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FlowSafe: Information Flow Security for the Browser

The central idea is to improve the default browser security model, which is "stuck" since 1995 at the Same-Origin Policy with its underlying and conflicting DOM access control and JavaScript object-capability security layers.

We aim to do this without breaking the web, and indeed with measurable improvements to safety property enforcement and security policy expressiveness.


  • Improve default cross-site script integrity (ads, analytics)
  • Systematically enforce the Same-Origin Policy and better security policies by pervasive mediation
  • Reduce existing "caps", DOM, and JavaScript engine patch-work / leaky reference monitor code
  • Guarantee termination-insensitive non-interference for better confidentiality
  • Explore timing and termination channel mitigations


Implement dynamic-only, fail-stop "no sensitive upgrade" or better, information flow security for JS, the DOM, and other parts of the browser. See this paper on part of the work.

  1. Add JSTrustLabel to the JS API, a union of JSPrincipals (trust labels replace principals)
  2. JSScript has a JSTrustLabel
  3. Interpreter pc has a JSTrustLabel
  4. Extend JSExtendedClass to delegate typeof so we can build LabeledValue wrappers for primitives
    1. Also need to rebox results of all primitive operators, e.g. -x for x = new LabeledValue(42) should be a LabeledValue instance, not the number -42
  5. Have shapes imply trust labels so that distinct origins get different shapes for standard objects, equivalent property list patterns, etc.
    1. So objects have trust labels because objects have shapes
    2. Beware shapeless objects (dense arrays, certain "host objects")
    3. Cormac's semantics requires labeled references too, another use-case for LabeledValue
  6. Add policy JS API that allows custom assignment, control flow branching, and input/output policy decision points -- does this mean SSA in one pass?
  7. Add shell functions for testing and write tests
  8. DOM, other host objects have trust labels
  9. Exceptions, etc.
  10. Declassify primitive TBD, defer for now

--Brendan 01:43, 31 October 2009 (UTC)


Efficient Purely-Dynamic Information Flow Analysis (PLAS '09)