GlobalScopeViaThis

(legacy summary: this is often bound to the global scope.) (legacy labels: Attack-Vector)

Global scope reachable via this from functions not invoked as methods

Effect

Untrusted functions invoked naively by trusted code can steal access to the global scope.

Background

EcmaScript 262 section 10.1.5 defines a Global Object which is the source of all references not satisfied by local function variables, properties of an object in an enclosing with block, or exception variables for an enclosing catch block.

When javascript is executing in multiple frames, it may be hard to determine whether an object is a global scope.

EcmaScript 5 changes the reflective myFunction.{call,apply} methods to not coerce null to the global scope in strict mode. Technically, this is undefined when null or undefined is passed to call or apply but when code references this, the global object is substituted for null or undefined. In strict mode, the interpreter will not perform this coercion.

Assumptions

An untrusted function that references this can be invoked not as a method (or in ES5, a property accessor), and not via call or apply without the global scope (or null or undefined) as the first input.

AND

Untrusted functions that reference this are not rewritten to abort execution if this is the global object.

Versions

All

Example

(function () {
   alert('your cookie is ' + this.document.cookie);
 })();

setTimeout(
    function () {
      alert('your cookie is ' + this.document.cookie);
    }, 0);