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Is XSS Solvable?

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Slides from my LayerOne 2009 presentation on cross site scripting.

Slides from my LayerOne 2009 presentation on cross site scripting.

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  • 1. IS XSS SOLVABLE? Don Ankney • LayerOne 2009 Saturday, May 23, 2009
  • 2. DISCLAIMER This presentation represents personal work and has not been approved or vetted by Microsoft. I am solely responsible for its content. Saturday, May 23, 2009
  • 3. OUTLINE • XSS as a generic injection attack. • What makes XSS unique, why it’s important. • Defending against XSS in general. • Web Application Defense • One effective architecture design pattern. • Tool to help. • Where to go from here. Saturday, May 23, 2009
  • 4. DEFINING THE PROBLEM Saturday, May 23, 2009
  • 5. INJECTION FLAWS • Any interpreted code can be injected: LDAP, XML, HTTP, XAML, PHP, Python, Ruby, and most infamously, SQL. • XSS is simply another form of interpreter injection, usually using Javascript. • We can solve it just like we would solve any other injection problem*. * except SQL injection -- we have a better solution there. Saturday, May 23, 2009
  • 6. PREVENTING INJECTION • Always keep track of your trust boundaries. • Validate/sanitize your inputs. • Properly encode your outputs. • Use white lists, not black lists. • Exercise the principal of least privilege. • Validate your assumptions. Saturday, May 23, 2009
  • 7. TRUST BOUNDARIES User System Forms Files headers services cookies Processes ajax files Application Database SQL LDAP web data stores Saturday, May 23, 2009
  • 8. WHY XSS IS DIFFERENT • Most injection flaws attack your server. • XSS attacks the end user -- it runs arbitrary code in their browser. • The browser is behind your firewall and is acting within the user’s security context. Saturday, May 23, 2009
  • 9. YOU CAN DO SOME NASTY THINGS WITH JAVASCRIPT • Javascript can control what appears on screen. • Javascript has access to your history. • Sites often store session tokens in GET request. • Javascript can intercept cookies. • Javascript can enumerate your network. Saturday, May 23, 2009
  • 10. XSS TAXONOMY Saturday, May 23, 2009
  • 11. REFLECTED XSS Attack Victim Attacker Attack Web App URL string Browser XSS • Victim browser submits an “evil” request that contains javascript. • The web application then reflects that javascript back to the victim browser, which then executes it. Saturday, May 23, 2009
  • 12. REFLECTED XSS • Attack does not persist across users, sessions, or pages. • It is only reflected back to the user that submits the malicious url. • This is relatively easy to detect remotely via a scanner/fuzzer. Saturday, May 23, 2009
  • 13. PERSISTENT XSS Attack string Attacker Webpage Database Victim Webpage XSS • An attacker stores an attack string in a web application. • Visitors to that site access infected pages causing javascript execution. Saturday, May 23, 2009
  • 14. PERSISTENT XSS • Can persist across user, sessions, and pages. • It depends on application context and who can access the infected pages. • Very difficult to detect remotely via scanning/fuzzing. • There are many paths through an application, scanning is too noisy for a production system. • Best identified by code analysis. Saturday, May 23, 2009
  • 15. HYBRID XSS Attack Webpage Attack A Attack Victim Attacker Session Data URL Browser XSS Webpage Attack B • Victim browser submits “evil” url. • Attack string is stored as session data. • Session data is returned to browser on another page, executing javascript. Saturday, May 23, 2009
  • 16. HYBRID XSS • Attacks are not persistent across sessions. • Attacks do persist across pages. • Attacks may persist across users. • Think about “who’s online” functionality. • Extremely difficult to detect remotely. • May require specific request sequences. Saturday, May 23, 2009
  • 17. DETECTING INJECTION FLAWS REMOTELY Saturday, May 23, 2009
  • 18. SCANNING FOR REFLECTED XSS VULNERABILITIES • This requires a simple fuzzer. • Send an attack string. • Check for appropriately encoded returns. • The most difficult part is maintaining a current attack string list. Saturday, May 23, 2009
