XSRF and the Same-Origin Policy

XSRF attacks, the Same-Origin Policy, and XSRF tokens

The Attack

Auth Tokens are Automatic

• Recall: After you log in, your auth token is stored in a cookie
• The browser attaches that cookie to every request sent to your server
  • No matter which page caused the request to be sent
  • The request is authenticated as you, even if you didn't choose to send it
• Your browser has many tabs open to many different sites at once
  • Some you trust (Your bank, Autolab)
  • Some you shouldn't (Clicking a link in a shady email)

Cross-Site Request Forgery

• Cross-Site Request Forgery (XSRF) - A malicious site causes your browser to send a request to another site
  • The browser attaches your cookies
  • The server sees an authenticated request from you and processes it
  • You never intended to send it
• Used to transfer your money, make an Autolab submission, make a post/DM in your name, change your email address

XSRF - The Attack

• You log in to bank.com and get an auth token in a cookie

XSRF - The Attack

• You click a link in an email: ALL YOUR DREAMS WILL COME TRUE!! JUST CLICK HERE!!!
• Your browser loads the attacker's HTML/JavaScript

XSRF - The Attack

• The attacker's page sends a transfer request to bank.com, and your browser attaches your auth cookie
  • The attacker knows the format of the request. Anyone can read bank.com's front end

XSRF - The Attack

• bank.com sees a valid auth token and processes the request. Hacked!

Sending the Forged Request

<form id="attack" action="https://bank.com/transfer" method="post">
  <input type="hidden" name="to" value="attacker">
  <input type="hidden" name="amount" value="10000">
</form>
<script>document.getElementById("attack").submit();</script>

• Navigation: A hidden form that submits itself as soon as the page loads. No click needed
• Embed: Or an image tag, which sends a GET request
  • <img src="https://bank.com/transfer?to=attacker&amount=10000">
  • This is why images are often blocked in emails
• Fetch: Or sends a fetch request
• Will any of these work?
  • It depends on the browser and the Same-Origin Policy

The Same-Origin Policy

Same-Origin Policy

• The Same-Origin Policy (SOP) is a set of rules enforced by every modern browser
• Content from one origin cannot read content from a different origin
  • freemoney4you.com's JavaScript cannot read any response from bank.com
• SOP sometimes blocks the entire request
• The SOP only affects cross-origin requests
• To understand the full rules, we first need to define "origin"

Origin

• An origin is the combination of:
  • Protocol - http, https, ws, ftp
  • Host - cse312.com
  • Port - 443
• Two URLs have the same origin if all three match exactly
• The path is not part of the Origin
• Origin is what the SOP checks

Site

• A site is broader than an origin
• Site = protocol + domain (Not host)
• Ports and subdomains are ignored
  • https://api.cse312.com and https://cse312.com are the same site
  • https://autograder.cse.buffalo.edu and https://buffalo.edu are the same site
  • http://localhost:8080 and http://localhost:5000 are the same site
• For local testing, you can use http://localhost:8080 and http://127.0.0.1:8080 which are different sites, but both reach your server
• This is the definition of site used by the SameSite cookie directive

SOP Blocks All Reads

• Navigation: Attacker sends a request that navigates the browser to another origin
  • The response replaces the page that sent the request
  • The page that sent the request never sees the response
• Embeds: Embeds can be used, but not read [via JS. You can still read them in your console]
  • The page can display the image, run the script, and apply the styles
  • The page's JavaScript can't read the image's pixels or the script's source
• Fetch: SOP might allow the request to be sent, but the response is always blocked
  • Attacker still sends the request, and the other origin will still send a response
  • The attackers JS is not allowed to read the response

The Request Still Arrives

• Try it: From a page on one origin, fetch a URL on another origin
  • Console: Access to fetch at '...' from origin '...' blocked by CORS
  • Misleading error message: Devs often blame CORS for SOP behavior
• Now check the Network tab and your server's logs
  • The request was sent
  • Your server processed it and sent a 200 response
  • The browser received the response and refused to give it to the JavaScript
• The SOP protects the response. It does not protect your server

SOP Blocks Some Requests

• If the request is still sent, can freemoney4you.com send bank.com:
  • A DELETE request?
  • A JSON body?
  • Any headers they want?
• No! The browser splits cross-origin requests into 2 kinds:
  • Simple requests - Always sent. The SOP only blocks reading the response
  • Preflighted requests - The browser asks the server for permission before sending the request

Simple Requests

A cross-origin request is simple if all of these are true:

• The method is GET, POST, or HEAD (A request for only headers)
• Only sets "CORS-safelisted" headers
  • Accept, Accept-Language, Content-Language, Content-Type
• The Content-Type, if set, is one of:
  • application/x-www-form-urlencoded
  • multipart/form-data
  • text/plain

Every other request is "to be preflighted"

Why These Rules?

