Why does Google prepend while(1); to their JSON responses?
Why does Google prepend
while(1); to their (private) JSON responses?
For example, here's a response while turning a calendar on and off in Google Calendar:
while(1);[['u',[['smsSentFlag','false'],['hideInvitations','false'],
['remindOnRespondedEventsOnly','true'],
['hideInvitations_remindOnRespondedEventsOnly','false_true'],
['Calendar ID stripped for privacy','false'],['smsVerifiedFlag','true']]]]
I would assume this is to prevent people from doing an
eval() on it, but all you'd really have to do is replace the while and then you'd be set. I would assume the eval prevention is to make sure people write safe JSON parsing code.
I've seen this used in a couple of other places, too, but a lot more so with Google (Mail, Calendar, Contacts, etc.) Strangely enough, Google Docs starts with
&&&START&&& instead, and Google Contacts seems to start with while(1); &&&START&&&.
What's going on here?
Solution:
It prevents JSON hijacking.
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.
