pygrub (tools/pygrub/src/GrubConf.py) in Xen 3.0.3, when booting a guest domain, allows local users with elevated privileges in the guest domain to execute arbitrary commands in domain 0 via a crafted grub.conf file whose contents are used in exec statements.
The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.