In the past day or so, a serious security issue was uncovered in two sliders which come packaged with a variety of themes sold on ThemeForest. The sliders were also available for sale on their own through CodeCanyon. The sliders in question are Revolution Slider and Showbiz Pro.
Envato (who run ThemeForest and CodeCanyon) have written a blog post about which themes are affected and what to do if you have one of them. They are keeping the post updated with the most recent information, so you should look there for the latest information on what to do if you have either of these sliders.