Pictures, presentations, or videos – for users of iPhones and macBooks it is extremely comfortable to share files via AirDrop, a service that enables the direct transfer of data between Apple devices. As sensitive data is typically exclusively shared with people who users already know, AirDrop only shows receiver devices from address book contacts by default. To determine whether the other party is a contact, AirDrop uses a mutual authentication mechanism that compares a user's phone number and email address with entries in the other user's address book.
A team of researchers from the Secure Mobile Networking Lab (SEEMOO) and the Cryptography and Privacy Engineering Group (ENCRYPTO) at TU Darmstadt took a closer look at this mechanism and discovered a severe privacy leak.
As an attacker, it is possible to learn the phone numbers and email addresses of AirDrop users – even as a complete stranger. All they require is a Wi-Fi-capable device and physical proximity to a target that initiates the discovery process by opening the sharing pane on an iOS or macOS device.
The discovered problems are rooted in Apple's use of hash functions for “obfuscating” the exchanged phone numbers and email addresses during the discovery process. However, researchers from TU Darmstadt already showed that hashing fails to provide privacy-preserving contact discovery as so-called hash values can be quickly reversed using simple techniques such as brute-force attacks.
New cryptographic protocol guarantees privacy
The research team also developed a solution named “PrivateDrop” to replace the flawed original AirDrop design. PrivateDrop is based on optimized cryptographic private set intersection protocols that can securely perform the contact discovery process between two users without exchanging vulnerable hash values. The researchers' iOS/macOS implementation of PrivateDrop shows that it is efficient enough to preserve AirDrop's exemplary user experience with an authentication delay well below one second.
Apple users are still vulnerable
The researchers informed Apple about the privacy vulnerability already in May 2019 via responsible disclosure. So far, Apple has neither acknowledged the problem nor indicated that they are working on a solution. This means that the users of more than 1.5 billion Apple devices are still vulnerable to the outlined privacy attacks. Users can only protect themselves by disabling AirDrop discovery in the system settings and by refraining from opening the sharing menu.
The researchers described their results in a scientific paper that will be presented in August at the renowned USENIX Security Symposium.
Alexander Heinrich, Matthias Hollick, Thomas Schneider, Milan Stute, and Christian Weinert. PrivateDrop: Practical Privacy-Preserving Authentication for Apple AirDrop. 30th USENIX Security Symposium, 2021. Link
Further publications on the security of the Apple ecosystem and mobile contact discovery
Milan Stute, Alexander Heinrich, Jannik Lorenz, and Matthias Hollick. Disrupting Continuity of Apple's Wireless Ecosystem Security: New Tracking, DoS, and MitM Attacks on iOS and macOS Through Bluetooth Low Energy, AWDL, and Wi-Fi. 30th USENIX Security Symposium, 2021. Link
Alexander Heinrich, Milan Stute, Tim Kornhuber, and Matthias Hollick. Who Can Find My Devices? Security and Privacy of Apple’s Crowd-Sourced Bluetooth Location Tracking System. Proceedings on Privacy Enhancing Technologies (PoPETs), 2021. Link
Christoph Hagen, Christian Weinert, Christoph Sendner, Alexandra Dmitrienko, and Thomas Schneider. All the Numbers are US: Large-scale Abuse of Contact Discovery in Mobile Messengers. 28th Network and Distributed System Security Symposium (NDSS), 2021. Link
Milan Stute, Sashank Narain, Alex Mariotto, Alexander Heinrich, David Kreitschmann, Guevara Noubir, and Matthias Hollick. A Billion Open Interfaces for Eve and Mallory: MitM, DoS, and Tracking Attacks on iOS and macOS Through Apple Wireless Direct Link. 28th USENIX Security Symposium, 2019. Link
Daniel Kales, Christian Rechberger, Thomas Schneider, Matthias Senker, and Christian Weinert. Mobile Private Contact Discovery at Scale. 28th USENIX Security Symposium, 2019. Link
Matthias Hollick, David Kreitschmann, and Milan Stute. One Billion Apples' Secret Sauce: Recipe for the Apple Wireless Direct Link Ad hoc Protocol. Proceedings of the 24th Annual International Conference on Mobile Computing and Networking (MobiCom '18), 2018. Link