qemu-kvm before 0.11.0 disables VNC authentication when the password is cleared, which allows remote attackers to bypass authentication and establish VNC sessions.
https://exchange.xforce.ibmcloud.com/vulnerabilities/65215
https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/697197
http://www.openwall.com/lists/oss-security/2011/01/12/2
http://www.openwall.com/lists/oss-security/2011/01/11/1
http://www.openwall.com/lists/oss-security/2011/01/10/3
http://ubuntu.com/usn/usn-1063-1
http://secunia.com/advisories/44393
http://secunia.com/advisories/43733
http://secunia.com/advisories/43272