which a second

   decryption key can be associated with a user's primary key in a

   certificate. All data encrypted for the primary key would also be

   encrypted with the second key. This configuration might be used, for

   example, in environments where data encrypted with an individual's key

   also needs to be available to their employer.

   

   The ADK feature is intended to only be available on those certificates

   where the user specifically consented to having an additional key

   associated with theirs. However, because of an implementation flaw in

   some versions of PGP, ADKs added to a victim's certificate by an

   attacker may be used for encryption in addition to the victim's key

   without their consent.

   

   Since a user's public key certificate is often widely distributed, an

   attacker could make this modification to a specific copy of the

   certificate without the legitimate user's knowledge. When a vulnerable

   version of PGP uses the modified certificate for encryption, it fails

   to detect that the ADK is contained in the unsigned portion of the

   certificate. Because PGP does not report an invalid signature, senders

   using the modified certificate have no way to detect the modification

   without complicated manual inspection.

   

   No legitimately produced PGP certificate will exhibit this

   vulnerability, nor is this an inherent weakness in the ADK

   functionality. Your exposure to this vulnerability is independent of

   whether or not you legitimately employ ADKs.

   

   The PGP Software Development Kit (PGP SDK) has this vulnerability,

   implying that PGP plugins and other PGP enabled applications may be

   vulnerable as well. We will provide additional information as it

   becomes available.

   

II. Impact



   Attackers who are able to modify a victim's public certificate may be

   able to recover the plaintext of any ciphertext sent to the victim

   using the modified certificate.

   

   For this vulnerability to be exploited, the following conditions must

   hold

     * the sender must be using a vulnerable version of PGP

     * the send must be encrypting data with a certificate modified by

       the attacker

     * the sender must acknowledge a warning dialog that an ADK is

       associated with the certificate

     * the sender have the key for the bogus ADK already on their local

       keyring

     * the bogus ADK must be signed certificate by a CA that the sender

       trusts

     * the attacker be able to obtain the ciphertext sent from the sender

       to the victim

       

   Taken together, these factors limit reasonable exploitation of this

   vulnerability to those situations in which the key identified as the

   ADK is known valid key. This might occur when the attacker is an

   insider known to the victim, but is unlikely to occur if the attacker

   is a completely unrelated third party.

   

   Viewing the keys in a GUI interface clearly shows that an ADK is

   associated with a given recipient, as shown in this [26]image.

   

   Since the key associated with the ADK is clearly listed as one of the

   recipients of the ciphertext, it is likely that the sender might

   notice this and be able to identify the attacker.

   

   The recipient may use any type of PGP key, including RSA and

   Diffie-Hellman. The version of PGP used by the recipient has no impact

   on the attack.

   

III. Solution



Apply a patch



   Network Associates has produced a new version of PGP 6.5 which

   corrects this vulnerability by requiring that the ADK be included in

   the signed portion of the certificate.

   

   Appendix A contains information provided by vendors 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.

   

Check certificates for ADKs before adding them to a keyring.



   Users of PGP who want to ensure that they are not using a modified

   certificate should check for the existence of ADKs when adding new

   keys to their keyring. Certificates that do not have ADKs are not

   vulnerable to this problem. Certificates which do have ADKs may be

   legitimate or modified and should be confirmed using an out-of-band

   communication.

   

   Users of PGP 6.x for Windows and MacOS can test for the presence of

   ADKs in a certificate by right clicking on the certificate and

   selecting "Key Properties". If the ADK tab is present, the key has one

   or more ADKs and might be a malicious certificate. We are not aware of

   a way to identify ADKs in the UNIX command line version of PGP 5.x or

   6.x.

   

   Users of GnuPG can test for certificates with ADKs by running the

   command

   

   gpg --list-packet

          

   Certificates with legitimate ADKs will contain in the output

   

   hashed subpkt 10 len 23 (additional recipient request)

          

   while those missing the "hashed" keyword

   

   subpkt 10 len 23 (additional recipient request)

          

   appear to indicate maliciously modified certificates.

   

Make a reliable copy of your public certificate publicly available.



   Since the recipient of messages encrypted with a modified certificate

   cannot prevent the plaintext from being recovered by the attacker,

   their best course of action is to ensure that senders are able to

   easily obtain legitimate copies of their public certificate.

