Summary

• Errors in certain Cisco IOS software versions for certain routers can cause IP datagrams to be output to network interfaces even though access lists have been applied to filter those datagrams. This applies to routers from the Cisco 7xxx family only, and only when those routers have been configured for distributed fast switching (DFS).

  There are two independent vulnerabilities, which have been given Cisco bug IDs CSCdk35564 and CSCdk43862. Each vulnerability affects only a specialized subset of DFS configurations. Affected configurations are not believed to be extremely common, but neither are they extremely rare. More details of affected configurations are in the "Who is Affected" section of this document.

  These vulnerabilities may permit users to send packets to parts of the customer's network for which they are not authorized. This may permit unauthorized access or other attacks on customer computer systems or data. Cisco does not know of any incidents in which these vulnerabilities have actually been exploited by attackers.

  Neither vulnerability affects any Cisco product other than routers in the 70xx or 75xx series. Of 70xx routers, only routers with the optional route-switch processor (RSP) card are affected. Additional configuration conditions apply.

  This advisory is posted at https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-19981105-ios-dfs-acl.

Affected Products

• This section provides details on affected products.

  Vulnerable Products

  These vulnerabilities apply only to the Cisco 7xxx router family. The Cisco 7xxx family are large, rack-mounted backbone routers used primarily by Internet service providers and in large enterprise networks.

  Cisco 75xx routers are affected by both vulnerabilities. Cisco 70xx routers are affected only if they have RSP cards installed. Cisco 72xx routers are not affected by either vunerability; an earlier version of this notice erroneously mentioned 72xx routers, but affected hardware configurations are not possible on the 72xx platform, and DFS cannot be configured on 72xx routers.

  CSCdk35564 affected configurations

  CSCdk35564 is a defect in the 11.1CC and 11.1CT releases. Routers running Cisco IOS software versions other than 11.1CC and 11.1CT are not affected by CSCdk35564. Cisco 75xx routers are affected; Cisco 70xx routers are not supported with the affected hardware/software combinations.

  Note: If you are a registered CCO user and you have logged in, you can view bug information.

  • View CSCdk35564
    

  To be affected by CSCdk35564, your router must be configured to switch traffic from an interface with DFS enabled to an interface without DFS enabled. This most commonly happens when routers contain both versatile interface processor (VIP) interface cards and non-VIP interface cards. Since DFS is supported only on VIP interfaces, traffic from a VIP to a non-VIP interface may be going from DFS to non-DFS.

  CSCdk43862 affected configurations

  CSCdk43862 affects 11.1, 11.2, and 11.3 versions of Cisco IOS software on the Cisco 70xx and 75xx series; see the table later in this document for details.

  Note: If you are a registered CCO user and you have logged in, you can view bug information.

  • View CSCdk43862
    

  To be vulnerable, your router must be configured to switch traffic from an input interface with DFS enabled to a logical subinterface of a physical output interface. The output interface may or may not have DFS enabled; the important question for the output interface is whether or not subinterfaces are in use, and whether or not output traffic to subinterfaces is being filtered.

  Products Confirmed Not Vulnerable

  Although each of the vulnerabilities is different and manifests itself under different conditions, both involve DFS. DFS is not enabled by default in any Cisco product, and must be manually configured. If the command ip route-cache distributed does not appear in your router configuration file, then you are not affected by either vulnerability.

  Specifically, process switching (no ip route-cache), ordinary fast switching (ip route-cache), optimum switching (ip route-cache optimum), and CEF or dCEF switching (ip route-cache cef, ip cef distributed switch) are not affected. Flow switching is considered a form of fast switching, and is affected only in distributed mode. Interactions between flow switching and access lists reduce, but do not eliminate, the impact of both vulnerabilities when flow switching is enabled along with DFS.

  If DFS is enabled on all of the interfaces in your router, then you are not affected by CSCdk35564. If DFS is not enabled on any interface in your router, then you are not affected. If you do not use the ip access-group command to filter outgoing traffic on any non-DFS interfaces, then you are not affected.

  Subinterfaces are pseudo-interfaces associated with subsets of the traffic on physical interfaces. For instance, a physical Frame Relay interface might have a subinterface associated with each Frame Relay PVC. Subinterfaces do not exist by default; they are created as part of user configuration. Subinterface numbers always contain periods, as in "Serial 0/1.1". If your configuration file does not contain any such "dotted" interface numbers, then you are not vulnerable.

  If you do not use the ip access-group command to apply output access-list filtering to subinterfaces, then you are not vulnerable.

  CSCdk43862 causes the access list applied to one subinterface on a physical interface to be incorrectly used for traffic destined for a different subinterface. If you use the same access list to filter outbound traffic on all subinterfaces of any given physical interface, then you are not vulnerable.

  No other Cisco products are currently known to be affected by these vulnerabilities.

Details

• CSCdk43862 has a duplicate report, CSCdk43696. The bug ID CSCdk43862 should be used to refer to this defect.

Workarounds

• These vulnerabilities can be worked around by disabling DFS on network interfaces (with no ip route-cache distributed). Be aware that the purpose of DFS is to transfer computational load from the router's primary CPU to the CPUs on the VIP cards, and that disabling DFS may therefore cause overload of the primary CPU. Evaluate your traffic load and CPU usage before using this workaround.

