An industry source in Seoul told the Korea Herald that the phones could be unlocked by the face of a sleeping person, or even just by a photo.
"For now, the facial recognition technology is only intended for fun. It should not be considered as a foolproof security measure," the source said.
Spanish phone expert MarcianoTech demonstrated the use of a photo to fool the facial recognition on the phone, at the New York launch on Thursday morning, Australian time.
Tricking facial recognition software with photos is one of the first things that it is tested when devising such biometric security features.
The S8 and S8+ also have fingerprint and iris screening.