Cisco Telnet Denial of Service Vulnerability

A specifically crafted Transmission Control Protocol (TCP) connection to a telnet or reverse telnet port of a Cisco device running Internetwork Operating System (IOS)® may block further telnet, reverse telnet, Remote Shell (RSH), Secure Shell (SSH), and in some cases Hypertext Transport Protocol (HTTP) access to the Cisco device. Data Link Switching (DLSw) and protocol translation connections may also be affected. Telnet, reverse telnet, RSH, SSH, DLSw and protocol translation sessions established prior to exploitation are not affected.

All other device services will operate normally. Services such as packet forwarding (excluding DLSw and protocol translation per above), routing protocols and all other communication to and through the device are not affected.

Cisco has made free software available to address this vulnerability. Workarounds, identified below, are available that protect against this vulnerability.

This vulnerability is documented in Cisco bug ID CSCef46191 [“https://bst.cloudapps.cisco.com/bugsearch/bug/CSCef46191”] ( registered [“https://sec.cloudapps.cisco.comRPF/register/register.do”] customers only) .

This Advisory is available at https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20040827-telnet [“https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20040827-telnet”].