The AddToAny Share Buttons WordPress Plugin was, until recently, vulnerable to a DOM-based cross-site scripting issue. The file in question is hosted on the author’s site, so you’re not vulnerable anymore (you’re welcome). If you just want the vulnerability details, go here. Now for the story of this bug.
This particular journey started with an email from XSSposed, a site intended to allow security researchers to responsibly report cross-site scripting issues in sites that don’t run formal bug bounty programs like those seen here. Having never heard of it before, I was immediately suspicious. I like the concept of bug bounties just fine, but I was a little surprised to learn that I had unknowingly opened one on my site.

