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 device is vulnerable if it is running affected Cisco IOS version and has BGP configured, regardless of whether the device is configured with a 2 or 4 byte AS number under the router bgp configuration command

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. For the second vulnerability, configuring “bgp maxas-limit [value]” on the affected device does mitigate this vulnerability. Cisco is recommends using a conservative value of 100 to mitigate this vulnerability.

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

Cisco IOS Software BGP 4-Byte AS Number Vulnerabilities
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