The OpenSSH server, as used in Fedora and Red Hat Enterprise Linux 7 and when running in a Kerberos environment, allows remote authenticated users to log in as another user when they are listed in the .k5users file of that user, which might bypass intended authentication requirements that would force a local login.
https://exchange.xforce.ibmcloud.com/vulnerabilities/99090
https://bugzilla.redhat.com/show_bug.cgi?id=1169843
https://bugzilla.mindrot.org/show_bug.cgi?id=1867
http://www.securityfocus.com/bid/71420
http://www.openwall.com/lists/oss-security/2014/12/04/17
http://www.openwall.com/lists/oss-security/2014/12/02/3
http://thread.gmane.org/gmane.comp.encryption.kerberos.general/15855