driver/subprocs.c in XScreenSaver before 5.34 does not properly perform an internal consistency check, which allows physically proximate attackers to bypass the lock screen by hot swapping monitors.
https://www.jwz.org/blog/2015/10/xscreensaver-5-34/
https://twitter.com/Thaolia/status/656823859304398848
http://www.ubuntu.com/usn/USN-2789-1
http://www.securitytracker.com/id/1034052
http://www.oracle.com/technetwork/topics/security/bulletinjul2016-3090568.html
http://www.openwall.com/lists/oss-security/2015/10/29/12
http://www.openwall.com/lists/oss-security/2015/10/25/1
http://www.debian.org/security/2016/dsa-3438
http://lists.opensuse.org/opensuse-updates/2015-11/msg00102.html