A Researcher Figured Out How to Reveal Any Phone Number Linked to a Google Account

This article was produced with support from WIRED.

A cybersecurity researcher was able to figure out the phone number linked to any Google account, information that is usually not public and is often sensitive, according to the researcher, Google, and 404 Media’s own tests.

The issue has since been fixed but at the time presented a privacy issue in which even hackers with relatively few resources could have brute forced their way to peoples’ personal information.

“I think this exploit is pretty bad since it’s basically a gold mine for SIM swappers,” the independent security researcher who found the issue, who goes by the handle brutecat, wrote in an email. SIM swappers are hackers who take over a target’s phone number in order to receive their calls and texts, which in turn can let them break into all manner of accounts.

In mid-April, we provided brutecat with one of our personal Gmail addresses in order to test the vulnerability. About six hours later, brutecat replied with the correct and full phone number linked to that account.

“Essentially, it’s bruting the number,” brutecat said of their process. Brute forcing is when a hacker rapidly tries different combinations of digits or characters until finding the ones they’re after. Typically that’s in the context of finding someone’s password, but here brutecat is doing something similar to determine a Google user’s phone number.