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    Response Processors: Request Captcha Processor: Incident - Captcha Image Probing

    Complexity: Low (2.0)

    Default Response: 1x = Warn User. 2x = 5 Day Block.

    Cause: A captcha is a special technique used to differentiate between human users, and automated scripts. This is done through a Turing test, where the user is required to visually identify characters in a jumbled image and transcribe them into an input. If the user is unable to complete the challenge in a reasonable amount of time, they are not allowed to proceed with their original request. Because it is nearly impossible to script the deciphering of the image, automated scripts generally get stuck and cannot proceed. Additionally, an audio version is optionally available to allow users who have a visual handicap to complete the captcha successfully. Captchas are used in two different ways by the system. They can be explicitly added to any workflow within the protected web application (such as requiring a captcha to login, or checkout a shopping cart), and they can be used to test a suspicious user before allowing them to continue using the site (similar to blocking the user, but with a way for the user to unblock themselves if they can prove they are not an automated script). Captchas are generally used to resolve "Insufficient Anti-Automation" weaknesses in the protected web application. Regardless of which type of captcha is being used, this incident is generated when the user attempts to request a captcha image file for a request that is not being protected by a captcha.

    Behavior: In order to find a way to bypass the captcha mechanism, attackers will often attempt to collect a large number of captcha images for offline analysis. If the attacker can find a pattern in how the captcha images are issued, or how the filename relates to the value in the image, then they can effectively bypass the captcha mechanism at will. In this case, the attacker is guessing arbitrary captcha image filenames, but is attempting to keep the format of the names consistent with known captcha image URL's. Because the filename used and the values in the image have no correlation, this technique will not be successful and will simply waste the attacker's time and resources.

    Published: 2013-11-20