  • 19. SCANNING FOR PERSISTENT XSS VULNERABILITIES • This sort of scanner is immature. • Requires mapping input cause vs. output effect without knowledge of application state. •A persistent XSS scan is extremely noisy. • Each form has to have unique data submitted in order to map persistent data across the site. • Running an attack list against the mapped site creates one persistent record per attack string. Saturday, May 23, 2009
  • 20. DETECTING INJECTION FLAWS LOCALLY Saturday, May 23, 2009
  • 21. AVAILABLE TECHNIQUES • Static code analysis • Examines the source code • Dynamic analysis • Examines application state Saturday, May 23, 2009
  • 22. STATIC CODE ANALYSIS • Manual code review. • Static code analyzers: • Pixy (PHP) • CAT.NET (ASP.NET) • CodeSecure (Java/ASP.NET/PHP) Saturday, May 23, 2009
  • 23. WHAT STATIC ANALYSIS CAN DETECT • Input sanitization • Really only boolean, doesn’t evaluate quality. • Output encoding • Are both application (html) and character encoding explicit? • Data flow • Does anything skip sanitization or encoding? Saturday, May 23, 2009
  • 24. DYNAMIC CODE ANALYSIS • The only way to consider application state. • Remember that persistent and hybrid XSS are state- dependent. • Internal representation of data is tied to risk. • There aren’t a lot of existing tools. • Most existing are run-time IPS-style tools. Saturday, May 23, 2009
  • 25. DEFENDING AGAINST XSS Saturday, May 23, 2009
  • 26. THERE ARE THREE CONTROL POINTS FOR XSS Web Browser Application Database IT Infrastructure XSS can be mitigated at all three points. Saturday, May 23, 2009
  • 27. THE BROWSER • Different browsers will execute different code snippets. • O9, IE6: <BODY BACKGROUND=quot;javascript:alert('XSS');quot;> • FF2, O9: <META HTTP-EQUIV=quot;refreshquot; CONTENT=quot;0;url=data:text/ html;base64,PHNjcmlwdD5hbGVydCgnWFNTJyk8L3NjcmlwdD4Kquot;> • Current browsers are much better than previous generations. Saturday, May 23, 2009
  • 28. IT INFRASTRUCTURE • Web application firewalls. • Intrusion prevention systems. • Web application sandboxes. • All of these are signature-based defenses. Saturday, May 23, 2009
  • 29. WEB APPLICATION DEFENSE Saturday, May 23, 2009
  • 30. IT’S ALL ABOUT PROCESS Request Return Security Decode Logic Encode Checks • This requires a consistent and enforced architectural approach. • JSP, PHP 3/4, and classic ASP don’t lend themselves to this approach. Saturday, May 23, 2009
  • 31. DECODE YOUR INPUT Accept-Language: en-us,en;q=0.5 Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7 • If the request specified an encoding, you might want to assume that encoding for parameters. • HTTP defaults to ISO-8859-1 • Convert the request to your application’s internal representation. Saturday, May 23, 2009
  • 32. SECURITY CHECKS • This is where you apply your whitelists. • Remember the principal of least privilege and use the most restrictive patterns possible. • If you must accept markup tags, parse the tags and replace them with tokens. • Consider whitelisting any paths or URLs. Saturday, May 23, 2009
  • 33. SANITIZING HTML • There are a number of libraries available to do this for you. • Microsoft AntiXSS (.NET), • OWASP AntiSamy (Java, .NET, Python). • HTML Purifier (PHP) Saturday, May 23, 2009
  • 34. WRITING YOUR OWN SANITIZATION LIBRARY • Whitelist HTML tags. • Be as restrictive as possible. • If allowing high-risk tasks (such as <a> and <img>), consider whitelisting their targets. • Replace all the allowed tags with symbols. Parse them for properties. • Store them this way. • Replace the symbols prior to encoding. This way, everything is constructed in your code and not passed from the user. Saturday, May 23, 2009
  • 35. ENCODE YOUR OUTPUT • Most commonly, encode using html entities. • After entity encoding, replace your tokens with actual tags. • If the final output is not html, encode accordingly. • XML if outputting AJAX, XAML, etc. Saturday, May 23, 2009
  • 36. EXPLICITLY DECLARE YOUR CHARACTER SET • Your web server can do it for you: • AddCharset UTF-8 .php (Apache .htaccess) • If in doubt, override your web server: • header('Content-type: text/html; charset=utf-8'); (PHP .htaccess) • <%Response.charset=quot;utf-8quot;%> (ASP.NET) • print quot;Content-Type: text/html; charset=utf-8nnquot;; (Perl) Saturday, May 23, 2009
