1. CSC 482/582: Computer Security
Cross-Site Security

2. Topics Cross-Site Request Forgery Mitigating CSRF.
Cross-Site Scripting (XSS)
Mitigating XSS.

3. Cross-Site Attacks Target users of application.
Web browsers cache authentication credentials and re-use them with requests to same origin.
Attackers use application to reach target users.
Attack obtains assets of individual users rather than assets of entire application.
Most common type of attack.
Cross-Site Request Forgery (CSRF)
Cross-Site Scripting (XSS)

4. 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.

5. Example: DSL Modem Attack
Home network devices 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.
<img src= " alt="" width="1" height="1" />
Image from

6. Example: POST-based CSRF
Send following HTML to user:
<form name="csrfForm" action=" method="POST">
<input type="hidden" name="giveMoneyTo" value="hacker" />
<input type="hidden" name="giveAmount" value="1000" />
<input type="submit"/>
</form>
<script>
document.csrfForm.submit();
</script>

7. Ineffective CSRF Mitigations
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.

8. Effective CSRF Mitigations
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.
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.

9. 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.

10. XSS Examples FF.net worm (October 2018) Paypal (2018)
If you view an infected profile, your profile infected.
Similar to Samy’s MySpace XSS worm in 2006.
Paypal (2018)
Tech support site allowed anonymous (but not authenticated) users to upload files in any format without input validation, including XSS content.
Mass Defacement ( )
Billy Ribeiro Anderson pled guilty after illegally accessing and defacing 11,000 web sites, including using XSS to defect West Point’s Combating Terrorism Center web site.

11. XSS Key Steps Attacker sends code to web application.
Legitimate user accesses web app.
Web app sends attacker code to user.
User’s browser executes code.

12. XSS Example Client browser sends an error message to the web server.
+error+occurred

13. XSS Example
The error message is “reflected” back from the Web server to the client in a web page.

14. XSS Example We can replace the error with JavaScript

15. Exploiting the Example
User logins in and is issued a cookie
Attacker feed the URL to user

16. 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.

17. Reflected XSS Attack Scenario Limitations 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%).

18. 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.

19. XSS Testing Strings
Use strings that use JavaScript in different contexts.
<script>alert(document.cookie);</script>
<script src=
<img src="javascript:alert(‘xss');">
<img src=# onmouseover="alert(‘xss')">
… and many other ways to run JavaScript.
Use URL and other encodings to bypass input filters.
See XSS_Filter_Evasion_Cheat_Sheet

20. XSS URL Examples

21. 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.

22. DOM-based XSS Attack scenario Exploits vulnerability in client code.
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.

23. Browser Exploitation Framework
BeEF hooks browsers via XSS exploit
Can use as stored or reflected XSS.
Hooked browsers are bots controlled by BeEF.
Exploitation modules run on hooked browsers to
View browsing history.
Identify authenticated sessions.
Phishing and other social engineering attacks.
Port scans of network browser is running on.
Reverse proxy into network browser is running on.
Use Metasploit.

24. BeEF Screenshot

25. Mitigating XSS Two main approaches:
Output encoding. HTML entity encode all output to remove script tags and the like. Note that output encoding is context specific. If output goes inside a <script> tag or inside tags that can call JavaScript, different encoding is needed than outside such tags.,
Input validation. Reject user input with HTML or reject user input that isn’t limited to a safe subset of HTML.
Use encoding libraries where possible, like
Microsoft Anti-Cross Site Scripting Library for .NET
OWASP Java Encoder Project

26. References Daswani et. al., Foundations of Security, Apress, 2007.
Seth Fogie et. al., XSS Attacks: Cross-Site Scripting Exploits and Defense, Syngress, 2007.
Jakob Kallin and Irene Lobo Valbuena. Excess XSS: A comprehensive tutorial on cross-site scripting.
Nathan,
OWASP. XSS Filter Evasion Cheat Sheet.
Stuttart and Pinto, The Web Application Hacker’s Handbook, 2nd edition, Wiley, 2011.
Michal Zalewski, The Tangled Web: A Guide to Securing Modern Web Applications, No Starch Press, 2012.

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