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

Complexity: Low (2.0)

Default Response: 1x = Warn User. 2x = Slow Connection 2-6 seconds and 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 an arbitrary file (not a captcha image, but something else) from within the same fake directory as the captcha images are being served from.

Behavior: When attempting to either bypass the captcha mechanism, or find a vulnerability in the server, attackers will often try finding unlinked resources throughout the website. The captcha mechanism uses a fake directory in order to serve the images and audio files that contain the captcha challenge. If the attacker is requesting an arbitrary file within the same fake directory, they are likely trying to find a "Predictable Resource Location" vulnerability. For example, the attacker might be trying to find a source file in the captcha serving directory in hopes of actually being able to get the source code behind how captcha images are generated. Because the directory is fake, the attacker will never find any of the resources they are looking for.