Why does a self-referencing iframe not infinitely loop and crash my machine?

I created a simple HTML page with an iframe whose src attribute references the containing page -- in other words a self-referencing iframe.

this.html

<html>
<head></head>
<body>
<iframe src="this.html"></iframe>
</body>
</html>

Why does this not infinitely loop and crash my browser? Also, why doesn't even IE crash at this?

(Note: This spawned from a team discussion on the virtues and demerits of using iframes to solve problems. You know, the 'mirror of a mirror' sort.)

Answers


W3C took care of that in 1997 explaining how frames should be implemented in "Implementing HTML Frames":

Any frame that attempts to assign as its SRC a URL used by any of its ancestors is treated as if it has no SRC URL at all (basically a blank frame).


Iframe recursion bug/attack history

As kingdago found out and mentioned in the comment above, one browser that missed to implement a safeguard for this was Mozilla in 1999. Quote from one of the developers:

This is a parity bug (and a source of possible embarrasment) since MSIE5 doesn't have a problem with these kinds of pages.

I decided to dig some more into this and it turns out that in 2004 this happened again. However, this time JavaScript was involved:

This is the code, what causes it: <iframe name="productcatalog" id="productcatalog" src="page2.htm"></iframe> directly followed by a script with this in it: frames.productcatalog.location.replace(frames.productcatalog.location + location.hash);

...

Actual Results: The parent window gets recursively loaded into the iframe, resulting sometimes in a crash.

Expected Results: Just show it like in Internet Explorer.

Then again in 2008 with Firefox 2 (this also involved JavaScript).

And again in 2009. The interesting part here is that this bug is still open and this attachment: https://bugzilla.mozilla.org/attachment.cgi?id=414035 (will you restrain your curiosity?) will still crash/freeze your Firefox (I just tested it and I almost crashed the whole Ubuntu). In Chrome it just loads indefinitely (probably because each tab lives in a separate process).


As for the other browsers:

  • In 2005 Konqueror had a bug in it's safeguard that allowed to render iframes one inside another (but it seems that somehow it wasn't freezing/crashing the whole app).
  • IE6, Opera 7.54 and Firefox 0.9.3 are also reported to be susceptible to attacks basing on iframe recursion.

I'd like to add a little something to the "Also, why doesn't even IE crash at this?" part of the question. IE does not let us down...

If you add a simple iteration number as a query string to the nested iFrame's src Firefox and others will just stop after a certain iteration depth. IE - and we tested this with IE version 10 - just crashes :)

this.php

<html>
<head></head>
<body>
<iframe src="this.php?q=<?php echo (isset($_GET['q'])?$_GET['q']:1)+1?>" />
</body>
</html>

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