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AUSCERT External Security Bulletin Redistribution
ESB-2001.406 -- Cisco Security Advisory
Cisco Secure PIX Firewall SMTP Filtering Vulnerability
28 September 2001
AusCERT Security Bulletin Summary
Product: PIX Firewall 6.0(1)
PIX Firewall 5.2(5)
PIX Firewall 5.2(4)
Vendor: Cisco Systems
Impact: Inappropriate Access
Access Required: Remote
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Cisco Secure PIX Firewall SMTP Filtering Vulnerability
For Public Release 2001 September 26 08:00 AM US/Pacific (UTC+0800)
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 CSCdu47003.
The complete notice will be available at
All users of Cisco Secure PIX Firewalls with software versions 6.0(1),
5.2(5) and 5.2(4) that provide access to SMTP Mail services are at risk.
Please see the table below for affected versions.
The IOS Firewall feature set is not affected by the above defect.
The behavior is a failure of the command fixup protocol smtp [portnum],
which is enabled by default on the Cisco Secure PIX Firewall. The impact
and description of this defect is similar to a defect outlined in a
previous security advisory,
however, this instance of mail filtering bypass was re-introduced by the
If you do not have protected Mail hosts with the accompanying configuration
(configuration example below) you are not vulnerable to the attack.
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
conduit permit tcp host 192.168.0.1 eq 25 any
conduit permit tcp 192.168.0.1 255.255.255.0 eq 25 any
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
If the mail server itself is not properly secured, an attacker may be able
to collect information about existing e-mail accounts and aliases, or may
be able to execute arbitrary code on the mail server. 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
Please note that Cisco strongly recommends that security on all servers,
workstations and network infrastructure gear is maintained as part of
Standard Operating Procedures. Internet Firewalls do not protect against
risk factors internal to a Firewalled network such as social engineering,
rogue internal users or additional external access points to the internal
network (i.e. modem pools or network fax machines) and as such should not
be viewed as the only security measure necessary to ensure network
Software Versions and Fixes
Interim Release Release; available
Version Affected Fix will carry now
forward into all Fix will carry
later versions forward into all
4.4(7.202) 4.4(7.204) 4.4(8)
5.1(4.206) 5.1(4.209) 5.1(5)
5.2(3.210) 5.2(5.207) 5.2(6)
5.3(1.200) 5.3(1.206) 5.3(2)
6.0(1) 6.0(1.101) 6.1(1)
Obtaining 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 above, 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.
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 whose Cisco products are provided or
maintained through prior or existing agreement with third-party support
organizations such as Cisco Partners, authorized resellers, or service
providers should contact that support organization for assistance with the
upgrade, which should be free of charge.
Customers who purchase direct from Cisco but who do not hold a Cisco
service contract, and customers who purchase through third party vendors
but are unsuccessful at obtaining fixed software through their point of
sale, 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: email@example.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 "firstname.lastname@example.org" or
"email@example.com" for software upgrades.
There is not a direct workaround 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 discovered internally by Cisco, during expanded
regression testing. This vulnerability has been discussed on public forums
previously. This vulnerability has not been discussed recently, and has not
been discussed with reference to the current versions of the PIX software.
Status of This Notice: FINAL
This is a final 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:
* firstname.lastname@example.org (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 1.0 For public release 26-SEP-2001 08:00 AM
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
includes instructions for press inquiries regarding Cisco security notices.
All Cisco Security Advisories are available at
This notice is Copyright 2001 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, and include
all date and version information.
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