• Compatibility. Many sites rely on cross-origin simple requests
• A simple request is a request that an HTML form could already send without any JavaScript
  • Forms could send these cross-origin long before AJAX and fetch existed
  • Blocking them in fetch wouldn't make anything safer so it's allowed. An attacker would simply use a form if it were not
• Anything else (PUT, DELETE, a JSON body, a custom header) could not be sent cross-origin when the SOP rules were written
  • Servers were written assuming they would never receive these cross-origin
  • Easy to block these without breaking existing apps
  • Browsers won't send them without the server's permission

Preflight

• Before sending a non-simple cross-origin request, the browser sends a preflight request
  • Method OPTIONS for the intended path
  • Asks the server whether the real request is allowed
• If the server does not allow it, the real request is never sent
• The server can allow the request by responding with various Access-Control-Allow headers
  • Access-Control-Allow-Method: PATCH
  • Access-Control-Allow-Origin: https://cse312.com
  • Would allow PATCH requests for the path from cse312.com
• If any response is sent without these headers, the request is not allowed
  • For example, if you respond to the OPTIONS request with a 404

GET Requests Should Never Change State

• GETs only get data. They should never change that data
• Any page can send them: links, forms, redirects, image tags
  • If they are simple, the SOP allows the request
  • If they navigate to your page, SameSite cookies are sent
• So GET /transfer?to=attacker&amount=10000 is exploitable even with all the protections built into modern browsers
  • Do not perform any sensitive actions with a GET request
• A web crawler could even accidentally change the state of your server

XSRF Tokens

When the Browser Isn't Enough

• The SOP and SameSite are enforced by the user's browser. We have limited control
  • An outdated or obscure browser might not implement them properly
  • A browser extension might disable them
  • Browsers disagree on the default SameSite value
• They protect most users, but not 100%
• We need protection on our server that works for every user, in every browser
  • Never trust your users, even with their own security

XSRF Tokens

• The attacker can send requests, but they can't read your pages (SOP)
• So, put a secret in your pages that the request must send back
• On the server:
  • Generate a long random XSRF token when serving a page that can make state-changing requests
    • The attacker must not be able to guess the token (>=80 bits of entropy)
  • Store the XSRF token and associate it with the user who made the request
    • Identify the user by their auth token or session token
  • Embed this user's XSRF token in the page when you render it for them
• On the front end:
  • The XSRF token is a hidden input on the form
  • The token is sent with the form submission

XSRF Tokens - In the Form

• Add a new hidden input to your form for the token
• Generate and inject the token as the value using HTML templates
• Read the token from the request and verify it

<form action="/image-upload" id="image-form" method="post" enctype="multipart/form-data">
    <input type="hidden" name="xsrf_token" value="AQAAAjppCA8mhugn2UvwOTaKnVY">
    <label for="form-file">Image: </label>
    <input id="form-file" type="file" name="upload">
    <br/>
    <label for="image-form-name">Caption: </label>
    <input id="image-form-name" type="text" name="name">
    <input type="submit" value="Submit">
</form>

XSRF Tokens - In Fetch Requests

• If you want to send a fetch request with a XSRF token
  • Use an HTML template to embed the token anywhere in the HTML
    • Preferably in a meta tag
  • Use JS to read the token
  • Add it as a custom header in your request
    • Or as part of the body (In a JSON field)

XSRF Tokens - Verifying

• On every request that changes state:
  • Authenticate the user with their auth token, or session token if they are not logged in
  • Read the XSRF token from the request body
  • Verify that this XSRF token was issued to this user
• If the XSRF token was issued to this user
  • Accept the request as valid
• If the XSRF token is missing or was NOT issued to this user
  • This is an invalid request and might be an XSRF attack
  • Respond with 403 Forbidden

XSRF Tokens - The Attack Fails

• Even if the browser attaches sends a request with auth tokens, the forged request can't contain the correct XSRF token
  • The SOP stops freemoney4you.com from reading your pages
  • This is why it's very important to embed the token in your page, not in a cookie
• The server rejects the request
• The attacker can get a valid XSRF token for their own account
  • But that token was not issued to you, so it's rejected too
• This is server-side security (The only real form of security)
  • Works in every browser, with any SameSite setting

Why Not Put the XSRF Token in a Cookie?

• The auth tokens are what we're trying to protect
  • XSRF tokens in cookies would be like have 2 auth tokens (redundant)
• Cookies are attached to requests automatically, even forged requests
  • A token that only arrives in a cookie proves nothing. The forged request has it too
• The token must arrive in something the attacker's page can't produce:
  • A form field
  • A custom request header (Which also triggers a preflight)
• The user's page must be able to read the token to send it
• The attacker's page must not be able to read the token
  • This is exactly what the SOP gives us

Further Reading

• Same-Origin Policy (MDN)