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 CSCds30699.
A new aspect of this vulnerability has been assigned Cisco bug ID CSCds38708.
The complete advisory is available at http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20000927-pix-firewall-smtp-filter.
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 defects.
No other Cisco products are currently known to be affected by these vulnerabilities.
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
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 attacker.
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 vulnerability.
When considering software upgrades, also consult http://www.cisco.com/go/psirt and any subsequent advisories to determine exposure and a complete upgrade solution.
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 assistance.
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 http://www.cisco.com/public/sw-license-agreement.html, or as otherwise set forth at Cisco.com Downloads at http://www.cisco.com/public/sw-center/sw-usingswc.shtml.
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 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 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)
- e-mail: firstname.lastname@example.org
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.
Refer to http://www.cisco.com/warp/public/687/Directory/DirTAC.shtml for additional TAC contact information, including special localized telephone numbers and instructions and e-mail addresses for use in various languages.
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 http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20000927-pix-firewall-smtp-filter.
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.