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Hello,
This is a Cisco response to an advisory published by FX of Phenoelit
posted as of September 13, 2006 at:
http://www.securityfocus.com/archive/1/445896/30/0/threaded
and entitled "Cisco Systems IOS VTP multiple vulnerabilities".

An official response is located at:
http://www.cisco.com/warp/public/707/cisco-sr-20060913-vtp.shtml

These vulnerabilities are addressed by Cisco bug IDs:

• CSCsd52629/CSCsd34759 – VTP version field DoS

• CSCse40078/CSCse47765 – Integer Wrap in VTP revision

• CSCsd34855/CSCei54611 – Buffer Overflow in VTP VLAN name

We would like to thank FX and Phenoelit Group for reporting these
vulnerabilities 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.

Additional Information

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 you
configure a new VLAN on one VTP server, the VLAN configuration
information is distributed via the VTP protocol through all switches
in the domain. This reduces the need to configure the same VLAN
everywhere. VTP is a Cisco-proprietary protocol that is available on
most of the Cisco Catalyst series products in both Cisco IOS and
Cisco CatOS system software.

Products affected by these vulnerabilities:
±-----------------------------------------

• Switches running affected versions of Cisco IOS and have VTP
  Operating Mode as either "server" or "client" are affected by all
  three vulnerabilities.
• Switches running affected versions of Cisco CatOS and have VTP
  Operating Mode as either "server" or "client" are only affected
  by "Integer Wrap in VTP revision" vulnerability.

Products not affected by these vulnerabilities:
±---------------------------------------------

• Switches configured with VTP operating mode as "transparent".
• Switches running CatOS with VTP Operating Mode as either "server"
  or "client" are not affected by "Buffer Overflow in VTP VLAN
  name" or "VTP Version field DoS" vulnerabilities

To determine the VTP mode on the switch, log into the device and
issue the "show vtp status" (IOS) or "show vtp domain" (CatOS)
command. Switches that show either "Server" or "Client" as the VTP
operating mode are affected by these vulnerabilities.

An example is shown below for Cisco IOS with VTP operating in
"Server" mode:

ios_switch#sh vtp stat  
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#

An example is shown below for Cisco CatOS with VTP operating in
"Server" mode:

catos_switch> (enable) sh 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) 

• VTP Version field DoS:

  The VTP feature in certain versions of Cisco IOS software may be
  vulnerable to a crafted packet sent from the local network
  segment which may lead to a denial of service condition. When a
  switch receives a specially crafted VTP summary packet, the
  switch will reset with a Software Forced Crash Exception.
  Messages for either "watchdog timeout" or "CPU hog" for process
  VLAN Manager will be seen prior to the software reset within the
  syslog messages generated by the switch.
  The packets must be received on a trunk enabled port.

  Switches running CatOS are not affected by this vulnerability and
  will display a log message "%VTP-2-RXINVSUMMARY:rx invalid
  summary from [port number]" should a specially crafted summary
  packet be received.

  There are no workarounds for this vulnerability. Switches
  configured with a VTP domain password are still affected by this
  vulnerability. Cisco recommends that customer upgrade to a
  version of Cisco IOS that contains the fixes for either
  CSCsd52629 or CSCsd34759.

• Buffer Overflow in VTP VLAN name:

  The VTP feature in certain versions of Cisco IOS software is
  vulnerable to a buffer overflow condition and potential execution
  of arbitrary code. 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).

  Applying a VTP domain password to the VTP domain will prevent
  spoofed VTP summary advertisement message from advertising an
  incorrect VLAN name. See http://www.cisco.com/univercd/cc/td/doc/
  product/lan/c3550/12119ea1/3550scg/swvtp.htm#1035247 for further
  information on setting VTP domain passwords.

• Integer Wrap in VTP revision:

  The VTP feature in certain versions of Cisco IOS software and
  Cisco CatOS software will display statistic counters as a
  negative number due to an integer wrap. Normal VTP operation will
  occur if no changes are made within the VTP domain. With the
  addition of switches or resetting of a VTP server configuration
  revision, VTP updates potentially may not be processed by other
  VTP servers/clients within the domain. Should any switches be
  impacted by this vulnerability, customers should execute the
  recovery procedures as listed below.

  Once the VTP configuration revision exceeds 0x7FFFFFFF, the
  output for the VTP configuration revision in "show vtp status"
  (IOS) or "show vtp domain" (CatOS) 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.

  Example from Cisco IOS:

  ios_switch#sh vtp stat
  VTP Version                     : 2
  Configuration Revision          : -2147483648
  Maximum VLANs supported locally : 1005
  Number of existing VLANs        : 17
  VTP Operating Mode              : Client
  VTP Domain Name                 : psirt
  VTP Pruning Mode                : Disabled
  VTP V2 Mode                     : Disabled
  VTP Traps Generation            : Disabled
  MD5 digest                      : <removed> 
  Configuration last modified by 0.0.0.0 at 3-1-93 00:10:07
  ios_switch#
  

  Example from Cisco CatOS:

  catos_switch# (enable) sh vtp domain
  Version      : running VTP1 (VTP3 capable)
  Domain Name  : psirt             Password  : not configured
  Notifications: disabled          Updater ID: 0.0.0.0
  
  Feature        Mode           Revision
  -------------- -------------- -----------
  VLAN           Server         -2147483648
  
  Pruning             : disabled
  VLANs prune eligible: 2-1000
  

  Applying a VTP domain password to the VTP domain will prevent
  spoofed VTP summary advertisement messages from advertising
  0x7FFFFFFF as a configuration revision number. See http://
  www.cisco.com/univercd/cc/td/doc/product/lan/c3550/12119ea1/
  3550scg/swvtp.htm#1035247 for further information on setting VTP
  domain passwords

  To recover from the negative configuration revision due to
  exploitation, the following methods can be performed to recover
  the VTP domain operations:

  • Change VTP domain names on all switches.

  • Change all VTP servers/clients to transparent mode first. Then
    change back to their original server/client mode.

For further information on VTP please refer to:
http://www.cisco.com/warp/public/473/21.html

For further information on Layer 2 security practices please refer
to:
http://www.cisco.com/en/US/netsol/ns340/ns394/ns171/ns128/
networking_solutions_white_paper09186a008014870f.shtml#wp998892

Regards
Paul Oxman
PSIRT Incident Manager
Cisco Systems

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