Advisory ID: cisco-sa-20010926-pix-firewall-smtp-filter
For Public Release 2001 September 26 15:00 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 CSCdu47003.
The complete notice will be available at
This section provides details on affected products.
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.
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
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 defect
If you do not have protected Mail hosts with the accompanying
configuration (configuration example below) you are not vulnerable to the
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
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 vulnerability.
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 integrity.
The following table of affected software versions and fixes details ALL
affected versions, including engineering special builds that are rarely
provided to customers. The "Affected Products" section above mentions only the
versions that have been officially released. For example, in version 4.4(7) the
defect was introduced in 4.4(7.202) and fixed in 4.4(7.204). As there was no
officially released version between those two engineering builds, it is
unlikely that any customer would have the affected version, however, this is
documented as resolved in the Release Notes for version 4.4(8).
Fix will carry forward into all later versions
Fixed Regular Release; available now
Fix will carry forward into all later versions
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.
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 "firstname.lastname@example.org" or "email@example.com"
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 in obtaining fixed software through their point of sale should obtain software patches and bug fixes 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 provide the URL of this notice as evidence of entitlement to a software patch or bug fix. Customers without service contracts should request a software patch or bug fix 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 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.
THIS DOCUMENT IS PROVIDED ON AN "AS IS" BASIS AND DOES NOT IMPLY ANY KIND OF GUARANTEE OR WARRANTY, INCLUDING THE WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR USE. YOUR USE OF THE INFORMATION ON THE DOCUMENT OR MATERIALS LINKED FROM THE DOCUMENT IS AT YOUR OWN RISK. CISCO RESERVES THE RIGHT TO CHANGE OR UPDATE THIS DOCUMENT AT ANY TIME.
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.
firstname.lastname@example.org (includes CERT/CC)
Various internal Cisco mailing lists
Future updates of this advisory, 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
above URL for any updates.
In the section "Software Versions and Fixes" added an
explanation of affected software versions.
For public release