They should be enough to control the problem until Apple rolls out a bug fix.
Since Siri is not equipped with the capability to distinguish between different voices, it gives a free reign to anyone with access to an iPhone to command Siri to read out notifications. The operation was possible even though you have set your notifications hidden on the lock screen.
Apple is always quick to come up with fixes for bugs that are highly publicized or those that gain a lot of media attention. In a statement on the matter provided to MacRumors, Apple said: "We are aware of the issue and it will be addressed in an upcoming software update". 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 for other apps such as third-party email apps, Facebook Messenger, Slack, and many more, Siri will just spout off details about the notification without checking that it's the right person asking to hear them.
As pointed out by MacRumors, Apple iPhone X users have a reason to be anxious by this bug.
The bug might be particularly concerning to iPhone X users, as there is a feature that hides notifications away from other users.
Thankfully there are some basic settings that can prevent Siri from reading out your personal messages without authentication, but it takes a toll at some of your convenience.
We accept that there are regulatory reasons why a lockscreen isn't allowed to lock out absolutely everything: in an crisis, you want to be able to dial 112, 911, 999, 000 or whatever the relevant emergency number is without fumbling your way through an unlock code first. To do so, go to Settings Siri & Search Disable Allow Siri When Locked. From what I can tell, as long as they don't show up on the lock screen, Siri will say that you don't have any new notifications and ignore those arriving in the background.