Citrix XenServer 5.0 Update 2 and earlier, and 5.5 Update 1 and earlier, when using a pvops kernel, allows guest users to cause a denial of service in the host via unspecified vectors that trigger "incorrectly set flags."
http://www.vupen.com/english/advisories/2010/1613
http://www.securitytracker.com/id?1024157