The disclosure comes as messaging and other applications race to improve security and as Facebook addresses criticism for not safeguarding privacy.

WhatsApp's privacy feature allows iPhone users to require Touch ID or Face ID -- fingerprint or facial recognition -- to open the app, but users were able to bypass those log-in methods by using the iPhone's "share" function.

Users can set verification to be required immediately upon log-in, meaning they would need to supply Touch ID or Face ID each time they open WhatsApp, or at intervals of up to an hour, allowing them to toggle between apps.

The system fails when users select any interval option other than "immediately."

"We are aware of the issue and a fix will be available shortly. In the meantime, we recommend that people set the screen lock option to 'immediately,'" a WhatsApp spokesperson said by email.

Last month a user discovered a privacy flaw with Apple's FaceTime group video chat software, which allowed iPhone users to see and hear others before they accept a video call. Apple rolled out an iOS update to fix the issue.

Australian Associated Press