Impersonating users of ‘protest’ app Bridgefy was as simple as sniffing Bluetooth handshakes for identifiers

An instant messaging app whose creators promoted it as secure and end-to-end encrypted was in fact no such thing, according to researchers at Royal Holloway.

The University of London college found, according to a paper it published yesterday, that the app “permits its users to be tracked, offers no authenticity, no effective confidentiality protections and lacks resilience against adversarially crafted messages”.

As first reported by Ars Technica, Bridgefy was promoting itself earlier this year as the app of choice for protesters in Hong Kong and India to organise their activities without being easily spied upon by law enforcement agencies.

Yet Royal Holloway’s Martin Albrecht, Rikke Bjerg Jensen, Jorge Blasco, and Lenka Marekova found in a security analysis [PDF] that the app “implements no effective authentication mechanism between users (nor a state machine) to impersonate arbitrary users”.

The app uses both the internet and Bluetooth Low Energy (BLE) for passing messages between users, falling back to the latter as a mesh network if wider internet connectivity is unavailable.

The researchers continued: “All one-to-one messages sent over the mesh network contain the sender and receiver IDs in plaintext, so a passive adversary with physical presence can build a social graph of what IDs are communicating with whom.”

Not only that but on BLE, Bridgefy “does not utilise cryptographic authentication mechanisms”. Device and user account verification is achieved through an app-assigned user ID and “the lower level Bluetooth device address”, neither of which, as the Royal Holloway researchers noted, is “an actual authentication mechanism”. Impersonating a Bridgefy user is as simple as sniffing a BLE handshake to gather the user ID and device address before spoofing the latter.

Other attacks detailed by the researchers in their paper included classic man-in-the-middle and denial of service. On the latter, they noted “compressing a message of size 10MB containing a repeated single character results in a payload of size 10KB”, which was enough to crash Bridgefy to the point where the only way to get it working again was to delete the app and reinstall it from scratch.

In an unsigned statement posted on its website, Bridgefy’s developers acknowledged the paper by declaring that they were ditching their existing setup in favour of the tech underpinning Signal.

“We realized that Bridgefy’s security model was appropriate for a small startup, but not for the scale it has achieved today and the growth we want in the future,” said the company, adding that it intends to implement the Signal protocol (as used in the secure messaging app of the same name) within future releases instead of its current setup.

The Register has contacted Bridgefy for further comment. ®

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