[c-nsp] Cisco Security Advisory: FWSM URL Filtering Solution TCP ACL Bypass Vulnerability

Wed May 11 12:25:50 EDT 2005

The Cisco Firewall Services Module (FWSM) is a high-speed, integrated
firewall module for Catalyst 6500 series switches and Cisco 7600 series
routers. A vulnerability exists in the Cisco Firewall Services Module
when URL, FTP, or HTTPS filtering is enabled in which inbound TCP
packets can bypass access-list entries intended to explicitly filter
them.

Cisco has made free software available to address this vulnerability.
There are workarounds available to mitigate the effects of the
vulnerability.

This advisory will be posted at 
http://www.cisco.com/warp/public/707/cisco-sa-20050511-url.shtml.

Affected Products
=================

Vulnerable Products
+------------------

Only Cisco Catalyst 6500 Series Switches and Cisco 7600 Series Internet
Routers with a Firewall Services Module (FWSM) installed running version
2.3.1 or prior are affected when configured to allow exceptions for
content filtering.

An example configuration of a filter exception which allows internal
hosts to reach another network might be

    FWSM#show filter 
    filter https except 0.0.0.0 0.0.0.0 10.1.3.0 255.255.255.0
    filter ftp except 0.0.0.0 0.0.0.0 10.1.3.0 255.255.255.0
    filter url except 0.0.0.0 0.0.0.0 10.1.3.0 255.255.255.0
    filter url http 0.0.0.0 0.0.0.0 0.0.0.0 0.0.0.0
    filter ftp 21 0.0.0.0 0.0.0.0 0.0.0.0 0.0.0.0
    filter https 443 0.0.0.0 0.0.0.0 0.0.0.0 0.0.0.0 

If the resulting output includes a "filter" command with an argument of 
"except", you may be susceptible to the vulnerability outlined in this 
advisory.

Products with similar functionality, such as the Cisco PIX Security
Appliance and the Cisco Adaptive Security Appliance (ASA) 5500 Series,
are not affected.

No other Cisco products are known to be affected by this vulnerability.

To determine if you are running a vulnerable version of FWSM software,
issue the "show module" command in IOS or CatOS to identify what modules
and sub-modules are installed in the system.

The example below shows a system with a Firewall Service Module
(WS-SVC-FWM-1) installed in slot 4.

6506-B#show module
Mod Ports Card Type                              Model              Serial No.
 -- ----- -------------------------------------- ------------------ -----------
  1   48  SFM-capable 48 port 10/100/1000mb RJ45 WS-X6548-GE-TX     SAxxxxxxxxx
  4    6  Firewall Module                        WS-SVC-FWM-1       SAxxxxxxxxx
  5    2  Supervisor Engine 720 (Active)         WS-SUP720-BASE     SAxxxxxxxxx
  6    2  Supervisor Engine 720 (Hot)            WS-SUP720-BASE     SAxxxxxxxxx

After locating the correct slot, issue the "show module <slot number>" 
command to identify the version of software running:

6506-B#sho module 4
Mod Ports Card Type                              Model              Serial No.
 -- ----- -------------------------------------- ------------------ -----------
  4    6  Firewall Module                        WS-SVC-FWM-1       SAxxxxxxxxx

Mod MAC addresses                       Hw    Fw           Sw           Status
 -- ---------------------------------- ------ ------------ ------------ -------
  4  0003.e4xx.xxxx to 0003.e4xx.xxxx   3.0   7.2(1)       2.3(1)       Ok

In this example, the FWSM is running version 2.3(1) as indicated by the
column under "Sw" above.

Alternatively, the information may also be gained directly from the FWSM
via the "show version" command:

    FWSM#show version

    FWSM Firewall Version 2.3(1)

For customers managing their FWSM via the PIX Device Manager (PDM),
simply log into the application, and the version may be found either in
the table in the login window or in the upper left hand corner of the
PDM window indicated by a label similar to:

    FWSM Version: 2.3(1)

Details
=======

The Cisco Firewall Services Module is a high-speed, integrated firewall
module for Catalyst 6500 series switches and Cisco 7600 series routers.
A vulnerability exists in the Cisco Firewall Services Module when
configured for exceptions in content filtering in which inbound TCP
packets can bypass access-list entries intended to explicitly filter
them.

Although access lists (ACL) can be used to prevent outbound access to
specific websites or File Transfer Protocol (FTP) servers via IP address
and/or IP address/port pairs, configuring and managing web usage this
way is often not practical because of the size and dynamic nature of the
Internet. The FWSM may be used in conjunction with a Websense Enterprise
or N2H2 server to better manage filtering of Hypertext Transfer Protocol
(HTTP), HTTP over Secure Sockets Layer (HTTPS), and FTP connections to
and from the Internet.

