BLURtooth

BLUR attacks

BLURtooth (the BLUR attacks) exploits the lack of cross-transport key validation, allowing an attacker to bypass Bluetooth Classic and Bluetooth Low Energy security mechanisms.

Bluetooth’s cross-transport key derivation (CTKD) is vulnerable to attacks enabling to attack Bluetooth Classic from Bluetooth Low Energy and vice versa. A remote attacker in Bluetooth range may impersonate, man-in-the-middle, and establish malicious sessions with arbitrary devices.

Summary

Here, we provide more details about a set of novel and standard-compliant Bluetooth vulnerabilities affecting both Bluetooth Classic (BT) and Bluetooth Low Energy (BLE). The uncovered vulnerabilities affect a security mechanism called cross-transport key derivation (CTKD). CTKD is used to improve the usability of Bluetooth pairing by allowing to generate BT and BLE pairing keys just by pairing two devices either on BT or BLE (rather than pairing them two times).

However, we find that CTKD introduces cross-transport security issues and that an attacker can abuse those issues to attack BT from BLE and vice versa. In particular, our attacks enable to impersonate, man-in-the-middle, and establish malicious sessions with arbitrary devices by abusing CTKD, while defeating all the security mechanisms put in place by BT and BLE. Our work is named BLURtooth and the related attacks are called BLUR attacks as they blur the security boundary between BT and BLE.

The team behind this work consists of Daniele Antonioli and Mathias Payer from the HexHive group at École Polytechnique Fédérale de Lausanne (EPFL), Nils Ole Tippenhauer from Helmholtz Center for Information Security (CISPA), and Kasper Rasmussen from the University of Oxford.

In the remainder of this document, we provide information on technical details, disclosure, impact, our proposed mitigation, the response from the Bluetooth SIG.

Technical Details

The Bluetooth standard includes two technologies Bluetooth Classic (BT) (also known as Bluetooth BR/EDR) and Bluetooth Low Energy (BLE). The majority of mobile devices, including laptops, smartphones, tablets, headphones, and smartwatches, support both and are defined as dual-mode Bluetooth devices. To securely use dual-mode devices over BT and BLE a user has to pair her devices two times, once for BT and once for BLE. As pairing the same device is considered user-unfriendly, in 2014, with the release of Bluetooth version 4.2, the Bluetooth standard introduced a security mechanism that allows a user to pair dual-mode Bluetooth devices once (either over BT or BLE) and then securely use them both over BT and BLE. This security mechanism is called cross-transport key derivation (CTKD), and, as the name implies, it enables deriving pairing keys across different transports (i.e. derive a BT pairing key from BLE and vice versa).

Despite being a security-critical mechanism, CTKD is not part of the Bluetooth threat model and there are no security evaluations of CTKD. Those reasons motivated us to perform a security analysis of CTKD, resulting in our findings. In particular, CTKD is affected by 5 major issues (i.e. vulnerabilities) enabling an attacker to abuse Bluetooth roles, association, security modes, keys, and pairing states across BT and BLE. Such issues derive from the lack of a cross-transport threat model in the Bluetooth standard. The standard considers BT and BLE with separate threat models and security architectures while, through CTKD, opens avenues for cross-transport attacks (i.e., attacks that exploit BT by taking advantage of a vulnerability in BLE or vice versa).

We demonstrate that the identified CTKD issues can be exploited by a remote attacker in Bluetooth range with the victims. In particular, the attacker can perform impersonation, man-in-the-middle, and malicious session establishment attacks while bypassing all the security mechanisms provided by BT and BLE (including Secure Connections or strong association). Those are very serious attacks that violate the security guarantees promised by Bluetooth. We confirmed the feasibility of our attacks by testing them on 13 common Bluetooth devices using 10 unique Bluetooth chips. All of them were vulnerable.

You will find technical details about CTKD, our security analysis, a detailed discussion of the threads, a discussion, and potential mitigations in our BLURtooth preprint.

Disclosure

We discovered the vulnerability in March 2020 and responsibly disclosed our findings along with suggested countermeasures to the Bluetooth SIG in May 2020. We kept our findings private and the Bluetooth SIG publicly disclosed them, without informing us, on the 10th of September of 2020. Our work is assigned CVE-2020-15802.

Impact

The BLUR attacks are a significant threat for all Bluetooth users and the related vulnerabilities remain 0-days. Our claim is backed up by our experimental results where we successfully conducted impersonation, man-in-the-middle, and malicious sessions establishment attacks on 13 different devices. Our device sample include manufacturers such as Dell, Google, Lenovo, Samsung, and Sony, operating systems, such as Windows 10, Linux, and Android, and Bluetooth chip manufacturers such as Cypress, Qualcomm, Intel, Broadcom, and Cambridge Silicon Radio (CSR).

Our Mitigations

As part of our disclosure, we provided concrete fixes to combat the BLUR attacks. In particular, we recommended disabling the capability to overwrite keys via CTKD in certain circumstances, enforce strong association and Secure Connections and roles across BT and BLE, disable pairing over BT and/or BLE when not needed, and add user notifications in case of odd behaviors. Our fixes can be implemented at the standard level and do not require vendor-specific features.

Response from the Bluetooth SIG

At the time of writing, there are no deployed patches to address the BLUR attacks on actual devices. The Bluetooth SIG suggested that version 5.1 of the standard will contain guidelines to mitigate the BLUR attacks (e.g., disable key overwrites in certain circumstances as proposed in our countermeasures), but such guidelines are not (yet) public and we cannot comment on them. The Bluetooth SIG provides a public statement about BLURtooth and the BLUR attacks.