I’m sure you must be aware of the security flaw that bypasses your passcode and allows anyone to access the photos and contacts on your iPhone running iOS 12. This seems to work by issues with Siri. I’m sure Apple will fix the flaw in the next iOS update but for now, the steps to protect your device is actually quite simple.
If you have an iPhone X or newer then you are ok because the technique doesn’t seem to work on iPhones with Face ID enabled. Any other iPhone you will have to go into Settings, then tap Touch ID & Passcode. Then, under ‘Allow Access When Locked’ switch Siri off.
That’s it you should now be protected from this flaw, that’s until the next one comes up of course.