If URL, HTTPS, or FTP filtering exceptions has been configured via the
command

    filter < url | https | ftp > except

in order to exclude certain addresses from being filtered, then a
vulnerability exists where any TCP traffic that matches this exception
filter is also exempt from the inbound ACL inspection on any interface.
Since filtering is enabled for outbound connections from the inside
interface, a configuration may be common where any source address
coming from an internal network is able to reach servers placed on a
DMZ via a source address and mask of all zeros in order to simplify
configurations.

An example configuration of a filter exception which allows internal
hosts to reach another network might be:

    FWSM# show filter
    filter https except 0.0.0.0 0.0.0.0 10.1.3.0 255.255.255.0
    filter ftp except 0.0.0.0 0.0.0.0 10.1.3.0 255.255.255.0
    filter url except 0.0.0.0 0.0.0.0 10.1.3.0 255.255.255.0
    filter url http 0.0.0.0 0.0.0.0 0.0.0.0 0.0.0.0
    filter ftp 21 0.0.0.0 0.0.0.0 0.0.0.0 0.0.0.0
    filter https 443 0.0.0.0 0.0.0.0 0.0.0.0 0.0.0.0 

In this example, all TCP traffic from any interface destined to hosts
on the 10.1.3.0/24 network will bypass all FWSM interface input ACLs
including those that explicitly deny them.

This vulnerability is documented as Cisco bug ID CSCeh21590.

Impact
======

Successful exploitation of the vulnerability may result in TCP traffic
which would normally not be allowed past the FWSM on any interfaces to
be able to reach hosts which should be protected by the FWSM.

Software Versions and Fixes
===========================

Cisco FWSM users can upgrade to version 2.3(2) or later software to
resolve this vulnerability. If older version of software is required,
interim images are available from the TAC which also address this issue.
Installations running 2.2 may upgrade to 2.2(1)18 or later and 1.1 users
may upgrade to 1.1(4)4 or later.

When considering software upgrades, please also consult
http://www.cisco.com/en/US/products/products_security_advisories_listing.html 
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") for assistance.

Obtaining Fixed Software
========================

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 http://www.cisco.com.

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 assistance with the upgrade, which
should be free of charge.

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 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: tac at cisco.com

Please 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.

Please do not contact either "psirt at cisco.com" or
"security-alert at cisco.com" for software upgrades.

See 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.

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.

Workarounds
===========

The effectiveness of any workaround 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 is the most
appropriate for use in the intended network before it is deployed.

The workaround for this issue is to remove any filter exception rules 
which exist from the configuration. Filter exception rules would start with

    filter < url | ftp | https > except

and may be removed using the "no" form of the command.

For more information on configuring the content filtering commands
please consult:

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat6000/mod_icn/fwsm/fwsm_2_3/fwsm_ref/df.htm#wp1142003

Exploitation and Public Announcements
=====================================

The Cisco PSIRT has been made aware of an instance where a customer may
have been impacted through the vulnerability described in this advisory.

This vulnerability was reported to Cisco by a customer.

Status of This Notice: FINAL
============================

THIS ADVISORY IS PROVIDED ON AN "AS IS" BASIS AND DOES NOT IMPLY
ANY KIND OF GUARANTEE OR WARRANTY, INCLUDING THE WARRANTY OF
MERCHANTABILITY. YOUR USE OF THE INFORMATION ON THE ADVISORY OR
MATERIALS LINKED FROM THE ADVISORY IS AT YOUR OWN RISK. CISCO RESERVES
THE RIGHT TO CHANGE OR UPDATE THIS NOTICE AT ANY TIME.

A stand-alone copy or paraphrase of the text of this security advisory
that omits the distribution URL in the following section is an
uncontrolled copy, and may lack important information or contain factual
errors.

Distribution
============

This advisory will be posted on Cisco's worldwide website at 
http://www.cisco.com/warp/public/707/cisco-sa-20050511-url.shtml.

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.

  * cust-security-announce at cisco.com
  * first-teams at first.org (includes CERT/CC)
  * bugtraq at securityfocus.com
  * cisco at spot.colorado.edu
  * cisco-nsp at puck.nether.net
  * full-disclosure at lists.grok.org.uk
  * comp.dcom.sys.cisco at newsgate.cisco.com

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.

Revision History
================

+-------------------------------------------------------+
| Revision 1.0 | 2005-May-11 | 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 website at
http://www.cisco.com/en/US/products/products_security_vulnerability_policy.html.
This includes instructions for press inquiries regarding
Cisco security notices. All Cisco security advisories are available at
http://www.cisco.com/go/psirt/.

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All contents are Copyright 1992-2005 Cisco Systems, Inc. 
All rights reserved.
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