S21sec-034-en: Cisco VTP DoS vulnerability

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ID: S21SEC-034-en
Title: Cisco VTP Denial Of Service
Date: 26/01/2007
Status: Vendor contacted, bug fixed
Severity: Medium - DoS - remote from the local subnet
Scope: Cisco Catalyst Switch denial of service
Platforms: IOS
Author: Alfredo Andres Omella, David Barroso Berrueta
Location: http://www.s21sec.com/es/avisos/s21sec-034-en.txt
Release: Public
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S 2 1 S E C

http://www.s21sec.com

Cisco VTP Denial Of Service


About VTP
---------

VTP (VLAN Trunking Protocol) is a Cisco proprietary protocol used for VLAN centralized management.
For instance, when you configure a VLAN in a switch, the VLAN information (the VLAN name and its identifier)
will be configured automatically in all the switches that belong to the same VTP domain.


Description of vulnerability
----------------------------

VTP uses Subset-Advert messages to advertise the existing VLANs within a VTP domain,
sending a malformed crafted packet it is possible to force a switch "crash & reload". In order to trigger the vulnerability,
you need to previously set up the trunking (manually or using Yersinia DTP attack).


Affected Versions and platforms
-------------------------------

This vulnerability has been tested against Cisco Catalyst 2950T switches with IOS 12.1(22)EA3.
Other versions are probably vulnerable.


Solution
--------

According to Cisco PSIRT, it is already fixed. We don't know all the details because
Cisco tagged (back in 2005) the issue as an "internal bug", not as a security vulnerability.
Upgrade your IOS to the latest release.


Additional information
----------------------

This vulnerability has been found and researched by:

David Barroso Berrueta dbarroso@xxxxxxxxxx
Alfredo Andres Omella aandres@xxxxxxxxxx

It was found on January 2005 and shown in a real demo at BlackHat Europe Briefings 2005 (March 2005) (Yersinia, a framework for layer 2 attacks).
Some months later, FX from Phenoelit found other VTP vulnerabilities:
http://www.securityfocus.com/archive/1/445896/30/0/threaded
Cisco released then an answer to FX (http://www.cisco.com/warp/public/ 707/cisco-sr-20060913-vtp.shtml) but as there is no any comment about this
specific vulnerability we suppose that it is not related with this one.

This vulnerability has been implemented in the current Yersinia version, under the VTP attacks (see the src/vtp.c file) .
Yersinia homepage: http://www.yersinia.net

You can find this advisory at:
http://www.s21sec.com/en/avisos/s21sec-034-en.txt

Other S21SEC advisories availabe at http://www.s21sec.com/en/avisos/

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