Cisco IOS Software for Cisco Catalyst 6500 Series Switches and 7600 Series Routers ACL Bypass Vulnerability

Summary

• A vulnerability in the ternary content addressable memory (TCAM) share access control list (ACL) functionality of Cisco IOS Software running on Supervisor Engine 720 and Supervisor Engine 32 Modules for Cisco Catalyst 6500 Series Switches and Cisco 7600 Series Routers could allow an unauthenticated, remote attacker to bypass access control entries (ACEs) in a port access control list (PACL).
  
  The vulnerability is due to the improper implementation of PACL logic for ACEs that include a greater than operator, a less than operator, a tcp flag, the established keyword, or the range keyword. An attacker could exploit this vulnerability by sending packets that meet one or more filter criteria through an affected device. A successful exploit could allow the attacker to bypass the filters defined in the PACL for a targeted system.
  
  Cisco has released software updates that address this vulnerability. There are workarounds that address this vulnerability.
  
  This advisory is available at the following link:
  http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20161005-catalyst

Affected Products

• Vulnerable Products

  This vulnerability affects Cisco Catalyst 6500 Series Switches and Cisco 7600 Series Routers that have a Supervisor Engine 720 Module or Supervisor Engine 32 Module running a vulnerable release of Cisco IOS Software, if all the following conditions exist for the device:
  

  • The mls acl tcam share-acl command is configured.
  • A PACL includes an ACE that contains a greater than operator, a less than operator, a tcp flag, the established keyword, or the range keyword.
  • The PACL is applied to more than eight interfaces.

  If all these conditions are met, an affected device will not apply filters that are in the same PACL and also contain a greater than operator, a less than operator, a tcp flag, the established keyword, or the range keyword.
  
  For example, if the mls acl tcam share-acl command is configured for a device, a PACL is applied to nine of the device's interfaces, and the PACL contains the following ACE, the device will allow all TCP traffic without honoring the established keyword:
  

  permit tcp any any established

  Note that other ACEs in the PACL will work if they do not include an affected operator or keyword. The PACL will not be bypassed completely. Only additional use of the affected operators or keywords will not be applied.

  Products Confirmed Not Vulnerable

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

Workarounds

• Depending on the configuration, administrators can mitigate this vulnerability by changing the device configuration to ensure that it does not meet all the conditions specified for this vulnerability. For example, an administrator may apply the PACL to fewer than eight device interfaces.

Fixed Software

• Cisco provides information about fixed software in Cisco bugs, which are accessible through the Cisco Bug Search Tool.
  
  When considering software upgrades, customers are advised to regularly consult the advisories for Cisco products, which are available from the Cisco Security Advisories and Alerts page, to determine exposure and a complete upgrade solution.
  
  In all cases, customers should ensure that the devices to be upgraded contain sufficient memory and confirm that current hardware and software configurations will continue to be supported properly by the new release. If the information is not clear, customers are advised to contact the Cisco Technical Assistance Center (TAC) or their contracted maintenance providers.

Exploitation and Public Announcements

• The Cisco Product Security Incident Response Team (PSIRT) is not aware of any public announcements or malicious use of the vulnerability that is described in this advisory.

Source

• This vulnerability was found during the resolution of support cases.

URL

• http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20161005-catalyst

Revision History

• Version	Description	Section	Status	Date
  1.0	Initial public release.	-	Final	2016-October-05

  Show Less