So I ask you: What does it mean, why is it here, and is it supposed to be here?
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Stefano was illustrating a potential exploit where returning an
That message should probably be removed now that he's illustrated his point. Ideally the browser shouldn't be showing that dialog at all (Chrome started preventing the dialog from appearing in this case in May of last year), and Stefano filed a bug report for Firefox to that effect.
So, to more directly answer your question: It means that your browser is vulnerable to the exploit (if you consider it an exploit), it's there because Stefano wanted to bring it to the attention of the Stack Exchange developers, and it probably does not need to be there any longer, since the point has been proven.
I deleted the message, so your browser won't request the image anymore and you won't get the popup