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Cisco Security Advisory

Cisco Secure PIX Firewall FTP Vulnerabilities

Advisory ID: cisco-sa-20000316-pix-ftp

Revision 1.7

For Public Release 2000 March 16 16:00  UTC (GMT)


The Cisco Secure PIX Firewall interprets FTP (File Transfer Protocol) commands out of context and inappropriately opens temporary access through the firewall. This is an interim notice describing two related vulnerabilities.

The first vulnerability is exercised when the firewall receives an error message from an internal FTP server containing an encapsulated command such that the firewall interprets it as a distinct command. This vulnerability can be exploited to open a separate connection through the firewall. This vulnerability is documented as Cisco Bug ID CSCdp86352.

The second vulnerability is exercised when a client inside the firewall browses to an external server and selects a link that the firewall interprets as two or more FTP commands. The client begins an FTP connection as expected and at the same time unexpectedly executes another command opening a separate connection through the firewall. This vulnerability is documented as Cisco Bug ID CSCdr09226.

Either vulnerability can be exploited to transmit information through the firewall without authorization.

Both vulnerabilities are addressed more completely in this updated interim security advisory.

This advisory is posted at

Affected Products

This section provides details on affected products.

Vulnerable Products

All users of Cisco Secure PIX Firewalls with software versions up to and including 4.2(5), 4.4(4), and 5.0(3) that provide access to FTP services are at risk from both vulnerabilities.

Cisco Secure PIX Firewall with software version 5.1(1) is affected by the second vulnerability only.

Products Confirmed Not Vulnerable

Cisco Secure Integrated Software (formerly Cisco IOSĀ® Software Firewall Feature Set) is not affected by either vulnerability.

No other Cisco products are currently known to be affected by these vulnerabilities.


The first vulnerability has been assigned Cisco bug ID CSCdp86352. The second vulnerability has been assigned Cisco bug ID CSCdr09226.

The behavior is due to the command fixup protocol ftp [portnum], which is enabled by default on the Cisco Secure PIX Firewall.

If you do not have protected FTP hosts with the accompanying configuration (configuration example below) you are not vulnerable to the attack which causes a server to send a valid command, encapsulated within an error message, and causes the firewall to read the encapsulated partial command as a valid command (CSCdp86352).

To exploit this vulnerability, attackers must be able to make connections to an FTP server protected by the PIX Firewall. If your Cisco Secure PIX Firewall has configuration lines similar to the following:

fixup protocol ftp 21

and either

conduit permit tcp host eq 21 any


conduit permit tcp eq 21 any

It is possible to fool the PIX stateful inspection into opening up arbitrary TCP ports, which could allow attackers to circumvent defined security policies.

If you permit internal clients to make arbitrary FTP connections outbound, you may be vulnerable to the second vulnerability (CSCdr09226). This is an attack based on CERT advisory CA-2000-02: Malicious HTML Tags Embedded in Client Web Requests

The recommendation in the workarounds section of this document will provide protection against this vulnerability.

Response for the first vulnerability (CSCdp86352)

The following changes have been made to the "fixup protocol FTP" behavior of the PIX Firewall:

  • Enforce that only the server can generate a reply indicating the PASV command was accepted.
  • Enforce that only the client can generate a PORT command.
  • Enforce that data channel is initiated from the expected side in an FTP transaction.
  • Verify that the "227" reply code and the PORT command are complete commands and not part of a "500" error code string broken into fragments.
  • Enforce that the port is not 0 or in the range between [1,1024]

These or equivalent changes will be carried forward into all PIX Firewall software versions after version 5.1(1).

Response for the second vulnerability (CSCdr09226)

We have added an additional command keyword to address this problem:

fixup protocol ftp [strict] <port1>[-<port2>]

The "strict" keyword directs the fixup protocol ftp command to maintain strict command state, and may impact some FTP features such as command pipelining or command grouping. This will be fixed in version 5.1(2) and subsequent versions, as well as in version 4.4(5).

