Same-Origin Policy

Ep-3: Same-Origin Policy

Table Of Contents

• Ep-3: Same-Origin Policy
  • Key Questions
  • What Is The Origin?
  • “Remember Me” Login Functionality
  • Why We Need SOP
  • SOP Definition
  • SOP Definition (CONT)
  • SOP Edge Cases
  • SOP Rules
  • SOP Rules: Cross-Origin Embedding Examples
  • SOP Rules: Cross-Origin Embedding Examples (CONT.)
  • How To Prevent Cross-Origin Writes?
  • Synchronizer Token Pattern
  • How To Allow Cross-Origin Access?
  • Additional Resources
  • Error Log
  • Knowledge Dependency Tree

Key Questions

• What is the Same-Origin Policy (SOP)?
  • What attacks does it prevent?
  • How does SOP relate to CORS?
• 80/20 Analysis

What Is The Origin?

• Where did the request originate from?
• If you’re on google.com’s homepage
  • For outgoing requests, google.com is the “origin”
• What if google.com’s javascript could make requests to any site on the internet?
  • How could this be a problem?
    • Hint: “Remember Me” login functionality

“Remember Me” Login Functionality

1. You authenticate to bank.com
2. bank.com sends an authentication cookie to your browser
   • Set-Cookie: auth=RANDOM_CHARS; Domain=bank.com
3. You close your browser
4. When you revisit bank.com, you don’t have to reenter your credentials
   • Any request to bank.com will contain the authentication cookie

Why We Need SOP

1. Visit evil.com
2. What prevents evil.com’s javascript from interacting with bank.com?
   • Same-Origin Policy helps scope evil.com’s requests to evil.com
     • One layer of defense

SOP Definition

• Restricts how a document or script loaded from one origin (evil.com) can interact with a resource from another origin (bank.com)
• The Same-Origin Policy helps protect sites that use authenticated sessions
• SOP Focus
  • Keeping privileged information safe
    • Helps prevent unauthorized read access
  • Does other items
• Source: MDN

SOP Definition (CONT)

• Browser Url: http://store.company.com/dir/page.html
  • Origin: <scheme> "://" <hostname> [ ":" <port> ]
    • Origin: http://store.company.com
    • Port: TCP port number on which the server is listening
      • If no port is given, fallback to scheme port
• MDN Origin Comparison

URL	SOP Violation?
http://store.company.com/dir2/other.html	No
http://store.company.com/dir/inner/another.html	No
https://store.company.com/secure.html	Yes (Different scheme)
http://store.company.com:81/dir/etc.html	Yes (Different port)
http://news.company.com/dir/other.html	Yes (Different host)

SOP Edge Cases

• What?! I embed <img> tags that don’t follow the SOP all the time!
  • These requests are going cross-origin
• Ex:

<img src="http://catphotos.example.com/dir/cat.gif" alt="He so puuurrrdy" height="42" width="42">

SOP Rules

• Cross-origin writes are typically allowed
  • Ex: POST form submissions
• Cross-origin embedding is typically allowed
  • Examples to come
• Cross-origin reads are typically not allowed
  • SOP core functionality
    • Preventing unauthorized reads
• Source: MDN

SOP Rules: Cross-Origin Embedding Examples

• Javascript
  • <script src="..."></script>
• CSS
  • <link rel="stylesheet" href="...">
• Images
  • <img>
• Media files
  • <video>, <audio>
• Plug-ins
  • <object>, <embed>, <applet>
• Source: MDN

SOP Rules: Cross-Origin Embedding Examples (CONT.)

• Fonts
  • @font-face
    • Some browsers allow cross-origin fonts, others require same-origin fonts
• iframes
  • <frame>, <iframe>
    • X-Frame-Options header mitigation
      • Prevent cross-origin embedding
• Source: MDN

How To Prevent Cross-Origin Writes?

• What prevents evil.com javascript from submitting a bank transfer to bank.com?
  • Nothing from the SOP
    • If done via form POST
  • bank.com must implement a synchronizer token

Synchronizer Token Pattern

<form action="http://bank.com/transfer" method="post">
  <input type="hidden" name="sync_token" value="j/DcoJ2VZvr7vdf8CHKsvjdlDbmiizaOb5B8DMALg6s=" >
  <!-- Rest of form -->
</form>

• For every state changing action (Ex: POST bank transfer)
  1. bank.com embeds a unique sync_token
     • Unique on every response to the browser
  2. Before proceeding with action from browser request
     • bank.com validates the sync_token
• How does the SOP interact with this?
  • evil.com can’t read sync_token due to SOP

How To Allow Cross-Origin Access?

• Cross Origin Resource Sharing (CORS)
  • Header sent in HTTP Response
    • Instructs browser to relax the SOP

Additional Resources

Error Log

Knowledge Dependency Tree