This document discusses cross-site request forgery (CSRF) and how to prevent it. It begins by explaining what CSRF is, how it exploits user authentication to make requests on a victim's behalf. It then demonstrates CSRF by having the user unknowingly click a link to perform actions on another site they are logged into. The document discusses how GET requests, POSTs, and JavaScript can all enable CSRF. It recommends making requests unique and non-repeatable to prevent CSRF. For web forms, it suggests using viewstate tied to session ID, and for MVC using anti-forgery tokens from both cookie and form values. Code demos are provided for CSRF prevention in web forms and M

1. SEA SURFING
HOW VULNERABLE IS MY WEB APPLICATION
FROM A DEVELOPER’S ANGLE…
Dilan Warnakulasooriya Asanka Fernandopulle
Information Security Engineer Senior Software Engineer
99X Technology 99X Technology

2. What is it?
 Cross Site Request Forgery – Sea Surrrrrfff
 Attacker exploits the fact that the victim is authenticated to
a website
 Identifying the attacker can be difficult
 What can it do?
 Proxy requests/commands for the attacker from the victim’s
browser
 Even POSTS can be forged as GET requests in some
cases
 Web forms One Click Demo in module
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3. How it is exploited?
 Can be very simple – Image link in email, script on a blog,
simple link
 Attackers gets user to
 Click a specially crafted link (or inject JavaScript to a site victim visits)
 Execute a request (can be very simple as requesting an image url in email)
 Innocently browsing a web site
 Can users include hrefs or Image links to your site? Link to bad url
 Ever click “view images” in an email?
 All browsers happily send over credentials if already
logged on
 If already logged in (forms auth) the cookie is sent over even for an
image request
 Some are invisible! IE default setting
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4. CSRF – HOW IT IS EXPLOITED?
DEMO
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5. CSRF – HOW IT IS EXPLOITED?
DEMO – Repeatability is the key
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6. CSRF – HOW IT IS EXPLOITED?
DEMO – Piggyback with some other attack like XSS
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7. CSRF – POSTs protect me
 They do, don’t they? Don’t they? Hello?
 MVC CSRF via XSS
 Web Forms One Click attack
 Page.IsPostBack doesn’t always tell the truth
 A button click doesn’t always mean someone click the button
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8. How do you prevent it?
 All Web Apps
 Ensure GET only retrieves a resource (as per HTTP Spec)
 No state is modified
 POSTS/PUT/DELETE can be forged, must take additional precautions
 Try to make requests unique and non-repeatable
 Web forms specific
 ViewStateUserKey = Session.SessionId
 ViewState then acts as a form token
 Must protect the Session Ids(Using Encryption, Hashing)
 Pages inherit from the base web page
 SSL to prevent sniffing of ViewState & SessionId
 MVC Specific
 Anti-Forgery token uses form value AND cookie value
 SSL to prevent from sniffing Anti-Forgery token
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9. Web Forms – CSRF Prevention
DEMO
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10. MVC – CSRF Prevention
DEMO
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