Apple says it will fix an error that caused Siri to read hidden notifications from the lock screen to anyone who asked. Apple said in a statement to MacRumors: "We are aware of the problem and will address it in a future software update." The update could come in iOS 11.3, currently still in beta, or possibly in iOS 11.2.7 if Apple publishes a minor update
The original error allowed Siri to read screen lock notifications for third-party applications, even if you turn off message previews while the phone is locked. Siri did not check to make sure the owner of the phone was the person who was asking. IMessage messages and text messages were kept private from being read aloud, but Facebook Messenger, Slack, email applications such as Apple's own mail application, and more, all could have messages read by Siri. (And since the whisper mode has not yet reached Siri, even though Apple patents that technology, the voice can still be quite noisy).
Anyone can press and hold the home button on most iPhones or a side button for the iPhone X to activate Siri without voice recognition and request that notifications be read. The problem persists in iOS 11.3 beta and iOS 11.2.6 devices until Apple issues the solution.