Dr. Alexander Heinrich

Location-based systems allow us to perform many tasks: we can check the weather wherever we are, navigate unfamiliar environments, and track our running workouts as well as valuables such as laptops or luggage. However, all of these examples require access to highly sensitive and private location data. Misuse of this data allows adversaries to identify individuals based on a few locations, track them digitally without consent, or manipulate location data to bypass location-based access policies, such as those implemented in cars or smart door locks. In this thesis, we contribute to three emerging areas of location-based systems: (1) Security and misuse of Bluetooth offline-finding networks; (2) Security, reliability, and accuracy of localization with UWB; and (3) Secure location sharing via satellite. Our methodology is diverse. We reverse-engineer implementations to investigate their privacy and security protections, open up proprietary systems and provide open-source implementations. Furthermore, we develop tools to protect users from unwanted location tracking, research novel attack vectors and protections, and conduct user studies to understand the needs of victims of unwanted tracking. To summarize our main research results: (1) We analyze Apple’s Find My network, uncover vulnerabilities that expose individuals’ location data, and demonstrate that the network can be misused for tracking and stalking. Based on our results, we open up the proprietary network and publish a framework that allows any programmable Bluetooth device to be integrated into the Find My network, making it locatable. To protect users from unwanted tracking attacks, we develop and publish the AirGuard app, which identifies malicious trackers following a person and alerts them. We conduct a survey with over 5,000 participants and find that 44% of stalking victims have suffered from unwanted location tracking. With the study results and AirGuard user data, we identify patterns and methods used by stalkers, and where trackers are hidden. (2) We measure the accuracy and reliability of Ultra-Wide Band (UWB) distance measurements in consumer smartphones and find that, while results are accurate in most cases, all devices measured a small number of large outliers. Furthermore, we analyze the security of UWB against distance manipulation attacks and develop a novel physical-layer attack that can reduce the measured distance by several meters. (3) Encryption and privacy protection mechanisms are often neglected in satellite communication. We evaluate the iPhone’s location-sharing via satellite feature and find robust security mechanisms with multi-layer encryption in place. Nevertheless, we identify bypasses around Apple’s restrictions and manage to use satellite services from restricted areas and send text messages encoded in location updates. We propose countermeasures for all vulnerabilities we find. In addition to academic publications, our work has demonstrated real-world impact with the AirGuard apps for Android and iOS. They have been installed over 1.3million times in the last three and a half years. Several victims of unwanted tracking informed us that they had found trackers using our application, preventing further tracking. By pushing for better detection mechanisms, we influenced major companies: Apple and Google have proposed standardized tracking detection for all Bluetooth trackers.

Apple has integrated satellite communication into their latest iPhones, enabling emergency communication, road- side assistance, location sharing with friends, iMessage, and SMS. This technology allows communication when other wireless services are unavailable. However, the use of satellites poses restrictions on bandwidth and delay, making it difficult to use modern communication protocols with their security and privacy guarantees. To overcome these challenges, Apple designed and implemented a proprietary satellite communication protocol to address these limitations. We are the first to successfully reverse-engineer this protocol and analyze its security and privacy properties. In addition, we develop a simulation-based testbed for testing emergency services without causing emergency calls. Our tests reveal protocol and infrastructure design issues. For example, compact protocol messages come at the cost of missing integrity protection and require an internet-based setup phase. We further demonstrate various restriction bypasses, such as misusing location sharing to send arbitrary text messages on old iOS versions, and sending iMessages over satellite from region-locked countries. These bypasses allow us to overcome censorship and operator control of text messaging services.

Finder networks in general, and Apple's Find My network in particular, can pose a grave threat to users' privacy and even health if these networks are abused for stalking. Apple's release of the AirTag-a very affordable tracker covered by the nearly ubiquitous Find My network-amplified this issue. While Apple provides a stalking detection feature within its ecosystem, billions of Android users are still left in the dark. Apple recently released the Android app "Tracker Detect," which does not deliver a convincing feature set for stalking protection. We reverse engineer Apple's tracking protection in iOS and discuss its features regarding stalking detection. We design "AirGuard" and release it as an Android app to protect against abuse by Apple tracking devices. We compare the performance of our solution with the Apple-provided one in iOS and study the use of AirGuard in the wild over multiple weeks using data contributed by tens of thousands of active users.

We present the first over-the-air attack on IEEE802.15.4z High-Rate Pulse Repetition Frequency (HRP) Ultra-Wide Band (UWB)distance measurement systems. Specifically, we demonstrate a practical distance reduction attack against pairs of Apple U1 chips (embedded in iPhones and AirTags), as well as against U1 chips inter-operating with NXP and Qorvo UWBchips. These chips have been deployed in a wide range of phones and cars to secure car entry and start and are projected for secure contactless payments, home locks, and contact tracing systems. Our attack operates without any knowledge of cryptographic material, results in distance reductions from 12m (actual distance) to 0m (spoofed distance) with attack success probabilities of up to 4%, and requires only an inexpensive (USD 65) off-the-shelf device. Access control can only tolerate sub-second latencies to not inconvenience the user, leaving little margin to perform time-consuming verifications. These distance reductions bring into question the use of UWB HRPinsecurity-critical applications.

