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I'm in the process of adding CAPTCHA validation to one of my websites and need to know what open source solutions exist. Please note strengths and weaknesses and what platform they work with. I'm primarily interested in ASP.NET solutions but feel free to include PHP, Java, etc.

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  • thing is CAPTCHA is opensource... reCAPCHA is a service runned by google... you have almost no say in the graphic design of the service, even if it absolutely obtuse to your site design...
    – user661623
    Commented Mar 16, 2011 at 0:30

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ReCAPTCHA is the same one StackOverflow uses. It has an ASP.NET implementation.

It uses a webservice to provide the captcha images; this is an university trying to digitize difficult to OCR texts. I'm not sure how easy it would be to replace this portion. There are many open source client implementations, as the API is quite well documented.

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  • Has anyone ever found eny resources on how hard it is to crack (by OCR or other attempts) the captchas provided by ReCAPTCHA?
    – Nils Schmidt
    Commented Aug 17, 2011 at 9:27
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    ReCAPTCHA is owned by google now. Using it helps google to digitize books, and also now to build their geo address database from housenumbers in streetview photos. All of this is undeniably clever, but this may not be well aligned with the goals of some projects ...and it's not open source
    – Harry Wood
    Commented Jan 31, 2013 at 2:01
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Jeff Atwood has a captcha control + source hosted on his coding horror site

ReCaptcha might be another alternative but i don't think the source is open.

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  • blog post 404, wow, all these captcha projects are dead so we have to submit to our overlords?
    – akostadinov
    Commented Jun 27 at 9:04
  • you're commenting on a post that is 16 years old. there are still lots of active captcha projects incl. Recaptcha which has since been acquired by Google google.com/recaptcha/about
    – Eoin Campbell
    Commented Jun 28 at 12:25
  • Not really open source. But yes, I found some. Interesting is ALTCHA but there are some more.
    – akostadinov
    Commented Jul 1 at 14:55
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Have you seen JCaptcha? Not an ASP framework, but might be useful.

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  • Not updated since more than 10 years.
    – akostadinov
    Commented Jun 27 at 9:03
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Try this one... http://www.codeproject.com/aspnet/CaptchaImage.asp very nice article!

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One I just found but have not tried out is MyCaptcha

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  • Unavailable anymore.
    – akostadinov
    Commented Jun 27 at 9:02
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ReCaptcha has many benefits. The fact that google owns it is a big plus. We used to use ReCaptcha on my project, but the firewall in our production environment cannot handle add entries based on domain name, only based on IP addresses. ReCaptcha has switched to using Googles infrastructure, so the IP addresses are no longer publicly available. Our production environment is locked down extremely tightly, so there are no workarounds possible -- we have to start using a new captcha. If you're in my boat, ReCaptcha won't work for you.

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In the Java world, check out JCaptcha.

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  • Not updated since more than 10 years.
    – akostadinov
    Commented Jun 27 at 9:03
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Have you taken a look at ReCAPTCHA?

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