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Apple AirDrop Flaws Could Let Hackers Grab Users' Phone Numbers and Email Addresses

Graham CLULEY

April 23, 2021

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Apple AirDrop Flaws Could Let Hackers Grab Users' Phone Numbers and Email Addresses

Users of Apple products have long loved the ability to wirelessly share files with each other, using AirDrop to transmit files between their iPhones and Macbooks.

But researchers at the Technical University of Darmstadt in Germany have discovered that security weaknesses could allow an attacker to obtain a victim’s phone number and even email address.

And you know what’s worse? Apple hasn’t fixed the problem almost two years after being told about it, despite 1.5 billion devices worldwide being potentially vulnerable.

The researchers’ paper, entitled “PrivateDrop: Practical Privacy-Preserving Authentication for Apple AirDrop”, details what it describes as “two severe privacy vulnerabilities in the underlying authentication protocol” used by AirDrop.

According to the paper, the problem lies in how AirDrop determines if a nearby device belongs to somebody the user already knows.

To discover if two devices belong to mutual contacts, AirDrop transmits a SHA-256 hash of the sending user’s email address or phone number. Other devices in the vicinity examine the hash, and compare it to entries in their own address book – if a mutual match is made, the receiver sends back their own hash.

An attacker can brute-force the hash to determine users’ phone numbers – a technique which takes just seconds because of the relatively small number of possible phone numbers.

Email addresses are more complicated to easily reverse, but the researchers believe attackers could have some success if they used dictionary attacks that use common email formats (such aas firstname.lastname@gmail.com, yahoo.com, and so forth). In addition, hashed email addresses could be derived using data from past data breaches.

Responsibly, the researchers disclosed the flaw to Apple privately in May 2019, hoping that it would be fixed. Apple responded in July 2020, saying that it did “not have any updates on new features or any changes to mitigate the underlying issue.”

It’s worth remembering that for an attack to be successful, a malicious party would need to be in close physical proximity to their victims. And yes, there are probably easier ways to determine someone’s phone number rather than through this route – but that’s no reason not to harden the security of AirDrop.

Perhaps frustrated by Apple’s response, the research team developed its own proof-of-concept solution for AirDrop’s flawed design, which they called “PrivateDrop.” However, the researchers admit that the only practical way for it to be used in place of AirDrop is if Apple themselves integrated it into their devices’ operating systems.

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