The telnet URI handler in Microsoft Internet Explorer 6 through 9 does not properly launch the handler application, which allows remote attackers to execute arbitrary programs via a crafted web site, aka "Telnet Handler Remote Code Execution Vulnerability."
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A12684
https://docs.microsoft.com/en-us/security-updates/securitybulletins/2011/ms11-057
http://www.us-cert.gov/cas/techalerts/TA11-221A.html