Hacking HTTP Status Codes

The trick is to identify GET requests that will succeed only if the victim is logged into $SITE_OF_INTEREST, and bury them in an

   <img src="https://SITE_OF_INTEREST/more/stuff/here" 
        onload="is_logged_in()"
        onerror="not_logged_in()"
        ...>

If $SITE_OF_INTEREST doesn't have decent CSRF protection, this is an easy way for a rogue website to not only make a request, but observe the result.

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