I'm developing a Chrome plug-in that needs to access a REST web service that is hosted on the same domain as my web site. The web service requires that the user be logged in. The website provides account registration and authentication. I would like to be able access the web service from the plugin without requiring the user to log in again from the plugin when they have already logged in to the website. I'm not sure how to do this. I'm using the forms authentication provided by ASP.NET MVC4 for the website. And the Chrome plugin, of course, is using JavaScript's XMLHttpRequest client-side to call the web service.

There are many plugins that do this. 'Delicious Bookmarks Extension' and 'Add to Amazon.com Wishlist' are a couple examples.