In systemd 240, bus_open_system_watch_bind_with_description in shared/bus-util.c (as used by systemd-resolved to connect to the system D-Bus instance), calls sd_bus_set_trusted, which disables access controls for incoming D-Bus messages. An unprivileged user can exploit this by executing D-Bus methods that should be restricted to privileged users, in order to change the system's DNS resolver settings.