Bluetooth is a pervasive wireless technology specified in an open standard. The standard defines Bluetooth Classic (BT) for high-throughput wireless services and Bluetooth Low Energy (BLE) very low-power ones. The standard also specifies security mechanisms, such as pairing, session establishment, and cross-transport key derivation (CTKD). CTKD enables devices to establish BT and BLE security keys by pairing just once. CTKD was introduced in 2014 with Bluetooth 4.2 to improve usability. However, the security implications of CTKD were not studied carefully.

This work demonstrates that CTKD is a valuable and novel Bluetooth attack surface. It enables, among others, to exploit BT and BLE just by targeting one of the two (i.e., Bluetooth cross-transport exploitation). We present the design of the first cross-transport attacks on Bluetooth. Our attacks exploit issues that we identified in the specification of CTKD. For example, we find that CTKD enables an adversary to overwrite pairing keys across transports. We leverage these vulnerabilities to impersonate, machine-in-the-middle, and establish unintended sessions with any Bluetooth device supporting CTKD. Since the presented attacks blur the security boundary between BT and BLE, we name them BLUR attacks. We provide a low-cost implementation of the attacks and test it on a broad set of devices. In particular, we successfully attack 16 devices with 14 unique Bluetooth chips from popular vendors (e.g., Cypress, Intel, Qualcomm, CSR, Google, and Samsung), with Bluetooth standard versions of up to 5.2. We discuss why the countermeasures in the Bluetooth are not effective against our attacks, and we develop and evaluate practical and effective alternatives.