Update: OnePlus has issued a statement which you'll find in full below.
In a video posted to Twitter, @rikvduijn shows the phone being unlocked by a cut out photo of his face, held in front of his friend’s face. Worse still, it apparently works even if the photo is black and white.
You can see in the video that it appears to take a few attempts to unlock the phone with a photo, but that still leaves it worryingly insecure.
In a statement, OnePlus has said: "We designed Face Unlock around convenience, and while we took corresponding measures to optimise its security we always recommended you use a password/PIN/fingerprint for security. For this reason, Face Unlock is not enabled for any secure apps such as banking or payments. We’re constantly working to improve all of our technology, including Face Unlock."
The Twitter user who posted the video also claims to have heard from OnePlus that the company is now investigating the issue, so perhaps a patch will be issued, but there’s only so much a software update will be able to do, so we wouldn’t count on face unlock on the OnePlus 6 ever being anywhere close to as secure as a fingerprint scanner.
Still, as the statement above notes, fingerprint scanning is at least an option, which it isn’t on the iPhone X, and the OnePlus 6 isn’t alone in being fooled by a photo, as the Samsung Galaxy Note 8 is similarly easy to trick.