Bluetooth SIG Statement Regarding the Exploiting Cross-Transport Key Derivation in Bluetooth Classic and Bluetooth Low Energy (BLURtooth) and the Security implications of key conversion between BR/EDR and BLE Vulnerabilities
Researchers at the École Polytechnique Fédérale de Lausanne (EPFL), the University of Oxford, and the CISPA Helmholtz Center for Information Security, and researchers at Purdue University have independently identified vulnerabilities related to Cross-Transport Key Derivation (CTKD) in implementations supporting pairing and encryption with both Bluetooth BR/EDR and LE in Bluetooth Specifications 4.2 through 5.0. The researches identified that CTKD, when implemented to older versions of the specification, may permit escalation of access between the two transports with non-authenticated encryption keys replacing authenticated keys or weaker encryption keys replacing stronger encryption keys.
The researchers also identified that CTKD may permit a remote paired device to access some LE services if BR/EDR access is achieved or BR/EDR profiles if LE access is achieved. As this is the intended use of CTKD, these cross-transport procedures are not being considered vulnerabilities by the SIG.
For this attack to be successful, an attacking device would need to be within wireless range of a vulnerable Bluetooth device supporting both BR/EDR and LE transports that supports CTKD between the transports and permits pairing on either the BR/EDR or LE transport either with no authentication (e.g. JustWorks) or no user-controlled access restrictions on the availability of pairing. If a device spoofing another device’s identity becomes paired or bonded on a transport and CTKD is used to derive a key which then overwrites a pre-existing key of greater strength or that was created using authentication, then access to authenticated services may occur. This may permit a Man In The Middle (MITM) attack between devices previously bonded using authenticated pairing when those peer devices are both vulnerable.
The Bluetooth SIG is recommending that potentially vulnerable implementations introduce the restrictions on Cross-Transport Key Derivation mandated in Bluetooth Core Specification versions 5.1 and later.
The Bluetooth SIG is also broadly communicating details on this vulnerability and its remedies to our member companies and is encouraging them to rapidly integrate any necessary patches. As always, Bluetooth users should ensure they have installed the latest recommended updates from device and operating system manufacturers.
For more information, please refer to the statement from the CERT Coordination Center.
