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Recent versions of Cisco IOS Software support RFC4893 (“BGP Support for Four-octet AS Number Space”) and contain two remote denial of service (DoS) vulnerabilities when handling specific Border Gateway Protocol (BGP) updates.

These vulnerabilities affect only devices running Cisco IOS Software with support for four-octet AS number space (here after referred to as 4-byte AS number) and BGP routing configured.

The first vulnerability could cause an affected device to reload when processing a BGP update that contains autonomous system (AS) path segments made up of more than one thousand autonomous systems.

The second vulnerability could cause an affected device to reload when the affected device processes a malformed BGP update that has been crafted to trigger the issue.

Cisco has released free software updates to address these vulnerabilities.

No workarounds are available for the first vulnerability.

A workaround is available for the second vulnerability.

This advisory is posted at the following link: http://www.cisco.com/warp/public/707/cisco-sa-20090729-bgp.shtml.

Affected Products

These vulnerabilities affect only devices running Cisco IOS and Cisco IOS XE Software (here after both referred to as simply Cisco IOS) with support for RFC4893 and that have been configured for BGP routing.

The software table in the section “Software Versions and Fixes” of this advisory indicates all affected Cisco IOS Software versions that have support for RFC4893 and are affected by this vulnerability.

A Cisco IOS software version that has support for RFC4893 will allow configuration of AS numbers using 4 Bytes. The following example identifies a Cisco device that has 4 byte AS number support:

Router#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)#router bgp ?
  <1-65535>    Autonomous system number
  <1.0-XX.YY>  4 Octets Autonomous system number

Or:

Router#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)#router bgp ?
  <1-4294967295>  Autonomous system number
  <1.0-XX.YY>     Autonomous system number

The following example identifies a Cisco device that has 2 byte AS number support:

Router#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)#router bgp ?
  <1-65535>  Autonomous system number

A router that is running the BGP process will contain a line in the configuration that defines the autonomous system number (AS number), which can be seen by issuing the command line interface (CLI) command “show running-config”.

The canonical textual representation of four byte AS Numbers is standardized by the IETF through RFC5396 (Textual Representation of Autonomous System (AS) Numbers). Two major ways for textual representation have been defined as ASDOT and ASPLAIN. Cisco IOS routers support both textual representations of AS numbers. For further information about textual representation of four byte AS numbers in Cisco IOS Software consult the document “Explaining 4-Byte Autonomous System (AS) ASPLAIN and ASDOT Notation for Cisco IOS” at the following link: http://www.cisco.com/en/US/prod/collateral/iosswrel/ps6537/ps6554/ps6599/white_paper_c11_516829.html

Cisco IOS Software with support for RFC4893 is affected by both vulnerabilities if BGP routing is configured using either ASPLAIN or ASDOT notation.

The following example identifies a Cisco device that is configured for BGP using ASPLAIN notation:

router bgp 65536

The following example identifies a Cisco device that is configured for BGP using ASDOT notation:

router bgp 1.0

To determine the Cisco IOS Software release that is running on a Cisco product, administrators can log in to the device and issue the show version command to display the system banner. The system banner confirms that the device is running Cisco IOS Software by displaying text similar to “Cisco Internetwork Operating System Software” or “Cisco IOS Software.” The image name displays in parentheses, followed by “Version” and the Cisco IOS Software release name. Other Cisco devices do not have the show version command or may provide different output.

The following example identifies a Cisco product that is running Cisco IOS Software Release 12.3(26) with an installed image name of C2500-IS-L:

Router#show version
Cisco Internetwork Operating System Software
IOS (tm) 2500 Software (C2500-IS-L), Version 12.3(26), RELEASE SOFTWARE (fc2)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2008 by cisco Systems, Inc.
Compiled Mon 17-Mar-08 14:39 by dchih

The following example identifies a Cisco product that is running Cisco IOS Software Release 12.4(20)T with an installed image name of C1841-ADVENTERPRISEK9-M:

Router#show version
Cisco IOS Software, 1841 Software (C1841-ADVENTERPRISEK9-M), Version 12.4(20)T, RELEASE SOFTWARE (fc3)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2008 by Cisco Systems, Inc.
Compiled Thu 10-Jul-08 20:25 by prod_rel_team

The following Cisco products are confirmed not vulnerable:

• Cisco IOS Software not explicitly mentioned in this Advisory
• Cisco IOS XR Software
• Cisco IOS NX-OS

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

ObtainingFixed Software

Cisco has released free software updates that address these vulnerabilities. Prior to deploying software, customers should consult their maintenance provider or check the software for feature set compatibility and known issues specific to their environment.

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/en/US/docs/general/warranty/English/EU1KEN_.html, or as otherwise set forth at Cisco.com Downloads at http://www.cisco.com/public/sw-center/sw-usingswc.shtml.

Do not contact psirt@cisco.com or security-alert@cisco.com for software upgrades.