  • 37. DO YOUR SECURITY CHECKS ACTUALLY WORK? foreach ($_GET as $secvalue) { if ((eregi(quot;<[^>]*script*quot;?[^>]*>quot;, $secvalue)) || (eregi(quot;<[^>]*object*quot;?[^>]*>quot;, $secvalue)) || (eregi(quot;<[^>]*iframe*quot;?[^>]*>quot;, $secvalue)) || (eregi(quot;<[^>]*applet*quot;?[^>]*>quot;, $secvalue)) || (eregi(quot;<[^>]*meta*quot;?[^>]*>quot;, $secvalue)) || (eregi(quot;<[^>]*style*quot;?[^>]*>quot;, $secvalue)) || (eregi(quot;<[^>]*form*quot;?[^>]*>quot;, $secvalue)) || (eregi(quot;([^>]*quot;?[^)]*)quot;, $secvalue)) || (eregi(quot;quot;quot;, $secvalue))) { die (quot;<center><img src=images/logo.gif><br><br><b>The html tags you attempted to use are not allowed</ b><br><br>[ <a href=quot;javascript:history.go(-1)quot;><b>Go Back</b></a> ]quot;); } } foreach ($_POST as $secvalue) { if ((eregi(quot;<[^>]*script*quot;?[^>]*>quot;, $secvalue)) || (eregi(quot;<[^>]*style*quot;?[^>]*>quot;, $secvalue))) { die (quot;<center><img src=images/logo.gif><br><br><b>The html tags you attempted to use are not allowed</ b><br><br>[ <a href=quot;javascript:history.go(-1)quot;><b>Go Back</b></a> ]quot;); } } This is actual code in a popular CMS. Saturday, May 23, 2009
  • 38. TESTING ARCHITECTURAL PATTERN IMPLEMENTATION Saturday, May 23, 2009
  • 39. VALIDATES INPUT SANITIZATION Web Fuzzer attack strings Sanitization Validator attack strings Stored strings Target Database App Virtual Machine Saturday, May 23, 2009
  • 40. VALIDATES OUTPUT ENCODING Endcoding Validator attack strings Database Fuzzer Stored strings attack strings Target Database App Virtual Machine Saturday, May 23, 2009
  • 41. [DEMO] Saturday, May 23, 2009
  • 42. TOOL BENEFITS • Finds fundamental problems in software design • More complex attacks rely on either sanitization or encoding to be broken. • Detects persistent XSS attack vectors. • In a dedicated VM, noise doesn’t matter. Saturday, May 23, 2009
  • 43. TOOL LIMITATIONS • Only works with database-resident state data • Cookies, header, etc are also valid XSS threats. • Only works against HTML forms • Can be extended to include AJAX, headers, cookies, etc. Saturday, May 23, 2009
  • 44. TOOL PLANS • Move it from PHP to C#. • Create a PM-friendly interface so that anyone can run it. • Add authentication integration: • Web forms, OpenID. • Expand it to include non-form fuzzing. Saturday, May 23, 2009
  • 45. WHY IS THIS HARD? • Legacy applications and languages often don’t lend themselves to this architectural pattern. • Browser behavior is often unexpected and gibberish code can be executed. • Application complexity makes it difficult to predict state, just like a client application. • Applications and developers change over time. Saturday, May 23, 2009
  • 46. CALL TO ACTION • Write better code. • Use current browser technology. • Honestly assess your enterprise. • Do you have lots of legacy code? Saturday, May 23, 2009
  • 47. I’M WEB 2.0 • Blog: http://hackerco.de • LinkedIn: http://www.linkedin.com/pub/don-ankney/6/213/651 • Twitter: http://twitter.com/dankney • E-mail: dankney@hackerco.de Saturday, May 23, 2009
  • 48. BIBLIOGRAPHY • Alshanetsky, Ilia. Php|Architect's Guide to Php Security|. Marco Tabini & Associates, Inc, 2005. • Ernst, M., Artzi, S., Kiezun, A., Dolby, J., Tip, F., and Dig, D. “Finding Bugs in Web Applications Using Dynamic Test Generation and Explicit State Model Checking.” dspace.mit.edu (2009): • Ernst, M., Kiezun, A., Guo, P. J., Jayaraman, K., and Ernst, M. D. “Automatic Creation of Sql Injection and Cross-Site Scripting Attacks.” dspace.mit.edu (2008): • Fogie, Seth, Jeremiah Grossman, Robert Hansen, Anton Rager, and Petko D. Petkov. Xss Attacks: Cross Site Scripting Exploits and Defense. Syngress, 2007. Saturday, May 23, 2009
  • 49. BIBLIOGRAPHY • Johns, M., Engelmann, B., and Posegga, J. “Xssds: Server-Side Detection of Cross- Site Scripting Attacks.” Proceedings of the 2008 Annual Computer Security Applications Conference (2008): 335-44. • Stuttard, Dafydd, and Marcus Pinto. The Web Application Hacker's Handbook: Discovering and Exploiting Security Flaws. Wiley, 2007. • Vogt, P., Nentwich, F., Jovanovic, N., Kirda, E., Kruegel, C., and Vigna, G. “Cross-Site Scripting Prevention With Dynamic Data Tainting and Static Analysis.” Proceeding of the Network and Distributed System Security Symposium (NDSS’07) (2007): • Wassermann, G., and Su, Z. “Static Detection of Cross-Site Scripting Vulnerabilities.” Proceedings of the 30th international conference on Software engineering (2008): 171-80. Saturday, May 23, 2009
  • 50. QUESTIONS? Saturday, May 23, 2009

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