How to secure Ajax link requests?

Question

Imagine the next scenario: a user wants to register to a webpage and fills a form. While he is filling the form, jQuery keeps checking through a regular expression if fields

Answer 1

First of all: I'd URL-encode the mail-address. 'example.com/check.php?email=' . urlencode(abcdefg@gmail.com)

Ad your question: when check.php is called, you can

• check, if the user's session and his IP have sent a request during the last seconds
• if not, write the user's session, the user's IP plus the current timestamp to a helper-table plus to a cookie and hit your DB
• if yes, block the request

But I'm afraid this won't help you from fraud because everyone can check your JavaScript and if someone want's to exploit this, he will find ways.