source: http://www.securityfocus.com/bid/25825/info

Xen is prone to a local command-injection vulnerability that can lead to privilege escalation.

This issue occurs because the application fails to validate input in the 'tools/pygrub/src/GrubConf.py' script.

This vulnerability affects Xen 3.0.3; other versions may be affected as well. 

An attacker can exploit this issue by including Python commands in a configuration file using filesystem utilities. The following proof of concept is available:

Change the 'default' statement in grub.conf to:
default "+str(0*os.system(" insert evil command here "))+"