pam_unix.so in Linux-PAM 0.99.7.0 allows context-dependent attackers to log into accounts whose password hash, as stored in /etc/passwd or /etc/shadow, has only two characters.
pam_wheel in Linux-PAM 0.78, with the trust option enabled and the use_uid option disabled, allows local users to spoof log entries and gain privileges by causing getlogin() to return a spoofed user name.
