Q:

Preventing Spoofing with Paypal

I have a scenario where i need to have a user checkout through PayPal and then once we receive the payment then we create a membership record in our membership site.  The PayPal requires a "return" field when you submit the form button that will take the user back to our page once the order has been successfully processed.  On that page is where we actually track the sale and fire a chunk of javascript to track the sale.
So my question is this, how can i make sure that the return page comes from PayPal and is not spoofed by a user in the system.  I know there is a way to use the notify_url to have PayPal post back to our system, but in this case i have to run a chunk of javascript.  Is there an easy way to do this, or would i have to write some looping/timeout function that checks to see if the notify post has come through?
Im using .net to do the posting to paypal

A:

You need to have PayPal call a script on your site in order to prevent spoofing. Anything that goes through the client/browser is open to the user for modification, so only things that come directly to your server from PayPal are secure.

