11/4/2022 0 Comments Recursive grep![]() ![]() While testing the WebSphere portal mentioned above, there were three distinct values that changed with each response from the server. The derived parameter can originate from any arbitrary location in the response and you can include as many recursive grep payloads as you need. This payload will formulate and insert a parameter into your request based on the previous server response. To accommodate for the token value present in the response, we can use the Burp Intruder Recursive Grep payload. Unsuccessful Intruder Attack Due to Invalid Token Value However, when the correct token value is submitted, the submitted values are reflected on the page along with an indication of success. If this form is submitted without the correct token value, the string “Invalid Token Value” is displayed. For simplicity, the current token value is displayed on the form. To demonstrate this problem, I have created a single ASP.NET web form with two fields. The server would simply respond by reloading the blank form with newly updated token values. ![]() When any form was submitted to the server, multiple values would change in the ensuing response which prevented automated tools like Burp Repeater and Burp Intruder from working properly on the site. I recently encountered a WebSphere portal that exhibited the behavior described above. Once the Recursive Grep payload is understood it can be used to solve nearly any dependency on a previous server response. In this post I will demonstrate how Burp Intruder’s Recursive Grep payload can be used to solve this problem. A side-effect of this behavior is that automated testing becomes nearly impossible without taking the token value into consideration. Some anti-CSRF implementations change the token value with each submission of the form. Since an attacker cannot know the value in advance and must have the value to successfully submit the form, this technique, when implemented properly, will thwart CSRF attacks. The token is a random value that is generated when the target form is loaded, and verified when the form is submitted. When the targeted user executes the attacker’s content, the transaction is executed on the target web application.Īs a defense against CSRF attacks, web applications employ Anti-CSRF tokens in sensitive forms. If this is possible, an attacker will use social engineering, craft a malicious URL, or post a malicious form on a third party site that will accept unfiltered HTML input. To be vulnerable to CSRF, an attacker must be able to determine and submit all of the values necessary to execute the target transaction in advance. Cross-Site Request Forgery (CSRF or XSRF) is an attack which is used to execute a transaction on behalf of a victim user against a vulnerable web application. ![]()
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