Advisory ID: cisco-sa-20000927-pix-firewall-smtp-filter
For Public Release 2000 September 27 08:00 UTC (GMT)
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
This section provides details on 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 feature set is not affected by either of the above
No other Cisco products are currently known to be affected by these
The behavior is a failure of the command fixup protocol
smtp [portnum], which is enabled by default on the Cisco Secure
If you do not have protected Mail hosts with the accompanying
configuration (configuration example below) you are not affected by this
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
Cisco has provided scores for the vulnerabilities in this advisory based on the Common Vulnerability Scoring System (CVSS). The CVSS scoring in this Security Advisory is done in accordance with CVSS version 2.0.
CVSS is a standards-based scoring method that conveys vulnerability severity and helps determine urgency and priority of response.
Cisco has provided a base and temporal score. Customers can then compute environmental scores to assist in determining the impact of the vulnerability in individual networks.
Cisco has provided an FAQ to answer additional questions regarding CVSS at
Cisco has also provided a CVSS calculator to help compute the environmental impact for individual networks at
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
When considering software upgrades, also consult
and any subsequent advisories to determine exposure and a complete upgrade
In all cases, customers should exercise caution to be certain the
devices to be upgraded contain sufficient memory and that current hardware and
software configurations will continue to be supported properly by the new
release. If the information is not clear, contact the Cisco Technical
Assistance Center ("TAC") or your contracted maintenance provider for
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.
Cisco has made free software available to address this vulnerability
for affected customers. Prior to deploying software, customers should consult
their maintenance provider or check the software for feature set compatibility
and known issues specific to their environment.
Customers may only install and expect support for the feature sets they
have purchased. By installing, downloading, accessing or otherwise using such
software upgrades, customers agree to be bound by the terms of Cisco's software
license terms found at
or as otherwise set forth at Cisco.com Downloads at
Do not contact either "email@example.com" or "firstname.lastname@example.org"
for software upgrades.
Customers with contracts should obtain upgraded software through their
regular update channels. For most customers, this means that upgrades should be
obtained through the Software Center on Cisco's worldwide website at
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 guidance and assistance with the appropriate course of
action in regards to this advisory.
The effectiveness of any workaround or fix is dependent on specific
customer situations such as product mix, network topology, traffic behavior,
and organizational mission. Due to the variety of affected products and
releases, customers should consult with their service provider or support
organization to ensure any applied workaround or fix is the most appropriate
for use in the intended network before it is deployed.
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)
Have your product serial number available and 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.
for additional TAC contact information, including special localized telephone
numbers and instructions and e-mail addresses for use in various
This vulnerability was first reported to Cisco by a customer. This
vulnerability has been discussed on public forums.
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A stand-alone copy or Paraphrase of the text of this document that omits the distribution URL in the following section is an uncontrolled copy, and may lack important information or contain factual errors.
This advisory will be posted on Cisco's worldwide website 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.
email@example.com (includes CERT/CC)
Various internal Cisco mailing lists
Future updates of this advisory, if any, will be placed on Cisco's
worldwide website, but may or may not be actively announced on mailing lists or
newsgroups. Users concerned about this problem are encouraged to check the
above URL for any updates.
New defect ID reference, and revised the Fixed in versions to
reflect recent fixes.
Initial public release.