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Testing for Captcha (OWASP-AT-008)

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OWASP Testing Guide v3 Table of Contents

This article is part of the OWASP Testing Guide v3. The entire OWASP Testing Guide v3 can be downloaded here.

OWASP at the moment is working at the OWASP Testing Guide v4: you can browse the Guide here

Brief Summary

Captcha ("Completely Automated Public Turing test to tell Computers and Humans Apart") is a type of challenge-response test used by many web applications to ensure that the response is not generated by a computer. Captcha implementations are often vulnerable to various kinds of attacks even if the generated captcha is unbreakable. This section will help you to identify these kinds of attacks and propose possible solutions.

Description of the Issue

Implementation of good captcha mechanism can be very efficient against:

  • enumeration attacks (login, registration or password reset forms are often vulnerable to this kind of attacks - the attacker can in a short time gain a lot of valid usernames, phone numbers, etc)
  • automated sending of many GET/POST requests in a short time even if it is undesirable (e.g. SMS/MMS/email flooding)

(e.g. SMS/MMS/email messages, undesirable

  • any automated attacks than gain any sensitive information from the application

Captchas should not use:

  • as a CSRF protection (the attacker can still steal the generated captcha nad break it in a real time)

Many image captchas can be identified as weak by simple

  • decoded captcha is encrypted (usually by some "security-by-obscurity"

"home-made" algorithm) and this value is sent by client as a hidden field (yeah, it's unbelievable but some web applications really do it in this way). Often this can be easily decrypted by observing of multiple captcha values.

  • even if it is difficult to decrypt decoded captcha value, many captchas are

vulnerable to replay attacks (attacker simply send old values of encrypted decoded captcha value and decoded value of this captcha)

  • many captchas don't destroy the session when the correct phrase is entered -

by reusing the session id of a known captcha it is possible to bypass captcha protected page

  • many captchas can be identified as weak by simple comparison with already

broken captchas (e.g.,,

Black Box testing and example

Gray Box testing and example



Captcha Decoders
PWNtcha - opensource captcha decoder - Commercial captach decoder -

Breaking a Visual CAPTCHA -