This is definitely something Apple needs to fix in the next version of iOS 11.
A privacy bug currently allows someone to access hidden message notifications on a locked iPhone by asking Siri to read them.
As of iOS 11, users can set message notifications to be hidden on the iOS’ Lock screen, wherein you’ll see that a message has arrived, but its content is hidden. That setting is on by default on the iPhone X, where simply glancing at it will unlock the phone and allow the content to be displayed, while it can be set on other models.
Recently, a user discovered that Siri can be asked to read the content of the hidden message.
Where Apple’s own Messages app is concerned, Siri will only read the message after the iPhone has been unlocked. Unfortunately, for third-party apps like WhatsApp, Telegram and Skype, Siri will read the content of the messages to anyone who picks up the locked phone.
The site verified that the bug is in both iOS 11.2.6 and the beta of iOS 11.3. It has been reported to Apple.
To hide or show message content on the Lock screen, go into Settings > Notifications, choose the app, select Previews and then choose your option.
Stay tuned for additional details as they become available.
Via 9to5Mac and Mac Magazine