Significant Xen Security Bug
https://raw.githubusercontent.com/QubesOS/qubes-secpack/master/QSBs/qsb-022-2015.txt - found via https://news.ycombinator.com/item?id=10471912 where you might find useful discussion.
Looks like any guest VM can compromise an unpatched host (and therefore other guests) in a complete and untraceable manner.
If you use Xen, patch now or at least investigate to prove that the version/configuration you are running is not affected, if you have not already done so. If you use a service provider that uses Xen who has not already patched or announced plans to do so, drop them a line in case they have not yet been informed.