An HTML5 browser exploit was found recently and it can fill your hard drive with junk data. Usually browsers allow websites to store small files like cookies but in case of HTML5 they allow bigger files around 10MB. This can be exploited and flood a hard drive with such files. In a demonstration, a 22-year-old web developer was able to fill 1GB of hard drive space every 16 seconds. I will let you stop and calculate how long it would take to fill your entire hard disk. I was happy to learn that every browser is vulnerable to this exploit except of my favorite Firefox. The web developer has created a website FillDisk.com that you can see this exploit yourself.
An HTML5 browser exploit can fill you disk with junk