Univention Corporate Server 4.2 erratum 418 (security)

This update addresses the following issues:
* Xwayland allows unconditional open access to display (CVE-2015-3164)
* timing attack against MIT Cookie (CVE-2017-2624)
* Stack overflow in the endianness conversion of X Events (CVE-2017-10971) 
* Unitialized data in endianness conversion in the XEvent handling
  (CVE-2017-10972)
* Unvalidated extra length in ProcEstablishConnection (CVE-2017-12176)
* Unvalidated variable-length request in ProcDbeGetVisualInfo
  (CVE-2017-12177)
* wrong extra length check in ProcXIChangeHierarchy (CVE-2017-12178)
* Integer overflow and unvalidated length in (S)ProcXIBarrierReleasePointer
  (CVE-2017-12179)
* unvalidated lengths in XFree86 VidModeExtension (CVE-2017-12180)
* unvalidated lengths in XFree86 DGA extension (CVE-2017-12181)
* unvalidated lengths in XFree86 DRI extension (CVE-2017-12182)
* unvalidated lengths in XFIXES extension (CVE-2017-12183)
* unvalidated lengths in XINERAMA extension (CVE-2017-12184)
* unvalidated lengths in MIT-SCREEN-SAVER extension (CVE-2017-12185)
* unvalidated lengths in X-Resource extension (CVE-2017-12186)
* unvalidated lengths in RENDER extension (CVE-2017-12187)
* Insufficient validation of shmseg resource id (CVE-2017-13721)
* Global buffer overflow in xkbtext.c (CVE-2017-13723)