Univention Corporate Server 4.3 erratum 509 (security)

This update addresses the following issues:
* Cross-origin theft of images with ImageBitmapRenderingContext
  (CVE-2018-18511)
* Out of bounds read in Skia (CVE-2019-5798)
* Use-after-free in png_image_free in png.c (CVE-2019-7317)
* Cross-origin theft of images with createImageBitmap (CVE-2019-9797)
* Memory safety bugs fixed in Firefox 67 and Firefox ESR 60.7 (CVE-2019-9800)
* Type confusion with object groups and UnboxedObjects (CVE-2019-9816)
* Stealing of cross-domain images using canvas (CVE-2019-9817)
* Compartment mismatch with fetch API (CVE-2019-9819)
* Use-after-free of ChromeEventHandler by DocShell (CVE-2019-9820)
* Use-after-free in XMLHttpRequest (CVE-2019-11691)
* Use-after-free removing listeners in the event listener manager
  (CVE-2019-11692)
* Buffer overflow in WebGL bufferdata on Linux (CVE-2019-11693)
* Theft of user history data through drag and drop of hyperlinks to and from
  bookmarks (CVE-2019-11698)