It prevents JSON hijacking, a major JSON security issue that is formally fixed in all major browsers since 2011 with ECMAScript 5.

Contrived example: say Google has a URL like mail.google.com/json?action=inbox which returns the first 50 messages of your inbox in JSON format. Evil websites on other domains can't make AJAX requests to get this data due to the same-origin policy, but they can include the URL via a <script> tag. The URL is visited with your cookies, and by overriding the global array constructor or accessor methods they can have a method called whenever an object (array or hash) attribute is set, allowing them to read the JSON content.

The while(1); or &&&BLAH&&& prevents this: an AJAX request at mail.google.com will have full access to the text content, and can strip it away. But a <script> tag insertion blindly executes the JavaScript without any processing, resulting in either an infinite loop or a syntax error.

This does not address the issue of cross-site request forgery.

It prevents disclosure of the response through JSON hijacking.

In theory, the content of HTTP responses is protected by the Same Origin Policy: pages from one domain cannot get any pieces of information from pages on the other domain (unless explicitly allowed).

An attacker can request pages on other domains on your behalf, e.g. by using a <script src=...> or <img> tag, but it can't get any information about the result (headers, contents).

Thus, if you visit an attacker's page, it couldn't read your email from gmail.com.

Except that when using a script tag to request JSON content, the JSON is executed as JavaScript in an attacker's controlled environment. If the attacker can replace the Array or Object constructor or some other method used during object construction, anything in the JSON would pass through the attacker's code, and be disclosed.

Note that this happens when the JSON is executed as JavaScript, not when it's parsed.

There are multiple countermeasures:

Making sure the JSON never executes

By placing a while(1); statement before the JSON data, Google ensures that the JSON data is never executed as JavaScript.

Only a legitimate page could actually get the whole content, strip the while(1);, and parse the remainder as JSON.

Things like for(;;); have been seen on Facebook for instance, with the same results.

Making sure the JSON is not valid JavaScript

Similarly, adding invalid tokens before the JSON, like &&&START&&&, makes sure that it is never executed.

Always return JSON with an Object on the outside

This is OWASP recommended way to protect from JSON hijacking and is the less intrusive one.

Similarly to the previous counter-measures, it makes sure that the JSON is never executed as JavaScript.

A valid JSON object, when not enclosed by anything, is not valid in JavaScript, since the { } gets interpreted as a code block:

eval('{"foo":"bar"}')
// SyntaxError: Unexpected token :

This is however valid JSON:

JSON.parse('{"foo":"bar"}')
// Object {foo: "bar"}

So, make sure you always return an Object at the top level of the response and make sure that the JSON is not valid JavaScript, while still being valid JSON.

As noted by @hvd in the comments, the empty object {} is valid JavaScript, and knowing the object is empty may itself be valuable information.

Comparison of the above methods

The OWASP way is less intrusive, as it needs no client library changes, and transfers valid JSON. It is unsure whether past or future browser bugs could defeat this, however. As noted by @oriadam, it is unclear whether data could be leaked in a parse error through an error handling or not (e.g. window.onerror).

Google's way requires a client library in order for it to support automatic de-serialization and can be considered to be safer with regard to browser bugs.

Both methods require server-side changes in order to avoid developers accidentally sending vulnerable JSON.

This is to ensure some other site can't do nasty tricks to try to steal your data. For example, by replacing the array constructor, then including this JSON URL via a <script> tag, a malicious third-party site could steal the data from the JSON response. By putting a while(1); at the start, the script will hang instead.

A same-site request using XHR and a separate JSON parser, on the other hand, can easily ignore the while(1); prefix.

That would be to make it difficult for a third-party to insert the JSON response into an HTML document with the <script> tag. Remember that the <script> tag is exempt from the Same Origin Policy.

Note: as of 2019, many of the old vulnerabilities that lead to the preventative measures discussed in this question are no longer an issue in modern browsers. I'll leave the answer below as a historical curiosity, but really the whole topic has changed radically since 2010 (!!) when this was asked.

