I’ve helped a colleague of mine work on a cross domain widget we are building for our applications that allows a user from an application to add information to another application through a pop up widget. Implementation wise, we used a  a simple form implemented using ExtJS FormPanel class. Some of the form fields like State and County dropdowns are  dynamically loaded after rendering, again using ExtJS’s ScriptTagProxy for cross-domain retrieval, no issue here and for more information on cross-domain data retrieval you can check out this post here.

The issue rose when it came to posting the data back to the server on another domain for processing. I did a bit of googling around and stumbled upon this post which explains how it can be done using a combination of server side programming and a little bit of Javascript and browser knowledge. To make a long story short, basically you can post data to a remote domain, and get data back by using the window.name property of an hidden iframe within your form page. Simply ingenious if you ask me,  I knew GWT was using the technique but did not know the the inner workings. Check out this post here for the actual implementation.

By the way if you a looking for a good XDM library, check out EasyXDM.

Addendum: As noted by Oyvind Sean Kinsey in the comments below, what i was trying to accomplish here was XDM, Cross Domain Messaging and not XSS Cross Site Scripting which is actually a security exploit. Thanks again Sean for the clarification, i understand now why the EasyXDM library was renamed from being initially called EasyXSS.