Advisory ID: cisco-sa-20000316-pix-ftp
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
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
This section provides details on affected 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.
Cisco Secure Integrated Software (formerly Cisco IOS® Software Firewall
Feature Set) is not affected by either vulnerability.
No other Cisco products are currently known to be affected by these
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
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
conduit permit tcp host 192.168.0.1 eq 21 any
conduit permit tcp 192.168.0.1 255.255.255.0 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
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 http://www.cert.org/advisories/CA-2000-02.html.
The recommendation in the workarounds section of this document will provide
protection against this vulnerability.
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
Enforce that data channel is initiated from the expected side in an
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).
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).
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.
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
Interim Release**(fix will carry forward into all later
Available Now through the TAC
First fixed regular release (fix will carry forward into all
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(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
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 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.
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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 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:
email@example.com (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.
- Changed to FINAL status in the Status of This Notice section,
and changes to the first fixed regular releases in the Software Version and
Changes to date in Status of This Notice INTERIM section, and
date change in the Software Version and Fixes section.
Changes to Summary, Response for the second vulnerability
(CSCdr09226), Software Versions and Fixes, and Status of This Notice
Changes to dates for fixed software and Status of This Notice
Addition of 2nd vulnerability issues.
Link corrections, table head clarification.
Initial public release.