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Cisco patches DoS vuln pair in IOS

No exploit spotted

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Cisco has issued a pair of updates today patching two remote denial of service vulnerabilities affecting certain devices running its Internetwork Operating System (IOS).

The vulnerabilities are limited to kit running Cisco IOS software with support for four-octet Autonomous Systems number space (AKA: 4-byte AS number) and Border Gateway Protocal (BGP) routing configured. Attackers could use the exploits for repeated reloading of the device causing an extended denial of service blockage.

The security holes were confirmed in Cisco IOS and Cisco IOS XE with support for RFC4893.

The first vulnerability may cause an affected device to crash with memory corruption, but requires three conditions:

  • Cisco IOS Software device is a 4-byte AS number BGP speaker
  • BGP peering neighbor is a 2-byte AS number BGP speaker
  • BGP peering neighbor is capable of sending a BGP update with a series of greater than one thousand AS numbers

If an affected 4-byte AS number BGP speaker receives a BGP update from a 2-byte AS number BGP speaker containing AS path segments of more than one thousand autonomous systems, the device may crash with memory corruption and spit out the error "%%Software-forced reload."

Cisco says there is no workarounds on the affected device, but neighbors could be configured to discard routes that have more than one thousand AS numbers in the AS-path segment.

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

This security hole requires three conditions as well:

  • Cisco IOS Software device is a 4-byte AS number BGP speaker
  • BGP peering neighbor is a 2-byte AS number BGP speaker
  • BGP peering neighbor is capable of sending a non-RFC complaint malicious BGP update message.

Cisco says configuring "bgp maxas-limited [value]" on the affected device will mitigate the vulnerability. The company suggests using a lowball value of 100 to best avoid the problem.

Updates have been issued to stop the vulnerabilities, and of course users are advised to update to the fixed version. Cisco said its not aware of anyone maliciously exploiting the security flaws, although some customers have accidentally triggered the first vulnerability within their infrastructures.

Additional details are available on the security notice here. ®

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