Ironically, the Apple Messages app seems to be somewhat immune from the bug because you need to say "Ok Siri read my messages". As we highlighted yesterday, a newly discovered Siri bug allows anyone to access hidden notifications on a device's lockscreen simply by asking Siri to read them aloud. However, with the bug within iOS 11.3, anybody can ask Siri things like "read my last note" or "read my last email" without any security blocks. But Siri can read out messages, even with this setting set to "When Unlocked", which can be a privacy faux pas, since anyone will be able to hear your messages.
It's particularly concerning that this affects Signal notifications, as the app is primarily favored for its encrypted messaging features that provide extra security. However, we have also experienced that Apple fixes its bugs and loopholes pretty quickly, patching whatever vulnerability or exploit which is present in the operating system.
Fortunately, any messages or notifications sent to Apple's own Messages app, iMessage, are still protected. It is unclear when the update will be released and whether or not users will have to wait until iOS 11.3 or if it could be released sooner. There is a toggle there that reads "Allow Siri When Locked".
Under the "When Unlocked" setting, you can choose from "Always" and 'Never'. This is a serious bug since it effectively removes a layer of security.
The digital assistant can create reminders, schedule calendar events and retrieve information on restaurants and weather.