The virDomainGetXMLDesc API in Libvirt before 1.2.11 allows remote read-only users to obtain the VNC password by using the VIR_DOMAIN_XML_MIGRATABLE flag, which triggers the use of the VIR_DOMAIN_XML_SECURE flag.
http://lists.opensuse.org/opensuse-updates/2014-11/msg00083.html
http://secunia.com/advisories/60010
http://secunia.com/advisories/60895
http://secunia.com/advisories/62058
http://secunia.com/advisories/62303
http://security.gentoo.org/glsa/glsa-201412-04.xml