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ESB-2000.268 -- Cisco Security Advisory -- Cisco Secure PIX Firewall Mailguard Vulnerability

Date: 28 September 2000
References: ESB-2000.276  

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              AUSCERT External Security Bulletin Redistribution
                   ESB-2000.268 -- Cisco Security Advisory
              Cisco Secure PIX Firewall Mailguard Vulnerability
                              28 September 2000


	AusCERT Security Bulletin Summary

Product:                Cisco Secure PIX Firewall
Vendor:                 Cisco Systems
Impact:                 Reduced Security
Access Required:        Remote

- --------------------------BEGIN INCLUDED TEXT--------------------


               Cisco Secure PIX Firewall Mailguard Vulnerability

Revision 1.0
  For public release 2000 Sept 27 08:00 AM 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
   The complete advisory is available at
Affected Products

   All users of Cisco Secure PIX Firewalls with software versions up to
   and including 4.4(5), 5.0(3), 5.1(2) and 5.2(1) 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(5) (including 2.7,   | 4.4(6)                           |
   | 3.0, 3.1, 4.0, 4.1)                 |                                  |
   | Version 5.0.x up to and including   |                                  |
   | version 5.0(3)                      | 5.1(3)                           |
   | All 5.1.x up to and including       |                                  |
   | version 5.1(2)*                     | 5.1(3)                           |
   | Version 5.2(1)                      | 5.2(2)                           |
   *For customers who may have engineering releases addressing specific
   unrelated defects, designated as 5.1(2)2xx, version 5.1(3) 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(2)207 or higher, which also
   includes the SMTP security 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:
   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: 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 1.0  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.

Version: PGP 6.0.2


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