Univention Corporate Server 4.3 erratum 415 (security)

This update addresses the following issues:
* Use-after-free in file transfer extension server code allows for potential
  code execution (CVE-2018-6307)
* Use-after-free in file transfer extension allows for potential code
  execution (CVE-2018-15126)
* Heap out-of-bounds write in rfbserver.c:rfbProcessFileTransferReadBuffer()
  allows for potential code execution (CVE-2018-15127)
* Multiple heap out-of-bound writes in VNC client code (CVE-2018-20019)
* Heap out-of-bound write inside structure in VNC client code allows for
  potential code execution (CVE-2018-20020)
* Infinite loop in VNC client code allows for denial of service
  (CVE-2018-20021)
* Improper initialization in VNC client code allows for information
  disclosure (CVE-2018-20022)
* Improper initialization in VNC Repeater client code allows for information
  disclosure (CVE-2018-20023)
* NULL pointer dereference in VNC client code allows for denial of service
  (CVE-2018-20024)