I’ve a forum where anonymous is allowed to post, protected by CAPTCHA. For users convenience, I set a Cookie for such a user which lasts about a month so the user does not get the CAPTCHA over and over again. In the simplest form the cookie is called no_captcha_for_one_month and it’s value is 1. When the user returns and posts anonymously, he gets not CAPTCHA.
Anyone seeing the flaw? A forum spammer just needs to fill out the CAPTCHA correctly once and use the cookie information for his bot and there he goes.
I thought about getting creative and using a server-side hash which includes e.g. users IP address and some secret salt to generate the cookie value, but it would still be valid for this IP address, of course.
Someone I get the impression the question is silly and I try to solve something unsolvable.
with such a solution it is always possible to use the cookie for a bot. no matter what you try.