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102 Web Application Penetration Testing References OWASP Resources • XSS Filter Evasion Cheat Sheet Books • Joel Scambray, Mike Shema, Caleb Sima - “Hacking Exposed Web Applications”, Second Edition, McGraw-Hill, 2006 - ISBN 0-07- 226229-0 • Dafydd Stuttard, Marcus Pinto - “The Web Application’s Handbook - Discovering and Exploiting Security Flaws”, 2008, Wiley, ISBN 978-0-470-17077-9 • Jeremiah Grossman, Robert “RSnake” Hansen, Petko “pdp” D. Petkov, Anton Rager, Seth Fogie - “Cross Site Scripting Attacks: XSS Exploits and Defense”, 2007, Syngress, ISBN-10: 1-59749-154-3 Whitepapers • CERT - Malicious HTML Tags Embedded in Client Web Requests: Read • Rsnake - XSS Cheat Sheet: Read • cgisecurity.com - The Cross Site Scripting FAQ: Read • G.Ollmann - HTML Code Injection and Cross-site scripting: Read • A. Calvo, D.Tiscornia - alert(‘A javascritp agent’): Read ( To be published ) • S. Frei, T. Dübendorfer, G. Ollmann, M. May - Understanding the Web browser threat: Read Testing for Stored Cross site scripting (OTG-INPVAL-002) Summary Stored Cross-site Scripting (XSS) is the most dangerous type of Cross Site Scripting. Web applications that allow users to store data are potentially exposed to this type of attack. This chapter illustrates examples of stored cross site scripting injection and related exploitation scenarios. Stored XSS occurs when a web application gathers input from a user which might be malicious, and then stores that input in a data store for later use. The input that is stored is not correctly filtered. As a consequence, the malicious data will appear to be part of the web site and run within the user’s browser under the privileges of the web application. Since this vulnerability typically involves at least two requests to the application, this may also called second-order XSS. This vulnerability can be used to conduct a number of browser-based attacks including: • Hijacking another user’s browser • Capturing sensitive information viewed by application users • Pseudo defacement of the application • Port scanning of internal hosts (“internal” in relation to the users of the web application) • Directed delivery of browser-based exploits • Other malicious activities • User visits vulnerable page • Malicious code is executed by the user’s browser This type of attack can also be exploited with browser exploitation frameworks such as BeEF, XSS Proxy and Backframe. These frameworks allow for complex JavaScript exploit development. Stored XSS is particularly dangerous in application areas where users with high privileges have access. When the administrator visits the vulnerable page, the attack is automatically executed by their browser. This might expose sensitive information such as session authorization tokens. How to Test Black Box testing The process for identifying stored XSS vulnerabilities is similar to the process described during the testing for reflected XSS. Input Forms The first step is to identify all points where user input is stored into the back-end and then displayed by the application. Typical examples of stored user input can be found in: • User/Profiles page: the application allows the user to edit/ change profile details such as first name, last name, nickname, avatar, picture, address, etc. • Shopping cart: the application allows the user to store items into the shopping cart which can then be reviewed later • File Manager: application that allows upload of files • Application settings/preferences: application that allows the user to set preferences • Forum/Message board: application that permits exchange of posts among users • Blog: if the blog application permits to users submitting comments • Log: if the application stores some users input into logs. Analyze HTML code Input stored by the application is normally used in HTML tags, but it can also be found as part of JavaScript content. At this stage, it is fundamental to understand if input is stored and how it is positioned in the context of the page. Differently from reflected XSS, the pen-tester should also investigate any out-of-band channels through which the application receives and stores users input. Note: All areas of the application accessible by administrators should be tested to identify the presence of any data submitted by users. Example: Email stored data in index2.php Stored XSS does not need a malicious link to be exploited. A successful exploitation occurs when a user visits a page with a stored XSS. The following phases relate to a typical stored XSS attack scenario: • Attacker stores malicious code into the vulnerable page • User authenticates in the application
103 Web Application Penetration Testing The HTML code of index2.php where the email value is located: In this case, the tester needs to find a way to inject code outside the tag as below: MALICIOUS CODE
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