Software Engineering Institute

Cisco's VLAN Trunking Protocol (VTP) provides the ability to manage the addition, deletion, and renaming of Virtual Local Area Networks (VLANs) across an entire network. VTP is supported on a number of different Cisco products in both the IOS and CatOS operating systems. According to Cisco Systems:

Once the VTP configuration revision exceeds 0x7FFFFFFF, the output for the VTP configuration revision in show vtp status (on an IOS device) or show vtp domain (on a CatOS device) will display as a negative number. Operation of the switch is not affected, however further changes to the VLAN database may not be properly propagated throughout the VTP domain.
This vulnerability may be triggered by sending a crafted VTP packet with a change in the configuration revision to an affected switch on a trunk enabled port. The crafted packet must include the correct domain name and VTP domain password to be effective.

Note that this vulnerability affects Switches and Ethernet Switch Modules for Cisco 1800/2600/2800/3600/3700/3800 Series Routers running Cisco IOS software that have VTP Operating Mode as either "server" or "client" and Switches running CatOS. See Cisco Security Response cisco-sr-20060913-vtp for more detailed version information. Switches configured with VTP operating mode as "transparent" are not affected by this vulnerability

This vulnerability may allow a remote, unauthenticated attacker to prevent changes to the VLAN database from being properly propagated throughout the VTP domain.

We are unaware of a complete solution to this problem. In the meantime, the following workarounds may help to mitigate this vulnerability.

Apply a VTP domain password

See Cisco white paper SAFE Layer 2 Security In-Depth — Version 2 for information about setting a VTP domain password to prevent spoofed VTP summary advertisement messages from advertising an incorrect VLAN name.

Exploitation Recovery

Cisco provides methods of recovery in Cisco Security Response cisco-sr-20060913-vtp. These include changing VTP domain names on all switches and temporarily setting all VTP servers/clients to transparent mode.