Vendor: Bluetooth SIG, Inc.
Vendor URL: https://www.bluetooth.com
Versions Affected: Specification versions 4.0 to 5.3
Systems Affected: Any systems relying on the presence of a Bluetooth LE connection as confirmation of physical proximity, regardless of whether link layer encryption is used
Author:  
Risk: An attacker can falsely indicate the proximity of Bluetooth LE (BLE) devices to   one another through the use of a relay attack. This may enable unauthorized access to devices in BLE-based proximity authentication systems.

Summary

Many products implement Bluetooth Low Energy (BLE) based proximity authentication, where the product unlocks or remains unlocked when a trusted BLE device is determined to be nearby. Common examples of such products include automotive Phone-as-a-Key systems, residential smart locks, BLE-based commercial building access control systems, and smartphones and laptops with trusted BLE device functionality. The possibility of relay attacks against BLE proximity authentication has been known for years, but existing public relay attack tooling (based on forwarding GATT requests and responses) introduces detectable levels of latency and is incapable of relaying connections employing link layer encryption. Thus, products commonly attempt to prevent relay attacks by imposing strict GATT response time limits and/or using link layer encryption. Some systems also try to block signal amplification relay attacks through various localization techniques involving triangulation.

NCC Group has developed a tool for conducting a new type of BLE relay attack operating at the link layer, for which added latency is within the range of normal GATT response timing variation, and which is capable of relaying encrypted link layer communications. This approach can circumvent the existing relay attack mitigations of latency bounding or link layer encryption, and bypass localization defences commonly used against relay attacks that use signal amplification.

Impact

If an attacker can place a relaying device within signal range of a target BLE device (Victim Device A) trusted for proximity authentication by another device (Victim Device B), then they can conduct a relay attack to unlock and operate Victim Device B.

Neither normal GATT response latency nor successful communications over an encrypted link layer can be used as indications that a relay attack is not in progress. Consequently, conventional mitigations to prior BLE relay attacks are rendered ineffective against link layer relay attacks.

Details

NCC Group has developed a tool for conducting a new type of Bluetooth Low Energy (BLE) relay attack that can forward link-layer responses within a single connection event and introduces as little as 8 ms of round-trip latency beyond normal operation. As typical connection intervals in proximity authentication system are 30 ms or longer, added latency can generally be limited to a single connection event. With further straightforward refinement of the tool, it would be possible to guarantee that the added response latency is one connection event or less for any connection interval permissible under the Bluetooth specification.

Real BLE devices commonly require multiple connection events to respond to GATT requests or notifications and have inherent variability in their response timing. Thus, the latency introduced by this relay attack falls within the range of normal response timing variation.

Since this relay attack operates at the link layer, it can forward encrypted link layer PDUs. It is also capable of detecting encrypted changes to connection parameters (such as connection interval, WinOffset, PHY mode, and channel map) and continuing to relay connections through parameter changes. Thus, neither link layer encryption nor encrypted connection parameter changes are defences against this type of relay attack.

Recommendation

The Bluetooth Core Specification does not make any claims of relay attack resistance. Furthermore, Section 6 of the Proximity Profile[1] (v1.0.1, updated in 2015) explicitly warns of the possibility of relay attacks, noting that proximity indicated by a BLE connection “should not be used as the only protection of valuable assets.” However, many members of the Bluetooth SIG have produced BLE proximity authentication systems intended for security critical applications, and some make claims of relay attack resistance while still being at risk. Makers of such systems and their applications are also commonly promoted [2],[3],[4],[5] on the Bluetooth SIG Blog despite the documented risks.

NCC Group recommends that the SIG proactively advise its members developing proximity authentication systems about the risks of BLE relay attacks. Moreover, documentation should make clear that relay attacks are practical and must be included in threat models, and that neither link layer encryption nor expectations of normal response timing are defences against relay attacks. Developers should be encouraged to either require user interaction on the mobile device to authorize unlock, or adopt a time-of-flight based secure ranging (distance bounding) solution using technologies such as Ultra-Wide Band (UWB). For existing systems where hardware modification is not feasible, NCC Group recommends that end users be educated about the risks of relay attacks and presented with an option to disable passive entry functionality that relies on inferred proximity alone. Risk can also be reduced by disabling passive unlock functionality when the user’s mobile device has been stationary for more than a minute (as measured by accelerometer readings).

Vendor Communication

April 4, 2022: Disclosure to Bluetooth SIG
April 19, 2022: Response from Bluetooth SIG confirming that relay attacks are a known   risk, and that more accurate ranging mechanisms are under development.
April 19, 2022: Follow up message to Bluetooth SIG clarifying certain details of relay   attack based on questions from the SIG.
May 15, 2022: Advisory released to public

Thanks to

Jeremy Boone for his support and guidance throughout the research process developing this attack.

About NCC Group

NCC Group is a global expert in cybersecurity and risk mitigation, working with businesses to protect their brand, value and reputation against the ever-evolving threat landscape. With our knowledge, experience and global footprint, we are best placed to help businesses identify, assess, mitigate respond to the risks they face. We are passionate about making the Internet safer and revolutionizing the way in which organizations think about cybersecurity.