Imagine the following scenario:

You work at Big Co. and your coworkers down the hall are on the web development team for Big Co's public blog system, which a lot of Big Co employees and some public people use. The blog system allows any HTML and JavaScript, and you've been told that it was a choice (not by accident) but you aren't sure if they realize the implications of this.

So you want to convince them that this is a bad idea. You write some demonstration code and plant a XSS script in your own blog, and then write some blog posts. Soon after, the head blog admin (down the hall) visits your blog post and the XSS sends his cookies to you. You copy them into your browser and you are now logged in as him.

Okay, now you're logged in as him... And you start realizing that it maybe wasn't such a good idea to go ahead and 'hack' the blog system. But you are a good guy! You don't touch his account after logging into it, and you definitely don't plan on publicizing this weakness; you just maybe want to show them that the public is able to do this, so that they can fix it before someone malicious realizes the same thing!

What is the best course of action from here?