libvirt, possibly before 0.9.12, does not properly assign USB devices to virtual machines when multiple devices have the same vendor and product ID, which might cause the wrong device to be associated with a guest and might allow local users to access unintended USB devices.
https://www.redhat.com/archives/libvir-list/2012-April/msg01494.html
http://www.openwall.com/lists/oss-security/2012/06/11/3
http://www.openwall.com/lists/oss-security/2012/06/11/2