Help us improve your experience.

Let us know what you think.

Do you have time for a two-minute survey?

Guide That Contains This Content
[+] Expand All
[-] Collapse All

    Response Processors: Request Captcha Processor: Incident - Expired Captcha Request

    Complexity: Suspicious (1.0)

    Default Response: None.

    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 after the allotted time for solving the captcha has elapsed.

    Behavior: When a hacker is attempting to establish an automated script that is capable of defeating the captcha, they may use various different techniques. One of these techniques is to run expensive image processing algorithms on the captcha image in order to identify what the represented value might be. Additionally, a user might attempt to send the captcha to a warehouse of human captcha solvers. These warehouses specialize in solving large volumes of captchas at a fairly low price (less then a penny per captcha). In either case, it can take several minutes to get the correct captcha answer, and will likely run out the amount of time the user is allowed for solving the captcha. If using a browser, the input would flat out stop accepting answers, but in a scripted scenario, the script will likely try and submit the value anyway, because it is unaware of the expiration. It is possible that this incident would be triggered by a legitimate user, if they were to refresh the page that was produced after the captcha was solved. This would effectively cause the captcha to be reprocessed after the expiration time had been exceeded. As such, this incident on its own is not considered malicious.

    Published: 2013-11-20