A security researcher whose Google Pixel battery died while he was composing a text message is probably grateful for the interruption, as turning it back on allowed him to find a lock screen bypass flaw that earned him a $70,000 bounty from Google.
The vulnerability, which has since been fixed, would have allowed anyone with a spare SIM card and access to a device to totally bypass the lock screen and gain full control of the device.
When turning on his Pixel 6 and forgetting his SIM’s PIN number, Hungarian security researcher David Schütz said he made the discovery. He then had to find the Personal Unlocking Key, or PUK, that would allow him to reset the PIN. His phone kept hanging on the “Pixel is starting” screen after a restart.
Schütz tried replicating the issue, but on one occasion he forgot to reboot the phone. “As I did before, I entered the PUK code and chose a new PIN. This time the phone glitched, and I was on my personal home screen,” Schütz said.
After a few additional attempts, Schütz said he was sure he had a “full lock screen bypass, on the fully patched [at the time] Pixel 6. I got my old Pixel 5 and tried to reproduce the bug there as well. It worked too.”
The problem stemmed from Android calling a .dismiss() function whenever the SIM PUK was reset. Schütz said that what Android seems to have done was to dismiss the screen prompting the PUK to be reset, while accidentally not sending that request until the PUK reset screen had already disappeared. Since the active security layer underneath was all that was left, Android dismissed it without realizing the mistake.
Schütz said Google triaged the issue quickly when he submitted it, but it then sat silently for several months. After asking for a follow up, he was told that the issue was a duplicate. Later on, Google admitted that, even though his bug was a duplicate, it was only because of his report that the company took action and patched it in Android’s November 5 security update.
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