Attacks that leverage USB as an attack vector are gaining popularity. While attention has so far focused on attacks that either exploit the host's USB stack or its unrestricted device privileges, it is not necessary to compromise the host to mount an attack over USB. This paper describes and implements a USB sniffing attack. In this attack a USB device passively eavesdrops on all communications from the host to other devices, without being situated on the physical path between the host and the victim device. To prevent this attack, we present UScramBle, a lightweight encryption solution which can be transparently used, with no setup or intervention from the user. Our prototype implementation of UScramBle for the Linux kernel imposes less than 15% performance overhead in the worst case.