We expose the vulnerability of an emerging wireless ranging technology, impulse radio ultra-wide band (IR-UWB), to distance-decreasing attacks on the physical communication layer (PHY). These attacks violate the security of secure ranging protocols that allow two wireless devices to securely estimate the distance between them, with the guarantee that the estimate is an upper-bound on the actual distance. Such protocols serve as crucial building blocks in security-sensitive applications such as location tracking, physical access control, or localization. Prior works show the theoretical possibility of PHY attacks bypassing cryptographic mechanisms used by secure ranging protocols. They also demonstrates that for physical layers used in ISO 14443 RFID and wireless sensor networks, some PHY attacks are indeed feasible. IR-UWB was proposed as a possible solution, but we show that the de facto standard for IR-UWB, IEEE 802.15.4a, does not automatically provide security against such attacks. We find that with the mandatory modes of the standard an external attacker can decrease the measured distance by as much as 140 meters with a high probability (above 99%).