    def check_xsrf_cookie(self):
        """Verifies that the ``_xsrf`` cookie matches the ``_xsrf`` argument.

        To prevent cross-site request forgery, we set an ``_xsrf``
        cookie and include the same value as a non-cookie
        field with all ``POST`` requests. If the two do not match, we
        reject the form submission as a potential forgery.

        The ``_xsrf`` value may be set as either a form field named ``_xsrf``
        or in a custom HTTP header named ``X-XSRFToken`` or ``X-CSRFToken``
        (the latter is accepted for compatibility with Django).

        See http://en.wikipedia.org/wiki/Cross-site_request_forgery

        Prior to release 1.1.1, this check was ignored if the HTTP header
        ``X-Requested-With: XMLHTTPRequest`` was present.  This exception
        has been shown to be insecure and has been removed.  For more
        information please see
        http://www.djangoproject.com/weblog/2011/feb/08/security/
        http://weblog.rubyonrails.org/2011/2/8/csrf-protection-bypass-in-ruby-on-rails
        """
        token = (self.get_argument("_xsrf", None) or
                 self.request.headers.get("X-Xsrftoken") or
                 self.request.headers.get("X-Csrftoken"))
        if not token:
            raise HTTPError(403, "'_xsrf' argument missing from POST")
        _, token, _ = self._decode_xsrf_token(token)
        _, expected_token, _ = self._get_raw_xsrf_token()
        if not _time_independent_equals(utf8(token), utf8(expected_token)):
            raise HTTPError(403, "XSRF cookie does not match POST argument")
The product behaves differently or sends different responses under different circumstances in a way that is observable to an unauthorized actor, which exposes security-relevant information about the state of the product, such as whether a particular operation was successful or not.