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?
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.