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TUCoPS :: Cisco :: ciacl002.htm

Cisco Secure PIX Firewall Mailguard Vulnerability
Cisco Secure PIX Firewall Mailguard Vulnerability Privacy and Legal Notice


L-002: Cisco Secure PIX Firewall Mailguard Vulnerability

October 11, 2000 21:00 GMT
PROBLEM:       A feature of the Cisco Secure PIX Firewall, "Mailguard", has a
               vulnerability. This vulnerability can be exploited to bypass
               SMTP command filtering.
PLATFORM:      Cisco Secure PIX Firewalls with versions up to and including
               4.4(6), 5.0(3), 5.1(3) and 5.2(2). This vulnerability only
               exists if access to SMTP Mail services are provided by the
DAMAGE:        Exploitation of the mail server is possible if the PIX firewall
               has been configured in a specific manner. Exploitation of the
               "Mailguard" feature effective removes firewall protection for
               the mail server. Failure of the "fixup protocol smtp [portnum]"
               command is responsible for the firewall failure. This command
               is enabled by default on the firewall.
SOLUTION:      The "Mailguard" feature incorporated into the firewall is a
               protection feature for weakly protected mail servers. The first
               stage protection should always be the fortification of the
               mailserver against intruders. The PIX firewall offers extra
               protection of the server. Cisco recommends that mailer server
               security features be fortified, or follow the advisory
               instructions for upgrading the PIX firewall code. Best
               operating practice is to do both.
VULNERABILITY  The risk is LOW. A malicious user would have to gain direct
ASSESSMENT:    access to a mailserver for exploitation to occur. The risk is
               further minimized if security measures have been taken on the
               server to protect it.

[******  Begin Cisco Advisory ******]

Cisco Security Advisory: Cisco Secure PIX Firewall Mailguard Vulnerability

Revision 1.1

Updated, for public release 2000 October 5 04:00 PM US/Pacific (UTC+0700)


   The Cisco Secure PIX firewall feature "mailguard," which limits SMTP
   commands to a specified minimum set of commands, can be bypassed.

   This vulnerability can be exploited to bypass SMTP command filtering.

   This vulnerability has been assigned Cisco bug ID CSCdr91002 and

   A new aspect of this vulnerability has been assigned Cisco bug ID

   The complete advisory is available at

Affected Products

   All users of Cisco Secure PIX Firewalls with software versions up to
   and including 4.4(6), 5.0(3), 5.1(3) and 5.2(2) that provide access to
   SMTP Mail services are at risk.

   The IOS Firewall featureset is not affected by either of the above


   The behavior is a failure of the command "fixup protocol smtp
   [portnum]", which is enabled by default on the Cisco Secure PIX

   If you do not have protected Mail hosts with the accompanying
   configuration (configuration example below) you are not affected by
   this vulnerability.

   To exploit this vulnerability, attackers must be able to make
   connections to an SMTP mail server protected by the PIX Firewall.  If
   your Cisco Secure PIX Firewall has configuration lines similar to the

             fixup protocol smtp 25

   and either

             conduit permit tcp host eq 25 any


             conduit permit tcp eq 25 any


             access-list 100 permit tcp any host eq 25
             access-group 100 in interface outside

   The expected filtering of the Mailguard feature can be circumvented by
   an attacker.


   The Mailguard feature is intended to help protect weakly secured mail
   servers.  The workaround for this issue is to secure the mail servers
   themselves, or upgrade to fixed PIX firewall code.

   In order to exploit this vulnerability, an attacker would need to also
   exploit the mailserver that is currently protected by the PIX.  If
   that server is already well configured, and has the latest security
   patches and fixes from the SMTP vendor, that will minimize the
   potential for exploitation of this vulnerability.