   

   Until this problem has been widely corrected, you may wish to make

   your legitimate certificate available in a location that is strongly

   authenticated using a different technology, or to make it available in

   more than one place.

   

   For example, the CERT/CC PGP certificate does NOT contain any ADKs,

   and a legitimate version can be obtained for our SSL secured web site

   at

   

   [27]https://www.cert.org/pgp/cert_pgp_key.asc

          

   You may also want to check that your public certificate has not been

   modified on the public certificate servers. Changes are likely to be

   made to the popular PGP certificate servers to detect and reject

   invalid certificates that attempt to exploit this vulnerability.

   

Appendix A. Vendor Information



Network Associates, Inc.



   We at NAI/PGP Security regret this important bug in the ADK feature

   that has been described on various Internet postings today (Thursday

   24 Aug). We were made aware of this bug in PGP early this morning.

   

   We are responding as fast as we can, and expect to have new 6.5.x

   releases out to fix this bug late Thursday evening. The MIT web site

   should have a new PGP 6.5.x freeware release early Friday, and the

   NAI/PGP web site should have patches out for the commercial releases

   at about the same time. As of this afternoon (Thursday), the PGP key

   server at PGP already filters out keys with the bogus ADK packets. We

   expect to have fixes available for the other key servers that run our

   software by tomorrow. We have also alerted the other vendors that make

   PGP key server software to the problem, and expect Highware/Veridis in

   Belgium to have their key servers filtering keys the same way by

   Friday.

   

   The fixes that we are releasing for the PGP client software filters

   out the offending ADK packets. We already warn the users whenever they

   are about to use an ADK, even in the normal case.

   

   We will have new information as soon as it becomes available at

   http://www.pgp.com.

   

   Philip Zimmermann

   prz@pgp.com

   19:00 PDT Thursday 24 Aug 2000

   

   A signed version of this statement is available at

   

   [28]CA-2000-18/pgp.asc

     _________________________________________________________________

   

   The CERT Coordination Center thanks Ralf Senderek for bringing this

   problem to light and Network Associates for developing a solution and

   assisting in the preparation of this advisory.

     _________________________________________________________________

   

   Authors: Cory Cohen, Shawn Hernan, Jeff Havrilla, and Jeff Lanza.

   Graphics developed by Matt DeSantis. [29]Feedback on this advisory is

   appreciated.

   ______________________________________________________________________

   

   This document is available from:

   [30]http://www.cert.org/advisories/CA-2000-18.html

   ______________________________________________________________________

   

CERT/CC Contact Information



   Email: [31]cert@cert.org

          Phone: +1 412-268-7090 (24-hour hotline)

          Fax: +1 412-268-6989

          Postal address:

          CERT Coordination Center

          Software Engineering Institute

          Carnegie Mellon University

          Pittsburgh PA 15213-3890

          U.S.A.

          

   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.

   

Using encryption



   We strongly urge you to encrypt sensitive information sent by email.

   Our public PGP key is available from

   

   [32]http://www.cert.org/CERT_PGP.key

       

   If you prefer to use DES, please call the CERT hotline for more

   information.

   

Getting security information



   CERT publications and other security information are available from

   our web site

   

   [33]http://www.cert.org/

       

   To be added to our mailing list for advisories and bulletins, send

   email to [34]cert-advisory-request@cert.org and include SUBSCRIBE

   your-email-address in the subject of your message.

   

   * "CERT" and "CERT Coordination Center" are registered in the U.S.

   Patent and Trademark Office.

   ______________________________________________________________________

   

   NO WARRANTY

   Any material furnished by Carnegie Mellon University and the Software

   Engineering Institute is furnished on an "as is" basis. Carnegie

   Mellon University makes no warranties of any kind, either expressed or

   implied as to any matter including, but not limited to, warranty of

   fitness for a particular purpose or merchantability, exclusivity or

   results obtained from use of the material. Carnegie Mellon University

   does not make any warranty of any kind with respect to freedom from

   patent, trademark, or copyright infringement.

     _________________________________________________________________

   

   [35]Conditions for use, disclaimers, and sponsorship information

   

   Copyright 2000 Carnegie Mellon University.

   

   Revision History

August 24, 2000:  Initial release



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