Data Leak with Cisco Express Forwarding Enabled

Summary

• Excluding Cisco 12000 Series Internet Routers, all Cisco devices running Cisco IOS® software that have Cisco Express Forwarding (CEF) enabled can leak information from previous packets that have been handled by the device. This can happen if the packet length described in the IP header is bigger than the physical packet size. Packets like these will be expanded to fit the IP length and, during that expansion, an information leak may occur. Please note that an attacker can only collect parts of some packets but not the whole session.

  No other Cisco product is vulnerable. Devices that have fast switching enabled are not affected by this vulnerability. Cisco 12000 Series Internet Routers are not affected by this vulnerability.

  The workaround for this vulnerability is to disable CEF.

  This advisory is available at the http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20020227-ios-cef.

Affected Products

• Vulnerable Products

  All Cisco IOS releases that are supporting CEF are vulnerable. In order to trigger this vulnerability CEF must be enabled on the device. The vulnerable Cisco IOS releases are (this is not an exhaustive list):

  • 11.1CC
    
  • 12.0, 12.0S, 12.0T, 12.0ST
    
  • 12.1, 12.1E, 12.1T
    
  • 12.2, 12.2T
    

  Products Confirmed Not Vulnerable

  No other Cisco products are affected. Specifically, the following products are not vulnerable:

  • Cisco 12000 Series Internet Routers
    
  • Catalyst 6000 with both Supervisor Engine I and II
    

Details

• When a router receives a packet where MAC level packet length is shorter than is indicated by the IP level, the router will "extend" the packet to the size indicated by the IP level. This extension will be done by padding the packet with an arbitrary data. The issue here is that padding may contain data from a previous packet that has not been erased.

  Although it is possible to trigger this vulnerability on command, it is not possible to predict what information would be collected this way. It is not possible for an attacker to selectively capture desired packets (for example, packets with username and password combination).

  This vulnerability is specific to CEF. Fast switching is not affected by it.

  This vulnerability is documented as Cisco Bug ID CSCdu20643. For the Cisco IOS 11.1CC image, this vulnerability is described as Cisco Bug ID CSCdp58360.

Workarounds

• The workaround is to disable CEF on a router.

Fixed Software

• Each row of the table describes a release train and the platforms or products for which it is intended. If a given release train is vulnerable, then the earliest possible releases that contain the fix and the anticipated date of availability for each are listed in the "Rebuild", "Interim", and "Maintenance" columns. A device running any release in the given train that is earlier than the release in a specific column (less than the earliest fixed release) is known to be vulnerable, and it should be upgraded at least to the indicated release or a later version (greater than the earliest fixed release label).

  When selecting a release, keep in mind the following definitions:

  • Maintenance - Most heavily tested and highly recommended release of any label in a given row of the table.
    
  • Rebuild - Constructed from the previous maintenance or major release in the same train, it contains the fix for a specific defect. Although it receives less testing, it contains only the minimal changes necessary to effect the repair.
    
  • Interim - Built at regular intervals between maintenance releases and receives less testing. Interims should be selected only if there is no other suitable release that addresses the vulnerability, and interim images should be upgraded to the next available maintenance release as soon as possible. Interim releases are not available via manufacturing, and usually they are not available for customer download from CCO without prior arrangement with the Cisco Technical Assistance Center (TAC).
    

  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 TAC for assistance as shown in the following section.

  More information on Cisco IOS software release names and abbreviations is available at http://www.cisco.com/warp/public/620/1.html.

  Train	Description of Image or Platform	Availability of Fixed Releases*
  11.1-based Releases		Rebuild	Interim**	Maintenance
  11.1CC	ED release for 7000 series	11.1(36)CC3
  12.0-based Releases		Rebuild	Interim**	Maintenance
  12.0	GD release for all platforms		12.0(20.4)
  12.0S	ED release for all platforms	12.0(17)ST4	12.0(18.3)S	12.0(19)S
  12.0ST	ED release for all platforms		12.0(18.3)ST	12.0(19)ST
  12.0T	ED release for all platforms	To be decided
  12.0W5	ED release for all platforms		12.0(20.4)W5(24.7)
  12.1-based Releases		Rebuild	Interim**	Maintenance
  12.1	LD release for all platforms		12.1(9.2)	12.1(10)
  12/1E	ED release for all platforms	12.1(8.5)E2	12.1(9.5)E	12.1(8a)E
  12.1EC	ED release for all platforms	12.1(7.5)EC1	12.1(9.5)EC
  12.1T	ED release for all platforms	To be decided
  12.1XM	ED release for all platforms	12.1(5) XM6
  12.2-based Releases		Rebuild	Interim**	Maintenance
  12.2	LD release for all platforms		12.2(2.5)	12.2(3)
  12.2S	LD release for all platforms		12.2(3.3)S
  12.2T	ED release for all platforms		12.2(2.4)T	12.2(4)T

Exploitation and Public Announcements

• The Cisco PSIRT is not aware of any public announcements or malicious use of the vulnerabilities described in this advisory.

URL

• http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20020227-ios-cef

Revision History

• Revision 1.3	2002-April-11	Added Catalyst 6000 with both Supervisor Engine I and II to products not affected in Affected Products section
  Revision 1.2	2002-February-28	Removed reference to dCEF in Affected Products section
  Revision 1.1	2002-February-28	Added Cisco 12000 Series Internet Routers to products not affected
  Revision 1.0	2002-February-27	Initial public release.

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