PAM (Pluggable Authentication Modules) is an architecture allowing the separation of the development of privilege granting software from the development of secure and appropriate authentication schemes. SELinux is an operating system based on Linux which includes Mandatory Access Control.
The SELinux patches for PAM introduce a vulnerability allowing a password to be checked with the unix_chkpwd utility without delay or logging. This vulnerability doesn't affect users who do not run SELinux.
A local attacker could exploit this vulnerability to brute-force passwords and escalate privileges on an SELinux system.
There is no known workaround at this time.
All SELinux PAM users should upgrade to the latest version:
# emerge --sync
# emerge --ask --oneshot --verbose ">=sys-libs/pam-0.78-r3"