November 2008

Revision 1.0
For Public Release 2008 November 5 1600 UTC (GMT)

Cisco Response
==============

This is the Cisco response to research done by 'showrun.lee' pertaining to a crafted VTP packet denial of service vulnerability. We would like to thank 'showrun.lee' for reporting this vulnerability to us. We greatly appreciate the opportunity to work with researchers on security vulnerabilities, and welcome the opportunity to review and assist in security vulnerability reports against Cisco products.

This vulnerability is being addressed by Cisco Bug IDs:

* CSCsv05934 - Crafted VTP packet crashes device running IOS
* CSCsv11741 - Crafted VTP packet crashes switch running CatOS

Additional Information
======================

The VLAN Trunking Protocol (VTP) is a Layer 2 messaging protocol that maintains VLAN configuration consistency by managing the addition, deletion, and renaming of VLANs on a network-wide basis. When a network administrator makes any configuration changes to the VLAN setup on one device working as a VTP server, said configuration is then distributed via the VTP protocol through all switches in the domain. This reduces the need for replicating this VLAN configuration manually across switches. VTP is a Cisco-proprietary protocol that is available on most of the Cisco Catalyst series products using both Cisco IOS and Cisco CatOS system software.

Cisco's VTP protocol implementation in some versions of Cisco IOS and CatOS may be vulnerable to a DoS attack via a specially crafted VTP packet sent from the local network segment when operating in either server or client VTP mode. When the device receives the specially crafted VTP packet, the switch may crash (and reload/hang). The crafted packet must be received on a switch interface configured to operate as a trunk port.

Devices without a VTP domain name configured are still vulnerable.

For devices not requiring the use of VTP, administrators should set the VTP mode as "transparent" via the CLI command "vtp mode transparent". Devices configured with a VTP domain password are still vulnerable to exploitation, without the malicious attacker knowing the VTP domain password. Switch configuration best practices limit exposure to exploitation, by disabling the Dynamic Trunking Protocol

(DTP) on all switch ports that are not required to operate as trunk ports. See "Best Practices for Catalyst 6500/6000 Series and Catalyst 4500/4000 Series Switches Running Cisco IOS Software" and "Best Practices for Catalyst 4500/4000, 5500/5000, and 6500/6000 Series Switches Running CatOS Configuration and Management" for further information.

Products affected by this vulnerability:

* Devices running affected versions of Cisco IOS or CatOS that have

VTP Operating Mode as either "server" or "client".

* Devices running affected versions of Cisco IOS with Ethernet

Switch Modules for Cisco 1800/2600/2800/3600/3700/3800 Series

Routers that have VTP Operating Mode as either "server" or

"client".

Products not affected by these vulnerabilities:

* Devices configured with VTP operating mode as "transparent".

* Devices configured with VTP version 3 (CatOS only)

To determine the current VTP operating mode on a Cisco device, log into the device and issue the show vtp status command on an IOS device or the show vtp domain command on a CatOS device. Switches that show either "server" or "client" as the VTP operating mode are affected by this vulnerability.

The following example shows a device running Cisco IOS and operating in VTP "server" mode:

ios_switch#show vtp status

VTP Version : 2

Configuration Revision : 0

Maximum VLANs supported locally : 1005

Number of existing VLANs : 5

VTP Operating Mode : Server

VTP Domain Name : test

VTP Pruning Mode : Disabled

VTP V2 Mode : Enabled

VTP Traps Generation : Disabled

MD5 digest : <removed>

Configuration last modified by 0.0.0.0 at 3-1-93 04:02:09

ios_switch#

The following example shows a device running Cisco CatOS and operating in VTP "server" mode:

catos_switch> (enable) show vtp domain

Version : running VTP1 (VTP3 capable)

Domain Name : test Password : not configured

Notifications: disabled Updater ID: 0.0.0.0

Feature Mode Revision

-------------- -------------- -----------

VLAN Server 2

Pruning : disabled

VLANs prune eligible: 2-1000

catos_switch> (enable)

Status of this Notice: FINAL
============================

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Revision History
================

+---------------------------------------+

| Revision | | Initial |

| 1.0 | 2008-November-5 | public |

| | | release |

+---------------------------------------+

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=========================

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