Response Processors: Request Captcha Processor: Incident - No Captcha Answer Provided

Complexity: Medium (3.0)

Default Response: 1x = Warn User. 2x = 1 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 forces the captcha interface to submit the request without a valid captcha solution. There is no way to do this without manipulating the logic that controls captcha protected requests.

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. In this specific case, the attacker attempted to submit the captcha protected page without actually solving the captcha. Instead they provided an empty value for the solution parameter. It is not possible to submit an empty solution using the provided captcha interface, so this is almost guaranteed to be a malicious attempt at generating an error and obtaining additional details about the captcha implementation though an "Information Leakage" weakness.

See http://projects.webappsec.org for information on attack types.