Impulse Radio Ultra-Wideband, in particular the recent standard IEEE 802.15.4a, is a primary candidate for implementing distance bounding protocols, thanks to its ability to perform accurate indoor ranging. Distance bounding protocols allow two wireless devices to securely estimate the distance between themselves, with the guarantee that the estimate is an upper-bound on the actual distance. These protocols serve as building blocks in security-sensitive applications such as tracking, physical access control, or localization. We investigate the resilience of IEEE 802.15.4a to physical-communication-layer attacks that decrease the distance measured by distance bounding protocols, thus violating their security. We consider two attack types: malicious prover (internal) and distance-decreasing relay (external). We show that if the honest devices use energy-detection receivers (popular due to their low cost and complexity), then an adversary can perform highly effective internal and external attacks, decreasing the distance by hundreds of meters. However, by using more sophisticated rake receivers, or by implementing small modifications to IEEE 802.15.4a and employing energy-detection receivers with a simple countermeasure, honest devices can reduce the effectiveness of external distance-decreasing relay attacks to the order of 10m. The same is true for malicious prover attacks, provided that an additional modification to IEEE 802.15.4a is implemented.