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.
www.openwall.com/lists/oss-security/2013/12/09/16
www.openwall.com/lists/oss-security/2013/12/09/5
www.securityfocus.com/bid/64180
www.ubuntu.com/usn/USN-2935-1
www.ubuntu.com/usn/USN-2935-2
www.ubuntu.com/usn/USN-2935-3
bugs.debian.org/cgi-bin/bugreport.cgi?bug=731368
bugzilla.redhat.com/show_bug.cgi?id=1038555
security.gentoo.org/glsa/201605-05