Security: Isolation

Lecture Notes for CS 142
Spring 2013
John Ousterhout

• Additional reading for this topic: none.
• The isolation problem:
  • Web content comes from many sources, not all equally trusted.
  • Trusted and untrusted content are in close proximity (frames, tabs, sequential visits).
  • Must separate various forms of content so that untrusted content cannot corrupt/misuse trusted content.
• Example: a "good" page displays a sponsored ad:
  • Attackers can buy advertisements, use them to attack good pages.
  • Advertiser gets to supply content for ad (e.g. "good" page links to advertiser site in <iframe>)
  • Ad can contain <script> elements that access DOM, submit forms, etc.

    parent.frames[0].forms[0].submit;
    

  • Even images aren't safe: browsers will accept non-image content.

Same-Origin Policy

• General idea: separate content with different trust levels into different frames, restrict communication between frames.
• One frame can access content in another frame only if they both came from the same origin. Origin =
  • Protocol
  • Domain name
  • Port (in some browsers).
• All modern browsers implement the same-origin policy.
• Same-origin applies to AJAX requests also.
• Where same-origin doesn't apply:
  • <script> tags: Javascript executes with full privileges of the enclosing frame.
  • <img> tags: may contain arbitrary HTML content.
• By itself, the same-origin policy is too restrictive: there are times when it is useful for frames with different origins to communicate in various ways.
• Domain relaxation:
  • Consider www.facebook.com, facebook.com, and chat.facebook.com.
  • If two frames each set document.domain to the same value, then they can communicate:
    • Must be a suffix of the actual domain.
    • Must explicitly set document.domain even if the value doesn't change (e.g., chat.facebook.com vs. facebook.com).
• New HTML5 feature: Access-Control-Allow-Origin header in HTTP responses:
  • Specifies one or more domains that may access this object's DOM.
  • Can use "*" to allow universal access.
• HTML5 postMessage mechanism:
  • Allows frames to send messages to each other in a controlled fashion.
  • Sender (from domain a.com):

    frames[0].postMessage("Hello world", "http://b.com/");
    

  • Receiver (domain b.com) can check origin:

    window.addEventListener("message", doEvent);
    function doEvent(e) {
      if (e.origin == "http://a.com") {
        ... e.data ... }
      }
    }
    
• Language-based isolation:
  • Frames are too restrictive for some situations:
    • Example: allow users to enter text that includes markup
  • An alternative:
    • Don't use frames: content from different sources intermixed.
    • Analyze content before including in page:
      • Don't allow some features (HTML, Javascript)
      • Modify code to include additional run-time checks for problems that can't be detected statically.
  • Example: Facebook Javascript (FBJS):
    • Subset of Javascript for use in Facebook applications.
    • Facebook analyzes/rewrites Javascript before it gets to your browser.
    • For example:
      • All identifiers get application-specific prefix to avoid conflicts:

        function foo(bar)
        

        is changed to

        function a123_foo(a123_bar)
        

      • Can't access the DOM directly: instead of

        element.parentNode... 
        

        must invoke FBJS method that enforces isolation:

        element.getParentNode()...
        

    • See http://developers.facebook.com/docs/fbjs/ for details on FBJS.
  • Language-based isolation is very tricky and prone to loopholes: beware!

Navigation

• Who is allowed to decide what content appears in a frame?

  frame.src = "http://www.attack.com/";
  

• Original policy was permissive: any frame could navigate any other frame.
• Guninski attack:
  • Password field on Citibank Web site contained within a frame.
  • Attacker could navigate this frame to an identical-looking one owned by the attacker; steals password.
  • Attack can come from any open window or tab.
• Solution:
  • Descendent policy: a frame can only navigate its children, and their children, etc.
  • Most current browsers implement this policy, but older versions such as IE6 and Firefox 2 are more permissive.

Cookie Security

• Cookies can be read and written from Javascript:

  alert(document.cookie);
  document.cookie = "name=value; expires=1/1/2011"
  

• Browsers use the same-origin policy to restrict access to cookies.