Code snippet to post a HTML5 Canvas image to Facebook

postcanvas.js

/*
 * Code snippet to post a HTML5 Canvas image to Facebook
 * H. Nagata
 *
 * Variables:
 *   accessToken: Facebook access token
 *   targetID: Posting target id such as user id or album id
 *   canvas: HTMLCanvasElement
 *   base64.decode: c.f. https://github.com/hnagata/js-base64
 *
 * It won't work in IE9 since using ArrayBuffer/FormData/Blob.
 */

// Convert canvas into binary data
var data_url = canvas.toDataURL();
var mime = data_url.slice(data_url.indexOf(":") + 1, data_url.indexOf(";"));
var data = base64.decode(data_url.slice(data_url.indexOf(",") + 1));

// Construct FormData
var fd = new FormData();
fd.append("access_token", accessToken);
fd.append("source", new Blob([data], {type: mime}));
  // And append some fields, such as "message" and "privacy"

// POST
var req = new XMLHttpRequest();
req.open("POST", "https://graph.facebook.com/" + targetID + "/photos");
req.send(fd);

I got a issue: SecurityError: The operation is insecure. for line15.

how to implement such as share button FB, so user doesn't need to authentication ?