Researcher Reports Zero-Day "Sign in with Apple" Bug that Could Allow Full Account Takeover

Infosec researcher Bhavuk Jain has pocketed a handsome $100,000 from Apple”s bug bounty program after reporting a critical flaw that could have allowed malicious actors to bypass authentication and take over a user”s account.

Released to much fanfare at the annual Worldwide Developers Conference (WWDC) in 2019, “Sign in with Apple” enables users to log into a third-party account without disclosing their email address.

“In the month of April, I found a zero-day in Sign in with Apple that affected third-party applications which were using it and didn”t implement their own additional security measures. This bug could have resulted in a full account takeover of user accounts on that third party application irrespective of a victim having a valid Apple ID or not,” Bhavuk writes on his blog.

Apple”s sign-in feature is similar to OAuth 2.0 in that it authenticates a user either via a JSON Web Token or a code generated on Apple”s end. With the JWT generated, Apple generates its own relay ID which goes into the JWT for authentication. However, Bhavuk found that he could request JWTs for any email ID from Apple, “and when the signature of these tokens was verified using Apple”s public key, they showed as valid.”

“This means an attacker could forge a JWT by linking any Email ID to it and gaining access to the victim”s account,” he explains.

If exploited correctly, the vulnerability could have allowed full account takeover. A lot of third-party apps and services are already integrated with “Sign in with Apple,” including Dropbox, Spotify, Airbnb, and Giphy.

Apple reportedly awarded the researcher $100,000 for his discovery. Bhavuk responsibly reported the flaw to the iPhone makers last month and waited until the company patched the vulnerability on their end before publishing his findings.