Comment 4 for bug 1665160

It is not dbus which is asking for authentication here. That's not really a thing that happens.

What does happen is that some services that own names on the bus will check that a method's caller is authorised to make the call before performing the action. This is usually achieved with PolicyKit - I think that you're seeing a PolicyKit denial here.

A notable user is systemd - am I right in saying that this is a response from a dbus call to a systemd method (StopUnit?)? If so, this behaviour is certainly nothing new.

The relevant polkit action id is "org.freedesktop.systemd1.manage-units". It is callable by admins (unix group admin or unix group sudo) after they have authenticated with their password. If you call the method as root, you won't get prompted and the action will be performed.

I would look at any changes to how you run the tests (which user?), then any changes to the systemd stuff (is it new? is it now being called from a non-root process whereas before it wasn't?). Note that policykit will output to the journal when it authorises or denies acition requests, which might be helpful.