It prevents it from being used as the target of a simple <script> tag. (Well, it doesn't prevent it, but it makes it unpleasant.) That way bad guys can't just put that script tag in their own site and rely on an active session to make it possible to fetch your content.

edit — note the comment (and other answers). The issue has to do with subverted built-in facilities, specifically the Object and Array constructors. Those can be altered such that otherwise innocuous JSON, when parsed, could trigger attacker code.

Since the <script> tag is exempted from the Same Origin Policy which is a security necessity in the web world, while(1) when added to the JSON response prevents misuse of it in the <script> tag.

As this is a High traffic post I hope to provide here an answer slightly more undetermined to the original question and thus provide further background on a JSON Hijacking attack and its consequences

JSON Hijacking as the name suggests is an attack similar to Cross-Site Request Forgery where an attacker can access cross-domain sensitive JSON data from applications that return sensitive data as array literals to GET requests. An example of a JSON call returning an array literal is shown below:

[{"id":"1001","ccnum":"4111111111111111","balance":"2345.15"}, 
{"id":"1002","ccnum":"5555555555554444","balance":"10345.00"}, 
{"id":"1003","ccnum":"5105105105105100","balance":"6250.50"}]

This attack can be achieved in 3 major steps:

Step 1: Get an authenticated user to visit a malicious page. Step 2: The malicious page will try and access sensitive data from the application that the user is logged into. This can be done by embedding a script tag in an HTML page since the same-origin policy does not apply to script tags.

<script src="http://<jsonsite>/json_server.php"></script>

The browser will make a GET request to json_server.php and any authentication cookies of the user will be sent along with the request. Step 3: At this point, while the malicious site has executed the script it does not have access to any sensitive data. Getting access to the data can be achieved by using an object prototype setter. In the code below an object prototypes property is being bound to the defined function when an attempt is being made to set the "ccnum" property.

Object.prototype.__defineSetter__('ccnum',function(obj){
    secrets =secrets.concat(" ", obj);
});

At this point, the malicious site has successfully hijacked the sensitive financial data (ccnum) returned byjson_server.php JSON

It should be noted that not all browsers support this method; the proof of concept was done on Firefox 3.x.This method has now been deprecated and replaced by the useObject.defineProperty There is also a variation of this attack that should work on all browsers where full-named JavaScript (e.g. pi=3.14159) is returned instead of a JSON array.

There are several ways in which JSON Hijacking can be prevented:

• Since SCRIPT tags can only generate HTTP GET requests, they only return JSON objects to POST requests.

• Prevent the web browser from interpreting the JSON object as valid JavaScript code.

• Implement Cross-Site Request Forgery protection by requiring that a predefined random value be required for all JSON requests.

so as you can see While(1) comes under the last option. In the most simple terms, while(1) is an infinite loop that will run till a break statement is issued explicitly. And thus what would be described as a lock for the key to be applied (google break statement). Therefore a JSON hijacking, in which the Hacker has no key will be consistently dismissed. Alas, If you read the JSON block with a parser, the while(1) loop is ignored.

So in conclusion, the while(1) loop can more easily be visualized as a simple break statement cypher that google can use to control the flow of data.

However, the keyword in that statement is the word 'simple'. The usage of authenticated infinite loops has been thankfully removed from basic practice in the years since 2010 due to its absolute decimation of CPU usage when isolated (and the fact the internet has moved away from forcing through crude 'quick-fixes'). Today instead the codebase has embedded preventative measures, and the system is not crucial or effective anymore. (part of this is the move away from JSON Hijacking to more fruitful data farming techniques that I won't go into at present)

After authentication is in place, JSON hijacking protection can take a variety of forms. Google appends while(1) into their JSON data, so that if any malicious script evaluates it, the malicious script enters an infinite loop.

Reference: Web Security Testing Cookbook: Systematic Techniques to Find Problems Fast