Security researchers from at the École Polytechnique Fédérale de Lausanne (EPFL) and Purdue University discovered vulnerabilities related to Cross-Transport Key Derivation (CTKD) with Bluetooth BR/EDR and LE in Bluetooth Specifications 4.0 through 5.0.

Bluetooth BR/EDR and LE that using Cross-Transport Key Derivation (CTKD) are vulnerable to key overwrite which allows attackers to gain additional access to profiles by reducing the encryption strength.

BLURtooth Attack

According to the specifications of 4.2 and 5.0 Bluetooth versions, CTKD is used for pairing by devices that support both Low Energy (BLE) and Basic Rate/Enhanced Data Rate (BR/EDR) transport methods.

By using the CTKD it let devices pair once by using the transport method while generating both the BR/EDR and LE Long Term Keys (LTK) without needing to pair a second time.

To launch a successful attack the device should reside within the wireless range of a vulnerable Bluetooth device supporting both BR/EDR and LE transports that support CTKD between the transports and permits pairing on either the BR/EDR or LE transport either with no authentication or no user-controlled access.

The vulnerability can be tracked as CVE-2020-15802, it is referred to as BLURtooth and the group of attacks is being referred to as the BLUR attacks.

Bluetooth SIG recommends additional conformance tests for authentication keys, disallow overwrite of the LTK, or LK.

The Bluetooth SIG further recommends that devices restrict when they are pairable on either transport to times when user interaction places the device into a pairable mode or when the device has no bonds or existing connections to a paired device.

Bluetooth users are recommended to have installed the latest updates from the device and operating system manufacturers.

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