Researchers Create Covert Channel Over Apple AirTag Network

The peer-to-peer network used by AirTags, Apple's just-released tracking and location devices, could be used by anyone to send small amounts of data, essentially creating a covert channel not limited to Apple's hardware, according to a recent analysis by two security researchers.

On April 30, Apple released its AirTags, button-sized devices that allow Apple customers to track and locate important possessions, such as backpacks and bikes. The devices use Apple's Find My network — a peer-to-peer mesh network of Apple devices — to send location data back to the owner. By analyzing Apple's new AirTags, however, two security researchers at consultancy Positive Security discovered that Apple's Find My network can be used to send data from non-AirTag devices as well, albeit at low data rates of a few bytes per second.

The covert channel is mostly of academic interest because the bandwidth is so low, but it could present attackers with a viable low-cost way to exfiltrate data from a corporate environment, says Fabian Bräunlein, co-founder at Positive Security.

"I think in certain situations it could be used to exfiltrate data from corporate settings," he says. "An example could be a USB keylogger that immediately broadcasts the sniffed keystrokes for upload. In most of those scenarios, however, a more expensive device with a SIM card and modem [could] also be a viable alternative."

Less than two weeks after Apple's released its AirTag $30 tracking and location tags, hardware hackers and security researchers have already found vulnerabilities and alternative uses for the devices. Hardware hacker Colin O'Flynn, for example, tore down the AirTags and mapped out the components. Another hardware hacker used the information to dump the firmware and modify it to send an Apple device an attacker-controlled URL.

In a May 12 blog post, Positive Security's Bräunlein and his co-founder Lukas Euler described experiments in trying to send data using the Find My network by broadcasting via Bluetooth Low Energy (BLE) to nearby Apple devices. The data can be retrieved from Apple's servers to reconstruct the data. The technique could be used to exfiltrate data from air-gapped sites, if someone enters with an iPhone or other Apple devices, the researchers wrote in their analysis.

The data encoding mechanism for communicating over the Find My network had to take into account the unreliable nature of its communications, with packets arriving out of order and dropped packets a real issue, they wrote.

"There is no guarantee as to when or whether at all specific broadcasts are uploaded to the Apple backend as location reports," they said. "This is because some packets might not reach any Apple device and the Finding devices can have highly variable delays between receiving a broadcast and uploading the location report, e.g., depending on their upstream connectivity or power mode."

The most likely use case of such a network is uploading data from low-powered, low-cost sensors, obviating the need for a cellular or Wi-Fi connection. This usage would reduce the cost of such sensors at the price of increasing the parasitic bandwidth used for Apple's Find My network. 

The covert communication channel is possible because of Apple's design of the network, which focuses on availability but also privacy. The system is designed to use data economically, so as to not overload connections between devices. Apple cannot read the packets, due to its pro-privacy design, so does not know when data is a location or exfiltrated data, the researchers stated.

"It turned out that security and privacy decisions in the design of the offline finding mechanism make our 'use case' quite efficient and almost impossible to protect against," Bräunlein says.

With the bandwidth of the BLE specification completely used up by Apple's design and AirTags using both low power and lacking a connection to the Internet, preventing communications over the network would be difficult, according to the researchers.

"I think Apple could implement some additional mechanisms such as rate limiting but it's unlikely that this misuse will be completely protected against," Bräunlein says.

A covert communication channel is not the only potential threat. By using a variety of devices to send data to a particular phone, an attacker could potentially use up a mobile data plan. Such a threat might be less likely as a targeted attack, and more likely if a variety of actors start pervasively using Apple's Find My network for low-bandwidth communications.

Overall, the threat posed by misuse of Apple's Find My as a communication channel currently remain low, Bräunlein says.

"For most company's threat models, it should not be relevant," he says. "Companies that allow iPhones around their air-gapped systems however might want to reconsider that."