Software Versions and Fixes

  Getting Fixed Software

   Cisco is offering free software upgrades to remedy this vulnerability
   for all affected customers. Customers with service contracts may
   upgrade to any software version. Customers without contracts may
   upgrade only within a single row of the table below, except that any
   available fixed software will be provided to any customer who can use
   it and for whom the standard fixed software is not yet available. As
   always, customers may install only the feature sets they have

   |                                     | Fixed Regular Release available  |
   | Version Affected                    | now; fix will carry forward into |
   |                                     | all later releases               |
   | All versions of Cisco Secure PIX up |                                  |
   | to version 4.4(6) (including 2.7,   | 4.4(7)                           |
   | 3.0, 3.1, 4.0, 4.1)                 |                                  |
   | Version 5.0.x up to and including   |                                  |
   | version 5.0(3)                      | 5.1(4)                           |
   | All 5.1.x up to and including       |                                  |
   | version 5.1(3)*                     | 5.1(4)                           |
   | Version 5.2(2)                      | 5.2(3)                           |

   *For customers who may have engineering releases addressing specific
   unrelated defects, designated as 5.1(2)2xx, version 5.1(4) only
   includes the SMTP security fixes and does not include any other
   bugfixes. Customers requiring engineering releases to address specific
   unrelated defects will need to use 5.1.4(200) or 4.4.7(200), which
   include all SMTP vulnerability fixes.

   Customers with contracts should obtain upgraded software through their
   regular update channels. For most customers, this means that upgrades
   should be obtained via the Software Center on Cisco's Worldwide Web
   site at

   Customers without contracts should get their upgrades by contacting
   the Cisco Technical Assistance Center (TAC). TAC contacts are as
     * +1 800 553 2447 (toll-free from within North America)
     * +1 408 526 7209 (toll call from anywhere in the world)
     * e-mail: HREF="">

   Give the URL of this notice as evidence of your entitlement to a free
   upgrade. Free upgrades for non-contract customers must be requested
   through the TAC. Please do not contact either "" or
   "" for software upgrades.


   There is not a direct work around for this vulnerability.  The
   potential for exploitation can be lessened by ensuring that mail
   servers are secured without relying on the PIX functionality.

Exploitation and Public Announcements

   This vulnerability was first reported to Cisco by a customer.  This
   vulnerability has been discussed on public forums.

Status of This Notice: Revised FINAL

   This is a final field notice. Although Cisco cannot guarantee the
   accuracy of all statements in this notice, all of the facts have been
   checked to the best of our ability.   Cisco does not anticipate
   issuing updated versions of this notice unless there is some material
   change in the facts. Should there be a significant change in the
   facts, Cisco may update this notice.


   This notice will be posted on Cisco's Worldwide Web site at
   In addition to Worldwide Web posting, a text version of this notice is
   clear-signed with the Cisco PSIRT PGP key and is posted to the
   following e-mail and Usenet news recipients:
     * (includes CERT/CC)
     * Various internal Cisco mailing lists

   Future updates of this notice, if any, will be placed on Cisco's
   Worldwide Web server, but may or may not be actively announced on
   mailing lists or newsgroups. Users concerned about this problem are
   encouraged to check the URL given above for any updates.

Revision History

   Revision 1.1  05-OCT-2000  New defect ID reference, and revised the Fixed
   in versions to reflect recent fixes.
   Revision 1.0  27-SEP-2000  Initial Public Release

Cisco Security Procedures

   Complete information on reporting security vulnerabilities in Cisco
   products, obtaining assistance with security incidents, and
   registering to receive security information from Cisco, is available
   on Cisco's Worldwide Web site at This
   includes instructions for press inquiries regarding Cisco security

   This notice is copyright 2000 by Cisco Systems, Inc. This notice may
   be redistributed freely after the release date given at the top of the
   text, provided that redistributed copies are complete and unmodified,
   including all date and version information.

[******  End Cisco Advisory ******]

CIAC wishes to acknowledge the contributions of Cisco Systems, Inc. for the information contained in this bulletin.
CIAC services are available to DOE, DOE Contractors, and the NIH. CIAC can be contacted at:
    Voice:          +1 925-422-8193 (7 x 24)
    FAX:            +1 925-423-8002
    STU-III:        +1 925-423-2604
    World Wide Web:
                     (same machine -- either one will work)
    Anonymous FTP:
                     (same machine -- either one will work)

This document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor the University of California nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial products, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation or favoring by the United States Government or the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or the University of California, and shall not be used for advertising or product endorsement purposes.
[Privacy and Legal Notice]

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