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It turns out that the "telnet" protocol can be used in the same manner as the "help" protocol to execute malicious code under OS X. The fix is the same as it was for the "help" and "disk" protocols. Use the More Internet Preference Pane to set TextEdit or some similarly harmless application as the default application for the "telnet" protocol.

A description of the telnet vulnerability is available here.

You may find some web sites recommending a program called RCDefaultApp to fix this problem instead of More Internet. Either application will do the job.