Rails 2.0 has forgery protection against CSRF attacks built in. If you stick to standard rails helpers you won't need to take any extra steps and the forgery protection will just work - reason being, the helpers will automatically add the appropriate parameters. For example form helpers will a hidden tag with the name "form_authenticity_token" and a value "big stinking authenticity token."
If you step into the land of raw Ajax, however, you will need to take care of passing this value yourself (unless you are sending GET requests, you don't need to send the token with a GET request). So where do you get the value? You use the helper form_authenticity_token of course. But I don't recommend using this value directly as it can throw and exception if you call it with forgery protection turned off (in tests for example). Instead, write your own helper in app/helpers/application_helper.rb, something like this:
Now when you make an Ajax request you just need to make sure you provide this value under the name "authenticity_token". For prototype.js, you can stuff this along in the options to the family of Ajax request functions:
If you are using the amazing Ext JS, you might send it along with something like this:
Oh, also, if you want to turn off this forgery protection you can remove it entirely by commenting out protect_from_forgery in your main application.rb controller or disable / enable it on an action by action basis in a controller with:
And of course that takes either an :except or :only value.