  If all interfaces in the router are DFS-capable, but DFS has for some reason been enabled only on some of the interfaces, it may be possible to work around CSCdk35564 by enabling DFS on all interfaces. This will not affect CSCdk43862.

  CSCdk43862 can sometimes be worked around by reconfiguring to use the same output access list on all the subinterfaces of a physical interface.

  Another possible workaround is to redesign the access lists structure on the router to avoid the need for output access lists on affected interfaces.

Fixed Software

• The following table summarizes the affected Cisco IOS software versions for both CSCdk35564 and CSCdk43862, and indicates which versions have been fixed. To use the table, look up the software release you're currently running (available from the show version command on your router) in the first column of the table. The other columns of the table tell you which Cisco IOS software versions from your major release have been fixed, and which versions Cisco recommends you install.

  The table lists both interim versions and regular released versions. Interim versions receive far less testing, and are generally of less certain quality, than regular released versions. Cisco recommends installing regular released software whenever possible. Interim versions are listed for reference, and for the convenience of customers who must upgrade before appropriate regular released versions are available.

  As always, a fix applied to one regular released version in a major release means that all later versions of that major release are also fixed. For instance, 11.2(17) is fixed, so 11.2(18) and later are also fixed.

  The table is designed to cover all supported software on all affected Cisco routers. If you are running distributed fast switching on a 75xx router, or a 70xx router with an RSP processor, and you are using an 11.1, 11.2, or 11.3 release not listed in the table, please contact the Cisco TAC for assistance.

  Cisco IOS Major Release (only 7xxx releases are listed)	Initial CSCdk35564 Fixes		Initial CSCdk43862 Fixes		Upgrade Path for 7xxx DFS Users
  	Interim (minimal testing; urgent upgrades only)	Regular (dates are subject to change)	Interim (minimal testing; urgent updates only)	Regular (dates are subject to change)
  11.0 and earlier, all variants	Unaffected	Unaffected	Unaffected	Unaffected	Unaffected
  11.1	Unaffected	Unaffected	-	-	Go to 11.1CA
  11.1CA (core ED)	Unaffected	Unaffected	11.1(22)CA	11.1(22)CA	11.1(22)CA or later
  11.1CC (CEF ED)	11.1(21.2)CC	11.1(21)CC1 11.1(22)CC	11.1(21.2)CC	11.1(21)CC1 11.1(22)CC	11.1(21)CC1, 11.1(22)CC or later
  11.1CT (tag switch ED)	11.1(21.2)CT	11.1(22)CT	11.1(21.2)CT	11.1(22)CT	11.1(22)CT or later
  11.2	Unaffected	Unaffected	11.2(16.1)	11.2(17), planned Jan-1999	11.2(17) or later; 11.2(16.1) or 11.3 if 11.2(17) schedule unacceptable
  11.2F	Unaffected	Unaffected	-	-	Go to 11.3
  11.2P (platform ED)	Unaffected	Unaffected	11.2(16.1)P	11.2(17)P, planned Jan-1999	11.2(17)P or later; 11.2(16.1)P or 11.3 if 11.2(17)P schedule unacceptable.
  11.2BC (CIP ED)	Unaffected	Unaffected	11.2(16.1)BC	11.2(17)BC, planned Jan-1999	11.2(17)BC or later; 11.2(16.1)BC if 11.2(17)BC schedule unacceptable.
  11.3	Unaffected	Unaffected	11.3(6.2)	11.3(7), planned Nov-1998	11.3(7) or later
  11.3T	Unaffected	Unaffected	11.3(6.2)T	11.3(7)T, planned Nov-1998	11.3(7)T or later
  11.3NA (voice ED)	Unaffected	Unaffected	11.3(6.2)NA	11.3(7)NA, Planned Dec-1998	11.3(7)NA or later; 11.3(6.2)NA if 11.3(7)NA schedule unacceptable.
  11.3(2)XA	Unaffected	Unaffected	-	-	11.3(7) or later
  12.0(1) and later, all variants	Unaffected	Unaffected	Unaffected	Unaffected	Unaffected

  Because of restricted port adapter support, Cisco does not believe that many, if any, customers are using DFS with 11.1 mainline software. 11.1CA is recommended for both functionality and stability reasons.

  The 11.1(21)CC1 release is a special release of 11.1CC; the 11.1CC release sequence runs from 11.1(21)CC through 11.1(21)CC1, then to 11.1(22)CC.

  11.3(2)XA was a special one-time release based on 11.3(2). The functionality of 11.3(2)XA was carried into the 11.3(3) release.

Exploitation and Public Announcements

• Cisco knows of no public announcements or discussion of these vulnerabilities prior to the date of this notice.

  CSCdk35564 was found by a Cisco customer during installed-system testing. CSCdk43862 was found by Cisco during internal testing.

  Because of the nature of these vulnerabilities, attackers would rarely be expected to exploit them directly. In most cases, attackers would simply find themselves with access to network resources to which administrators thought they had denied access. Cisco has had no actual reports of malicious attacks succeeding because of this vulnerability, nor of anyone deliberately trying to create "vulnerable" conditions.

URL

• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-19981105-ios-dfs-acl

Revision History

• Revision 1.3

  1998-November-05

  Initial public release.

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