title: How to check the post variables sent to return URL is in fact sent by PayPal?
body: <p>I have a problem, I have implemented PayPal IPN, but it takes time, user want to see immediately his pay status. So I need to implement using the post variables returned to return url, but how do I check the variables received are really sent by PayPal, because any body can create a form and send the variables to return URL. I don't want my application to be vulnerable.</p>