The rather convoluted bypass method was shared in a video by Jose Rodriguez, who has discovered iOS bugs in the past that Apple has subsequently fixed.
With physical access to the locked device, the attacker first asks Siri to activate VoiceOver, sleeps the device with the Side button, and then calls the iPhone using another device. Once the call screen shows up, the attacker taps the Message button, opts to create a custom message, and then taps the plus (+) icon in the top right.
Next, on the other phone, the attacker sends a text or iMessage to the target iPhone, whose screen is then double-tapped when the message notification appears. This causes an odd behavior in the UI, since it highlights the plus icon underneath.
After a short wait, the screen goes white and the notification disappears, but the VoiceOver's text selection box is apparently still tappable and can now be used to access the Messages interface. Following multiple screen swipes, the VoiceOver is heard to say "Cancel," which reveals the original Messages screen.
Adding a new recipient to the message and selecting a numeral from the virtual keyboard then reveals a list of recently dialed or received phone numbers and contacts. Further, if one of the numbers or contacts includes an info ("i") button, disabling VoiceOver and tapping the button shows the contact's information. Performing a 3D Touch action on the contact also brings up call and message options, along with options to Add to Existing Contact or Create New Contact.
In a similarly complicated set of steps involving an invisible user menu, an attacker can eventually access a locked iPhone's Camera Roll and other photo folders, which can then be used to add profile pictures to contact cards.
The bypass methods work on all iPhones including the iPhone XS lineup, but Apple doesn't appear to have fixed the vulnerabilities in the latest iOS 12.1 beta. Thankfully however, all of the above can be easily prevented by disabling access to Siri from the lock screen.
Concerned users can do so by navigating to Settings > Face ID & Passcode (that's Settings > Touch ID & Passcode on iPhones with Touch ID) and disabling the Siri toggle under the "Allow access when locked" menu.