Why do people put code like "throw 1; <dont be evil>" and "for(;;);" in front of json responses?

Google returns json like this:

throw 1; <dont be evil> { foo: bar}

and Facebooks ajax has json like this:

for(;;); {"error":0,"errorSummary": ""}

Why do they put code that would stop execution and makes invalid json?
How do they parse it if it's invalid and would crash if you tried to eval it?
Do they just remove it from the string (seems expensive)?
Are there any security advantages to this?

In response to it being for security purposes:

If the scraper is on another domain they would have to use a script tag to get the data because XHR won't work cross-domain. Even without the for(;;); how would the attacker get the data? It's not assigned to a variable so wouldn't it just be garbage collected because there's no references to it?

Basically to get the data cross domain they would have to do

<script src="http://target.com/json.js"></script>

But even without the crash script prepended the attacker can't use any of the Json data without it being assigned to a variable that you can access globally (it isn't in these cases). The crash code effectivly does nothing because even without it they have to use server sided scripting to use the data on their site.

Solution

Even without the for(;;); how would the attacker get the data?

Attacks are based on altering the behaviour of the built-in types, in particular Object and Array, by altering their constructor function or its prototype. Then when the targeted JSON uses a {...} or [...] construct, they'll be the attacker's own versions of those objects, with potentially-unexpected behaviour.

For example, you can hack a setter-property into Object, that would betray the values written in object literals:

Object.prototype.__defineSetter__('x', function(x) {
    alert('Ha! I steal '+x);
});

Then when a <script> was pointed at some JSON that used that property name:

{"x": "hello"}

the value "hello" would be leaked.

The way that array and object literals cause setters to be called is controversial. Firefox removed the behaviour in version 3.5, in response to publicised attacks on high-profile web sites. However at the time of writing Safari (4) and Chrome (5) are still vulnerable to this.

Another attack that all browsers now disallow was to redefine constructor functions:

Array= function() {
    alert('I steal '+this);
};

[1, 2, 3]

And for now, IE8's implementation of properties (based on the ECMAScript Fifth Edition standard and Object.defineProperty) currently does not work on Object.prototype or Array.prototype.

But as well as protecting past browsers, it may be that extensions to JavaScript cause more potential leaks of a similar kind in future, and in that case chaff should protect against those too.