A use-after-free vulnerability was found in systemd. This issue occurs due to the on_stream_io() function and dns_stream_complete() function in 'resolved-dns-stream.c' not incrementing the reference counting for the DnsStream object. Therefore, other functions and callbacks called can dereference the DNSStream object, causing the use-after-free when the reference is still used later.

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.