Cisco IOS Telnet Vulnerability (Critical) – Over 300 switch models affected

Summary of the vulnerability

A vulnerability in the Cisco Cluster Management Protocol (CMP) processing code in Cisco IOS and Cisco IOS XE Software could allow an unauthenticated, remote attacker to cause a reload of an affected device or remotely execute code with elevated privileges.

How it works

The Cluster Management Protocol utilizes Telnet internally as a signalling and command protocol between cluster members. The vulnerability is due to the combination of two factors:

  • The failure to restrict the use of CMP-specific Telnet options only to internal, local communications between cluster members and instead accept and process such options over any Telnet connection to an affected device
  • The incorrect processing of malformed CMP-specific Telnet options.

Affected products

Please see the following URL for a list of the affected Cisco devices:

Further information

There is currently no workaround available, but Cisco have advised that a software release will be available to address the vulnerability. Unfortunately, there is no timescale associated with this release.


Disabling the Telnet protocol as an allowed protocol for incoming connections would eliminate the exploit vector. Disabling Telnet and using SSH is recommended by Cisco.