A security vulnerability in the Cross-Transport Key Derivation (CTKD) of devices supporting both Bluetooth BR/EDR and LE could allow an attacker to overwrite encryption keys, researchers have discovered.
Dubbed BLURtooth, the issue was identified independently by researchers at the École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland and Purdue University. The flaw is related to CTKD in implementations where pairing and encryption with both Low Energy (LE) and Basic Rate/Enhanced Data Rate (BR/EDR) are supported, in Bluetooth specifications 4.0 through 5.0.
The implementation of CTKD in 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 Bluetooth Special Interest Group (SIG) explains.
The researchers also discovered that CTKD could allow “a remote paired device to access some LE services if BR/EDR access is achieved or BR/EDR profiles if LE access is achieved.” However, this is considered normal behavior, and the SIG does not consider the cross-transport procedures as being security bugs.
The BLURtooth attack, the SIG explains, requires for the attacker to be within wireless range of a vulnerable product that permits pairing on either the BR/EDR or LE transport (with no authentication or no user-controlled access restrictions).
“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,” the Bluetooth SIG reveals.
This may allow an adversary to mount a Man-In-The-Middle (MITM) attack between devices that have been paired and authenticated, provided that both of them are vulnerable.
In a vulnerability note on Wednesday, the CERT Coordination Center (CERT/CC) revealed that the issue, which is tracked as CVE-2020-15802, may allow an attacker to gain access to profiles or services that should otherwise be restricted.
Restrictions on CTKD that were included in Bluetooth Core Specification 5.1 and later should be introduced in potentially vulnerable implementations as well, the SIG recommends.
“Implementations should disallow overwrite of the LTK or LK for one transport with the LTK or LK derived from the other when this overwrite would result in either a reduction of the key strength of the original bonding or a reduction in the MITM protection of the original bonding (from authenticated to unauthenticated). This may require that the host track the negotiated length and authentication status of the keys in the Bluetooth security database,” CERT/CC explains.
The Bluetooth SIG also recommends performing additional conformance tests so as to make sure that overwriting an authenticated encryption key is not allowed on devices that feature support for Bluetooth Core Specification version 5.1 or newer. Furthermore, devices should restrict when they are pairable, as well as the duration of pairing mode.
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