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).
https://bugs.launchpad.net/ubuntu/+source/pam/+bug/874565
http://www.ubuntu.com/usn/USN-1237-1
http://security.gentoo.org/glsa/glsa-201206-31.xml
http://secunia.com/advisories/49711