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    Response Processors: Request Captcha Processor: Incident - Multiple Captcha Disallow Multipart

    Complexity: Low (2.0)

    Default Response: 1x = 1 Day Clear Inputs

    Cause: A CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) is a special technique used to differentiate between human users, and automated scripts. The user is required to visually identify characters in a jumbled image and transcribe them into a text box. An audio version is also available, for users with a visual handicap. 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. 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 submit dozens of CAPTCHA-protected requests that contain binary files, and the CAPTCHAs are explicitly configured to not allow binary file submission (it has been configured to disallow multi-part form submissions).

    Behavior: When a hacker is attempting to establish an automated script that is capable of defeating the CAPTCHA, they can use various 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 submitted dozens of binary files in the requests that are being protected. The CAPTCHA in this case has been explicitly configured to not allow Multi-Part form submissions, so this represents unexpected and undesired activity. Using Multi-Part forms, the attacker can more easily accomplish a "Buffer Overflow" attack, which would produce potentially sensitive error data and possibly open the server up to further exploitation. Additionally, some web applications do not handle the encoding used for multi-part forms gracefully, so error information can also be obtained from conflicts arising from the submission type. Because this is happening so frequently from the same user, it is also possible that the user is attempting to execute a "Denial of Service" attack.

    Published: 2014-06-27