How can spammers manage to circumvent captcha?

Question

I have put captcha on my blog, I still get spammers, is there a script somewhere which allows them to do this or do they do this by hand ?

Answer 1

It depends on what type of CAPTCHA you're using. Some methods for generating CAPTCHA challenges are easily circumvented with optical character recognition. Some methods have inherent flaws that let spammers through without ever passing the challenge.

"Secure" or "good" CAPTCHA schemes that haven't yet been beaten by automated means can still be beaten by humans. One popular technique is to let the spamming software retrieve the challenge and then display it on a different website where unsuspecting humans solve it in order to gain access to some other resource.

Finally, some spammers just enter solutions by hand, because they're just that determined to annoy you.

Wikipedia has a good article on CAPTCHAs including their circumvention.