X.Org has disclosed a long list of vulnerabilities that have been fixed in
the X Window System client libraries; most of them expose clients to
attacks by a hostile server. "Most of the time X clients & servers
are run by the same user, with the server more privileged from the clients,
so this is not a problem, but there are scenarios in which a privileged
client can be connected to an unprivileged server, for instance, connecting
a setuid X client (such as a screen lock program) to a virtual X server
(such as Xvfb or Xephyr) which the user has modified to return invalid
data, potentially allowing the user to escalate their privileges."
See the advisory for details.