Response Processors: Request Captcha Processor: Incident - Captcha Signature Tampering

Complexity: High (4.0)

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

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 provides a solution to a captcha which is correct, but they have modified the integrity checking signature passed along with the captcha solution.

Behavior: When a hacker is attempting to establish an automated script that is capable of defeating the captcha, they can use various different techniques. One of these techniques is to try changing various values used by the web application in the captcha mechanism in an effort to see if an error can be generated, or an unexpected outcome can be achieved. This type of probing and reverse engineering is generally performed by advanced hackers. Depending on the value they submitted for the original request data, this can also fall under one of the other attack categories involving manipulating general inputs, such as a "Buffer Overflow", "XSS", "Denial of Service", "Fingerprinting", "Format String", "HTTP Response Splitting", "Integer Overflow", or "SQL injection" attack among many others.