A new Bluetooth vulnerability has come to light. Researchers at the École Polytechnique Fédérale de Lausanne (EPFL) & the Purdue University have independently identified vulnerabilities related to Cross-Transport Key Derivation (CTKD) in implementations supporting pairing & encryption with both Bluetooth BR/EDR and LE in Bluetooth Specifications 4.2 through 5.0.
All devices using the Bluetooth standard 4.0 through 5.0 are vulnerable. The Bluetooth 5.1 standard comes with features that can be activated & prevent BLURtooth attacks.
A post put out by the organizations behind Bluetooth said the researchers identified that CTKD, when implemented to older versions of the specification, may permit escalation of access between the 2 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 & LE transports that supports CTKD between the transports & 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 & 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 & later.
The Bluetooth SIG is also broadly communicating details on this vulnerability & its remedies to our member companies & is encouraging them to rapidly integrate any necessary patches.
