Testing for Captcha (OWASP-AT-012)

This article is part of the new OWASP Testing Guide v4.

Back to the OWASP Testing Guide v4 ToC:
   https://www.owasp.org/index.php/OWASP_Testing_Guide_v4_Table_of_Contents
Back to the OWASP Testing Guide Project:
   https://www.owasp.org/index.php/OWASP_Testing_Project

WARNING: Captcha protection is considered to be an ineffective security mechanism!

Most current used Captcha images can be easily cracked in a fully automated way using many commercial or opensource services. Commercial services are usually very cheap and provide a simple API for most programming languages.
It is not recommended to use Captcha protection for security-critical applications, in this case it is more suitable to use SMS text messages or OTP tokens instead.

Example of Google reCAPTCHA cracked in 7 seconds by commercial automated cracking service:

Price: US¢1.390
Uploaded: Sun Nov 17 20:03:13 2013
Solved: Sun Nov 17 20:03:23 2013
Text: 270 35524452

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.

Description of the Issue

Despite CAPTC

Black Box testing and example

Testing for Topic X vulnerabilities:
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Result Expected:
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References

Whitepapers
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Tools
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