A flaw in the Samsung Galaxy S10 smartphone means any fingerprint can unlock it.
The tech giant has promised a software patch that will fix the problem.
The issue was spotted by a woman whose husband was able to unlock her phone with his thumbprint when it was stored in a cheap case.
When the S10 was launched, in March, Samsung described the fingerprint authentication system as "revolutionary".
Today the company said it was "aware of the case of S10's malfunctioning fingerprint recognition and will soon issue a software patch".
The scanner recognises users by using ultrasounds to detect 3D ridges of fingerprints.
Previous reports have suggested certain screen protectors are incompatible with the device's reader due to a small air gap which interfered with the scanning.
Biometric scanners such as fingerprint and facial recognition systems have increasingly featured on smartphones because they are seen as less vulnerable and harder to discover or copy than passwords or number-based unlock codes
Meanwhile, in the fallout of the security flaw, the online-only South Korean KaKao Bank has urged its customers to disabled the fingerprint recognition option - which can be used to log on to its services - until Samsung has announced a solution.
