Cisco Security Advisory-
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 https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20050511-url.
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This section provides details on 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.
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)
Products Confirmed Not Vulnerable
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.
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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 ( registered customers only) .
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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/en/US/docs/security/fwsm/fwsm23/command/reference/df.html#wp1142003
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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.
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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.
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To learn about Cisco security vulnerability disclosure policies and publications, see the Security Vulnerability Policy. This document also contains instructions for obtaining fixed software and receiving security vulnerability information from Cisco.
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Revision 1.0
2005-May-11
Initial public release.
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