From http://www.x.org/wiki/Development/Security/Advisory-2013-05-23:

Ilja van Sprundel, a security researcher with IOActive, has discovered
a large 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.

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 & 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.

Fixes are available in git commits listed on
http://www.x.org/wiki/Development/Security/Advisory-2013-05-23

Fixes will also be included in these module releases from X.Org:
     libX11 1.5.99.902 (1.6 RC2)
     libXcursor 1.1.14
     libXext 1.3.2
     libXfixes 5.0.1
     libXi 1.7.2
     libXinerama 1.1.3
     libXp 1.0.2      
     libXrandr 1.4.1
     libXrender 0.9.8
     libXRes 1.0.7
     libXv 1.0.8
     libXvMC 1.0.8
     libXxf86dga 1.1.4
     libXxf86vm 1.1.3
     libdmx 1.1.3
     libxcb 1.9.1
     libFS 1.0.5                                                         
     libXt 1.1.4
or releases to be determined from our sister projects:
     xf86-video-openchrome    OpenChrome project - http://www.openchrome.org/
     Mesa                     Mesa3D project - http://www.mesa3d.org/