Tobias Stoeckmann from the OpenBSD project has discovered a
number of issues in the way various X client libraries handle
the responses they receive from servers, and has worked with
X.Org's security team to analyze, confirm, and fix these issues.
These issue come in addition to the ones discovered by Ilja van
Sprundel in 2013.
Most of these issues stem from the client libraries trusting
the server to send correct protocol data, and not verifying
that the values will not overflow or cause other damage. Most
of the time X clients and servers are run by the same user, with
the server more privileged than 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.