12.17
Summary
The new Google Groups interface contained an XSS vulnerability in its search functionality. The vulnerability required some user interaction to be activated.
How Did It Work?
The search box at the top of the new interface is designed to provide some type-ahead functionality: as the user types, his/her input is scanned and used to create a drop down menu of choices. Unfortunately, user input was not properly sanitized; for instance, typing <u> into the search box caused drop down menu items to become underlined. As a result of this oversight, it was possible to type arbitrary HTML/JavaScript into the search box and have it executed by the page.
The new interface’s type-ahead functionality, hard at work
My next step was to find a way to pass a malicious search string to a user. As it turned out, the interface provided a way to link to search results: visiting https://groups.google.com/forum/?fromgroups#!searchin/googlegroups-announce/<script>alert(document.cookie)<$2Fscript> would put <script>alert(document.cookie)</script> into the user’s search box.
Executing that malicious string required a little user interaction, however. The drop down box (and accordingly, the XSS) would only be activated when the user interacted with the search field. It was possible for a user to avoid the XSS by clicking in the box, highlighting the text, and deleting the entire string (or changing a character in the string so that the script failed to run). However, any other kind of interaction would have triggered the script’s execution.
More Information
The vulnerability mentioned here has been confirmed patched by the
Google Security Team. I owe them a ton of thanks for organizing this
program and giving me a chance to improve my skills. 
Interested readers are encouraged to take a look at other vulnerabilities I’ve reported under Google’s Vulnerability Reward Program.