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. Some versions of IOS and CatOS contain a buffer overflow vulnerability in their handling of certain VTP summary advertisements. According to Cisco Systems:

If a VTP summary advertisement is received with a Type-Length-Value (TLV) containing a VLAN name greater than 100 characters, the receiving switch will reset with an Unassigned Exception error. The packets must be received on a trunk enabled port, with a matching domain name and a matching VTP domain password (if configured).
This vulnerability may be triggered by sending a switch running certain versions of Cisco IOS software a VTP summary advertisement with a VLAN name greater than 100 bytes. VLAN names in VTP are limited to 255 bytes in length.

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". See Cisco Security Response cisco-sr-20060913-vtp for more detailed version information. Switches running CatOS and those configured with VTP operating mode as "transparent" are not affected by this vulnerability

This vulnerability may allow a remote, unauthenticated attacker to execute arbitrary code or cause a denial-of-service condition on an affected device.

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.