User's pasword is not checked in telnet session if F flag is set. On older versions defining TTYPROMPT variable allows unauthorized access with bin group privileges. Vulnerability is used by internet worm.
CVE-2007-0882 (Argument injection vulnerability in the telnet daemon (in.telnetd) in Solaris 10 and 11 (SunOS 5.10 and 5.11) misinterprets certain client "-f" sequences as valid requests for the login program to skip authentication, which allows remote attackers to log into certain accounts, as demonstrated by the bin account.)
