The pyGrub boot loader in Xen 3.0.3, 3.3.0, and Xen-3.3.1 does not support the password option in grub.conf for para-virtualized guests, which allows attackers with access to the para-virtualized guest console to boot the guest or modify the guest's kernel boot parameters without providing the expected password.
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A9466
http://www.securitytracker.com/id?1022950
http://www.redhat.com/support/errata/RHSA-2009-1472.html
http://www.openwall.com/lists/oss-security/2009/09/25/1
http://secunia.com/advisories/36908
http://lists.opensuse.org/opensuse-security-announce/2010-05/msg00002.html