So I ask you: What does it mean, why is it here, and is it supposed to be here?
Stefano was illustrating a potential exploit where returning an
401 Authorization Required status for an image resource (which you can embed, say, in chat) will cause certain browsers to show the authorization dialog, which could cause some unaware user to mistakenly disclose some sort of credentials.
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.
@balpha here's a security vulnerability
I can't think of a fix though (: (possibly do a HEAD on images, remove if it looks suspicious)
(this is the code, it's completely trivial: paste.ubuntu.com/788593)
and sorry to everyone else for the annoying pop-up
I deleted the message, so your browser won't request the image anymore and you won't get the popup