Wednesday, 14 June 2017

javascript - Why does Google prepend while(1); to their JSON responses?

Answer


Answer




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?


Answer



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

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