Bug in Apple’s Find My Feature Could’ve Exposed Users’ Location Histories

Cybersecurity researchers on Thursday disclosed two distinct
design and implementation flaws in Apple’s crowdsourced Bluetooth
location tracking system that can lead to a location correlation
attack and unauthorized access to the location history of the past
seven days, thereby by deanonymizing users.

The findings^[1]
are a consequence of an exhaustive review undertaken by the Open
Wireless Link (OWL) project, a team of researchers from the Secure
Mobile Networking Lab at the Technical University of Darmstadt,
Germany, who have historically taken apart Apple’s wireless
ecosystem with the goal of identifying security and privacy
issues.

In response to the disclosures on July 2, 2020, Apple is said to
have partially addressed the issues, stated the researchers, who
used their own data for the study citing privacy implications of
the analysis.

How Find My Works?

Apple devices come with a feature called Find My^[2] that makes it easy for
users to locate other Apple devices, including iPhone, iPad, iPod
touch, Apple Watch, Mac, or AirPods. With the upcoming iOS 14.5,
the company is expected to add support for Bluetooth tracking
devices — called AirTags^[3]
— that can be attached to items like keys and wallets, which in
turn can be used for tracking purposes right from within the Find
My app.

What’s more interesting is the technology that undergirds Find
My. Called offline finding and introduced in 2019, the location
tracking feature broadcasts Bluetooth Low Energy (BLE) signals from
Apple devices, allowing other Apple devices in close proximity to
relay their location to Apple’s servers.

Put differently, offline loading turns every mobile device into
a broadcast beacon designed explicitly to shadow its movements by
leveraging a crowdsourced location tracking mechanism that’s both
end-to-end encrypted and anonymous, so much so that no third-party,
including Apple, can decrypt those locations and build a history of
every user’s whereabouts.

This is achieved via a rotating key scheme, specifically a pair
of public-private keys that are generated by each device, which
emits the Bluetooth signals by encoding the public key along with
it. This key information is subsequently synchronized via iCloud
with all other Apple devices linked to the same user (i.e., Apple
ID).

A nearby iPhone or iPad (with no connection to the original
offline device) that picks up this message checks its own location,
then encrypts the information using the aforementioned public key
before sending it to the cloud along with a hash of the public
key.

In the final step, Apple sends this encrypted location of the
lost device to a second Apple device signed in with the same Apple
ID, from where the owner can use the Find My app to decrypt the
reports using the corresponding private key and retrieve the last
known location, with the companion device uploading the same hash
of the public key to find a match in Apple’s servers.

Issues with Correlation and Tracking

Since the approach follows a public key encryption (PKE^[4]) setup, even Apple
cannot decrypt the location as it’s not in possession of the
private key. While the company has not explicitly revealed how
often the key rotates, the rolling key pair architecture makes it
difficult for malicious parties to exploit the Bluetooth beacons to
track users’ movements.

But OWL researchers said the design allows Apple — in lieu of
being the service provider — to correlate different owners’
locations if their locations are reported by the same finder
devices, effectively allowing Apple to construct what they call a
social graph.

“Law enforcement agencies could exploit this issue to
deanonymize participants of (political) demonstrations even when
participants put their phones in flight mode,” the researchers
said, adding “malicious macOS applications can retrieve and decrypt
the [offline finding] location reports of the last seven days for
all its users and for all of their devices as cached rolling
advertisement keys are stored on the file system in cleartext.”

In other words, the macOS Catalina vulnerability (CVE-2020-9986)
could allow an attacker to access the decryption keys, using them
to download and decrypt location reports submitted by the Find My
network, and ultimately locate and identify their victims with high
accuracy. The weakness was patched by
Apple^[5] in November 2020
(version macOS 10.15.7) with “improved access restrictions.”

A second outcome of the investigation is an app that’s designed
to let any user create an “AirTag.” Called OpenHaystack^[6], the framework allows
for tracking personal Bluetooth devices via Apple’s massive Find My
network, enabling users to create their own tracking tags that can
be appended to physical objects or integrated into other
Bluetooth-capable devices.

This is not the first time researchers from Open Wireless Link
(OWL) have uncovered flaws in Apple’s closed-source protocols by
means of reverse engineering.

In May 2019, the researchers disclosed^[7]
vulnerabilities in Apple’s Wireless Direct Link (AWDL) proprietary
mesh networking protocol that permitted attackers to track users,
crash devices, and even intercept files transferred between devices
via man-in-the-middle (MitM) attacks.

This was later adapted by Google Project Zero researcher Ian
Beer to uncover a critical “wormable” iOS
bug^[8] last year that could
have made it possible for a remote adversary to gain complete
control of any Apple device in the vicinity over Wi-Fi.