Vulnerability Scoring Details

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 .


Any Cisco Secure PIX Firewall that has enabled the fixup protocol ftp command is at risk of unauthorized transmission of data through the firewall.

Software Versions and Fixes

When considering software upgrades, also consult 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.

Version Affected

Interim Release**(fix will carry forward into all later versions)

Available Now through the TAC

First fixed regular release (fix will carry forward into all later versions)

All versions of Cisco Secure PIX up to version 4.2(5) (including 2.7, 3.0, 3.1, 4.0, 4.1)


Upgrade to 4.4(5) available now.

All 4.3.x and 4.4.x up to and including version 4.4(4)


4.4(5) available as of 30 May, 2000.

All 5.0.x up to and including version 5.0(1)


Upgrade to 5.1(2) available now.

Version 5.1(1) - not affected by CSCdp86352


5.1(2) available as of 9 June, 2000.

** Interim releases are subjected to less internal testing and verification than are regular releases, may have serious bugs, and should be installed with great care.

Schedules have been updated to include released versions that fix both vulnerabilities addressed by this interim security advisory.

If version 4.3 or 4.4 is utilized on a PIX 'Classic' (excludes PIX10000, PIX-510, PIX-520, and PIX-515)


If version 5.0 is utilized on a PIX 'Classic', PIX10000, or PIX-510 (excludes PIX-520 and PIX-515)

A 128MB upgrade for the PIX Firewall is necessary. As with any new software installation, customers planning to upgrade should carefully read the release notes and other relevant documentation before beginning any upgrade. Also, it is important to be certain that the new version of Cisco Secure PIX Firewall software is supported by your hardware, and especially that enough memory is available.


The behaviors described in this document are a result of the default command fixup protocol ftp [portnum]. To disable this functionality, enter the command no fixup protocol ftp. This will disable support of the fixup of the FTP protocol in the PIX, and will eliminate the vulnerabilities. The command fixup protocol ftp 21 is the default setting of this feature, and is enabled by default on the Cisco Secure PIX Firewall.

This workaround will force your clients to use FTP in passive mode, and inbound FTP service will not be supported. Outbound standard FTP will not work without fixup protocol ftp 21, however, passive FTP will function correctly with no fixup protocol ftp configured.

Obtaining Fixed Software

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 Downloads at

Do not contact either "" or "" for software upgrades.

Customers with Service Contracts

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 Using Third-Party Support Organizations

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 Without Service Contracts

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)
  • e-mail:

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.

Refer to for additional TAC contact information, including special localized telephone numbers and instructions and e-mail addresses for use in various languages.

Exploitation and Public Announcements

This vulnerability was proposed on the BUGTRAQ list, and in follow-ups to the article, the Cisco Secure PIX Firewall was also identified as susceptible. As the vulnerabilities have been widely discussed, Cisco is posting this advisory prior to having a full fix. We will update this notice again, when we have a full fix available.

Cisco has had no reports of malicious exploitation of this vulnerability. However, versions of exploit scripts have been posted to various security related lists.

This vulnerability was reported to Cisco via several sources, shortly after the time of the original supposition.

Status of This Notice: Final


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 notice will be posted on Cisco's Worldwide Web site at In addition to Worldwide Web posting, the initial version of this notice is being sent to the following e-mail and Usenet news recipients:

  • (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 History

Revision 1.7


- Changed to FINAL status in the Status of This Notice section, and changes to the first fixed regular releases in the Software Version and Fixes section.

Revision 1.6


Changes to date in Status of This Notice INTERIM section, and date change in the Software Version and Fixes section.

Revision 1.5


Changes to Summary, Response for the second vulnerability (CSCdr09226), Software Versions and Fixes, and Status of This Notice sections.

Revision 1.4


Changes to dates for fixed software and Status of This Notice section.

Revision 1.3


Addition of 2nd vulnerability issues.

Revision 1.1


Link corrections, table head clarification.

Revision 1.0


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 This includes instructions for press inquiries regarding Cisco security notices.

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