The qemu guest agent in Qemu 1.4.1 and earlier, as used by Xen, when started in daemon mode, uses weak permissions for certain files, which allows local users to read and write to these files.
Exploitability
AV:LAC:MAu:NImpact
C:CI:CA:C6.9/AV:L/AC:M/Au:N/C:C/I:C/A:C