User blog comment:Rappy 4187/Technical Update: January 28, 2014/@comment-1277243-20140130071755/@comment-11733175-20140131011154

As mentioned in the release notes, neither .pdf, .djvu, or .djv file extensions are enabled by default in MediaWiki installations. On Wikia, .pdf does appear to be enabled by default although the PdfHandler extension is not installed. The release notes seem to suggest (to me anyway) that the combination of .pdf being enabled and the extension being installed is a cause of the weakness, not just the file type being enabled.

There are better people than me to answer your question, but I would think there is only an issue for your wiki if the extension is installed or .djv/.djvu files are enabled. Reading through the report on wikimedia's bugzilla suggests it's something to do with incorrectly escaped shell command arguments which affects image thumbnails. If your wiki does not meet either of the criteria then I doubt there's anything to worry about. I believe PdfHandler was only recently approved for use on Wikia wikis (within a year or so) and I doubt it's found in many places even now.

As for the update being rolled out on Wikia, judging by the importance assigned to it on bugzilla I'd expect to see it within a few days if not sooner.