  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.
