Alban Crequy of Collabora Ltd. discovered a denial of service flaw in dbus-daemon.  If a client (C1) was prohibited from sending a message to a service (S1), and S1 was not currently running, then C1 could attempt to send a message to S1's well-known bus name, causing dbus-daemon to start S1.  When S1 had started and obtained its well-known bus name, the dbus-daemon evaluated its security policy, decided that it would not deliver the message to S1, and constructed an AccessDenied error.  Instead of sending that AccessDenied error reply to C1 as a reply to the denied message, dbus-daemon incorrectly sent it to S1 as a reply to the request to obtain its well-known bus name.  This would cause S1 to fail to initialize and exit, denying service to other legitimate clients of S1.

As well, this opens up the possibility of a side channel where, in an environment where C1 and S1 are untrusted and were administratively prohibited from communicating, S1 could use these incorrectly-directed error messages as a side channel to receive information from C1.  This scenario is only really relevant on systems where an LSM such as SELinux was used in a highly restrictive configuration, because in practice a system bus could typically communicate in other ways (such as non-D-Bus IPC mechanisms, files in /tmp, etc.).


Acknowledgements:

Red Hat would like to thank D-Bus upstream for reporting this issue.  Upstream acknowledges Alban Crequy of Collabora Ltd. as the original reporter.