Bluetooth Bug Could Allow MITM Attacks

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Security researchers have discovered a new vulnerability in Bluetooth which could allow attackers to perform man in the middle (MITM) attacks and access authenticated services.

The so-called “BLURtooth” vulnerability was independently discovered by teams at the École Polytechnique Fédérale de Lausanne (EPFL) and Purdue University.

It exists in the Cross-Transport Key Derivation (CTKD), which sets up authentication keys for dual-mode devices (i.e. smartphones) that support both Bluetooth Low Energy (BLE) and Basic Rate/Enhanced Data Rate (BR/EDR) transport methods.

Several attack scenarios were described using BLURtooth (CVE-2020-15802). It is possible for an attacker to exploit the bug to overwrite and lower the strength of the LTK or Link Key (LK) encryption keys used to pair devices.

“Vulnerable devices must permit a pairing or bonding to proceed transparently with no authentication, or a weak key strength, on at least one of the BR/EDR or LE transports in order to be susceptible to attack,” explained Carnegie Mellon University.

“For example, it may be possible to pair with certain devices using JustWorks pairing over BR/EDR or LE and overwriting an existing LTK or LK on the other transport. When this results in the reduction of encryption key strength or the overwrite of an authenticated key with an unauthenticated key, an attacker could gain additional access to profiles or services that are not otherwise restricted.”

Devices that had previously been paired but are vulnerable to the exploit may also be exposed to MITM by attackers within range.

“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,” explained the Bluetooth Special Interest Group (SIG).

There doesn’t appear to be a patch available for BLURtooth as yet, although the SIG said it is “encouraging” its member companies to roll one out when ready.

In the meantime, it  recommended that “potentially vulnerable implementations introduce the restrictions on CTKD mandated in Bluetooth Core Specification versions 5.1 and later.”

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