Every OS has security problems, and every new OS incorporates fixes to them. If you're looking for specifics, a quick Google search can help there: http://www.csoonline.com/article/3044089/data-protection/the...

Some of these: http://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=windows+8.1 though I'll grant most of the ones on the front page at least are basically just "everything from 7 on".