Let's say you don't want other sites to "frame" your site in an <iframe>
:
<iframe src="http://example.org"></iframe>
So you insert anti-framing, frame busting JavaScript into all your pages:
/* break us out of any containing iframes */
if (top != self) { top.location.replace(self.location.href); }
Excellent! Now you "bust" or break out of any containing iframe automatically. Except for one small problem.
As it turns out, your frame-busting code can be busted, as shown here:
<script type="text/javascript">
var prevent_bust = 0
window.onbeforeunload = function() { prevent_bust++ }
setInterval(function() {
if (prevent_bust > 0) {
prevent_bust -= 2
window.top.location = 'http://example.org/page-which-responds-with-204'
}
}, 1)
</script>
This code does the following:
- increments a counter every time the browser attempts to navigate away from the current page, via the
window.onbeforeunload
event handler - sets up a timer that fires every millisecond via
setInterval()
, and if it sees the counter incremented, changes the current location to a server of the attacker's control - that server serves up a page with HTTP status code 204, which does not cause the browser to navigate anywhere
My question is -- and this is more of a JavaScript puzzle than an actual problem -- how can you defeat the frame-busting buster?
I had a few thoughts, but nothing worked in my testing:
- attempting to clear the
onbeforeunload
event viaonbeforeunload = null
had no effect - adding an
alert()
stopped the process let the user know it was happening, but did not interfere with the code in any way; clicking OK lets the busting continue as normal - I can't think of any way to clear the
setInterval()
timer
I'm not much of a JavaScript programmer, so here's my challenge to you: hey buster, can you bust the frame-busting buster?
example.org
as specified in RFC 2606 ietf.org/rfc/rfc2606.txt – Baitsrc
of the iframe is set dynamically. – Lowrie$("a").click(function() { prevent_bust--; });
at the bottom of the page – Elburrvar prevent_bust = 0 window.onbeforeunload = function() { prevent_bust++ } var interval = setInterval(function() { if (prevent_bust > 0) { prevent_bust -= 2; window.top.location = 'example.org/204.php'; clearInterval(interval); } }, 1);
– Yahiya