The _expand_arg function in the pam_env module (modules/pam_env/pam_env.c) in Linux-PAM (aka pam) before 1.1.5 does not properly handle when environment variable expansion can overflow, which allows local users to cause a denial of service (CPU consumption).
http://secunia.com/advisories/46583
http://secunia.com/advisories/49711
http://security.gentoo.org/glsa/glsa-201206-31.xml