Apple's offline file-sharing service AirDrop is integrated into more than 1.5 billion end-user devices worldwide. We discovered two design flaws in the underlying protocol that allow attackers to learn the phone numbers and email addresses of both sender and receiver devices. As a remediation, we study the applicability of private set intersection (PSI) to mutual authentication, which is similar to contact discovery in mobile messengers. We propose a novel optimized PSI-based protocol called PrivateDrop that addresses the specific challenges of offline resource-constrained operation and integrates seamlessly into the current AirDrop protocol stack. Using our native PrivateDrop implementation for iOS and macOS, we experimentally demonstrate that PrivateDrop preserves AirDrop's exemplary user experience with an authentication delay well below one second. We responsibly disclosed our findings to Apple and open-sourced our PrivateDrop implementation.

OpenHaystack is an open-source framework for locating personal Bluetooth devices using Apple’s Find My Network. A user can integrate it into Bluetooth-capable devices, such as notebooks, or create custom tracking accessories that can be attached to personal items (key rings, backpacks, etc.). We provide firmware images for the Nordic nRF5 chips and the ESP32. We show that they consume little energy and run from a single coin cell for a year. Our macOS application can locate personal accessories. Finally, we make both application and firmware available on GitHub.

Apple’s file-sharing service AirDrop leaks phone numbers and email addresses by exchanging vulnerable hash values of the user’s own contact identifiers during the authentication handshake with nearby devices. In a paper presented at USENIX Security’21, we theoretically describe two attacks to exploit these vulnerabilities and propose “PrivateDrop” as a privacy-preserving drop-in replacement for Apple’s AirDrop protocol based on private set intersection. In this demo, we show how these vulnerabilities are efficiently exploitable via Wi-Fi and physical proximity to a target. Privacy and security implications include the possibility of conducting advanced spear phishing attacks or deploying multiple “collector” devices in order to build databases that map contact identifiers to specific locations. For our proof-of-concept, we leverage a custom rainbow table construction to reverse SHA-256 hashes of phone numbers in a matter of milliseconds. We discuss the trade-off between success rate and storage requirements of the rainbow table and, after following responsible disclosure with Apple, we publish our proof-of-concept implementation as “AirCollect” on GitHub.

Apple controls one of the largest mobile ecosystems, with 1.5 billion active devices worldwide, and offers twelve proprietary wireless Continuity services. Previous works have unveiled several security and privacy issues in the involved protocols. These works extensively studied AirDrop while the coverage of the remaining vast Continuity service space is still low. To facilitate the cumbersome reverse-engineering process, we describe the first guide on how to approach a structured analysis of the involved protocols using several vantage points available on macOS. Also, we develop a toolkit to automate parts of this otherwise manual process. Based on this guide, we analyze the full protocol stacks involved in three Continuity services, in particular, Handoff (HO), Universal Clipboard (UC), and Wi-Fi Password Sharing (PWS). We discover several vulnerabilities spanning from Bluetooth Low Energy (BLE) advertisements to Apple's proprietary authentication protocols. These flaws allow for device tracking via HO's mDNS responses, a denial-of-service (DoS) attack on HO and UC, a DoS attack on PWS that prevents Wi-Fi password entry, and a machine-in-the-middle (MitM) attack on PWS that connects a target to an attacker-controlled Wi-Fi network. Our PoC implementations demonstrate that the attacks can be mounted using affordable off-the-shelf hardware ($20 micro:bit and a Wi-Fi card). Finally, we suggest practical mitigations and share our findings with Apple, who have started to release fixes through iOS and macOS updates.

Apple Wireless Direct Link (AWDL) is a key protocol in Apple's ecosystem used by over one billion iOS and macOS devices for device-to-device communications. AWDL is a proprietary extension of the IEEE 802.11 (Wi-Fi) standard and integrates with Bluetooth Low Energy (BLE) for providing services such as Apple AirDrop. We conduct the first security and privacy analysis of AWDL and its integration with BLE. We uncover several security and privacy vulnerabilities ranging from design flaws to implementation bugs leading to a man-in-the-middle (MitM) attack enabling stealthy modification of files transmitted via AirDrop, denial-of-service (DoS) attacks preventing communication, privacy leaks that enable user identification and long-term tracking undermining MAC address randomization, and DoS attacks enabling targeted or simultaneous crashing of all neighboring devices. The flaws span across AirDrop's BLE discovery mechanism, AWDL synchronization, UI design, and Wi-Fi driver implementation. Our analysis is based on a combination of reverse engineering of protocols and code supported by analyzing patents. We provide proof-of-concept implementations and demonstrate that the attacks can be mounted using a low-cost ($20) micro:bit device and an off-the-shelf Wi-Fi card. We propose practical and effective countermeasures. While Apple was able to issue a fix for a DoS attack vulnerability after our responsible disclosure, the other security and privacy vulnerabilities require the redesign of some of their services.