The advisory recommends that users running affected versions, which is essentially anyone using Ruby on Rails, upgrade immediately to one of the fixed versions, 3.2.10, 3.1.9 or 3.0.18.
The vulnerability lies specifically in the Ruby on Rails framework, and it's presence doesn't mean that all of the apps developed on vulnerable versions are susceptible to the bug.
"The Rails session mechanism allows storing arbitrary Ruby objects, including hashes with symbol keys. Rails provides a variety of session stores, the default being the cookie store which stores session data in a cookie on the client. The cookie data is not encrypted, but is signed with an HMAC [hash-based message authentication cookie] to prevent tampering. The cookie store is fast, does not require any server-side maintenance, and is only meant for session data that do not contain sensitive information such as credit card numbers. Apps that store sensitive information in the session should use the database session store instead. Nevertheless, it turned out that 95% of all Rails apps only ever store the user authentication credentials in the session, so the cookie store was made the default," Hongli Lai of Phusion wrote in an analysis of the problem.