Cisco Secure PIX Firewall Mailguard Vulnerability
Privacy and Legal Notice
CIAC INFORMATION BULLETIN
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
Firewall.
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)
_________________________________________________________________
Summary
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
CSCds30699.
A new aspect of this vulnerability has been assigned Cisco bug ID
CSCds38708.
The complete advisory is available at
http://www.cisco.com/warp/public/707/PIXfirewallSMTPfilter-pub.shtml.
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
defects.
Details
The behavior is a failure of the command "fixup protocol smtp
[portnum]", which is enabled by default on the Cisco Secure PIX
Firewall.
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
following:
fixup protocol smtp 25
and either
conduit permit tcp host 192.168.0.1 eq 25 any
or
conduit permit tcp 192.168.0.1 255.255.255.0 eq 25 any
or
access-list 100 permit tcp any host 192.168.0.1 eq 25
access-group 100 in interface outside
The expected filtering of the Mailguard feature can be circumvented by
an attacker.
Impact
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
purchased.
+-------------------------------------+----------------------------------+
| | 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 http://www.cisco.com.
Customers without contracts should get their upgrades by contacting
the Cisco Technical Assistance Center (TAC). TAC contacts are as
follows:
* +1 800 553 2447 (toll-free from within North America)
* +1 408 526 7209 (toll call from anywhere in the world)
* e-mail: HREF="mailto:tac@cisco.com">tac@cisco.com
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 "psirt@cisco.com" or
"security-alert@cisco.com" for software upgrades.
Workarounds
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.
Distribution
This notice will be posted on Cisco's Worldwide Web site at
http://www.cisco.com/warp/public/707/PIXfirewallSMTPfilter-pub.shtml.
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:
* cust-security-announce@cisco.com
* bugtraq@securityfocus.com
* first-teams@first.org (includes CERT/CC)
* cisco@spot.colorado.edu
* comp.dcom.sys.cisco
* firewalls@lists.gnac.com
* 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
http://www.cisco.com/warp/public/707/sec_incident_response.shtml. This
includes instructions for press inquiries regarding Cisco security
notices.
_________________________________________________________________
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.
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can be contacted at:
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FAX: +1 925-423-8002
STU-III: +1 925-423-2604
E-mail: ciac@llnl.gov
World Wide Web: http://www.ciac.org/
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(same machine -- either one will work)
Anonymous FTP: ftp.ciac.org
ciac.llnl.gov
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UCRL-MI-119788
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