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    Response Processors: Request Captcha Processor: Incident - Captcha Disallowed MultiPart

    Complexity: Suspicious (1.0)

    Default Response: 10x = Multiple Captcha Disallow Multipart Incident.

    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 submit a captcha protected request that contains a binary file, and the captcha is 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 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 submitted a binary file in the request that is 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 useful error data and potentially 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. This is not necessarily a malicious incident on its own, because it is possible that the user is legitimately submitting a multi-part form, and just happened to have the captcha activated during the submission. However this is a very rare case, and still represents a somewhat suspicious client.

    Published: 2014-06-27