Oops! With just a few taps, Siri allows access to all your contacts even if your phone is locked with a passcode! The following video shows how someone could gain access to ALL your contacts even if they don’t know your four-digit passcode.
To stop this from happening to you, you should disable Siri from operating from your phone’s lockscreen.
1) Go to Settings / Touch ID & Passcode (depending on your iOS version, it might just say “Passcode”).
2) Enter your passcode.
3) Scroll to Allow Access When Locked and now toggle off Siri.
For more details on the hack, see this article from NakedSecurity, “Open the iPhone door, Siri!”
