How to defeat CAPTCHA systems

How to defeat CaptchaA captcha (an acronym for "completely automated public Turing test to tell computers and humans apart") is a type of challenge-response test used in computing to determine whether or not the user is human.If you are planning on implementing a CAPTCHA system (

If you plan on using a CAPTCHA system, you must be aware of the numerous ways to defeat such a system. Apparently one of the guys that are constantly defeating CAPTCHA is Sam Hocevar . His PWNtcha website presents a huge collection of CAPTCHA systems which have been or are in the process of being defeated.

Sam designed a tool (closed source for good reasons) which is able to analyze and decode even the trickiest images.

PWNtcha is simply a toolkit of image manipulation functions, and a list of known CAPTCHAs with the associated list of image operations to apply in order to decode each of them. If I have never seen your CAPTCHA, then PWNtcha does not know about it, and there is absolutely no way it could decode it.

Update May 29:
A vulnerability has been reported in the CAPTCHA plugin for Geeklog, which can be exploited by malicious people to disclose sensitive information or to compromise a vulnerable system. Secunia has the details.

Are you aware of any CAPTCHA system which is not listed on this site ?


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3 Responses to “How to defeat CAPTCHA systems

  • 1
    Nathan - Money Game
    April 27th, 2009 11:13

    I’ve never heard about such a long decryption of this word, so thank’s very much for the briefing. Any support in the battle against spam-bots can be useful and quite valuabe.

  • 2
    August 20th, 2009 07:40

    if you de-couple your CAPTCHA image generating code from the image displayed on the device (via encryption) you will have a very high degree of safety against this specific system designed to defeat CAPTCHA.

  • 3
    Dan Houser
    March 2nd, 2011 09:15

    J-Captcha isn't listed on that site.  Wondering how good it is…

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