The pam_userdb module for Pam uses a case-insensitive method to compare hashed passwords, which makes it easier for attackers to guess the password via a brute force attack.
https://security.gentoo.org/glsa/201605-05
https://bugzilla.redhat.com/show_bug.cgi?id=1038555
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=731368
http://www.ubuntu.com/usn/USN-2935-3
http://www.ubuntu.com/usn/USN-2935-2
http://www.ubuntu.com/usn/USN-2935-1
http://www.securityfocus.com/bid/64180