W3C home > Mailing lists > Public > public-web-security@w3.org > December 2009

Re: Seamless iframes + CSS3 selectors = bad idea

From: Maciej Stachowiak <mjs@apple.com>
Date: Mon, 07 Dec 2009 02:53:18 -0800
Cc: Ian Hickson <ian@hixie.ch>, sird@rckc.at, public-web-security@w3.org
Message-id: <FDD777BF-2F80-4B50-B7AC-D1C87CE94A1E@apple.com>
To: Adam Barth <w3c@adambarth.com>

On Dec 6, 2009, at 8:22 AM, Adam Barth wrote:

> On Sun, Dec 6, 2009 at 1:21 AM, Ian Hickson <ian@hixie.ch> wrote:
>> On Sat, 5 Dec 2009, Adam Barth wrote:
>>> I think you're missing the main attack that sird's worried about:
>>> Assumptions:
>>> 1) The attacker can injection content into the target web site, but
>>> cannot injection script.
>> If you grant the assumption that the page has a faulty filter, IMHO  
>> it
>> becomes easy to have all kinds of vulnerabilities. That filters  
>> should
>> make sure the user can't insert arbitrary CSS is not new. Selectors  
>> and
>> expressions get more and more expressive with each year, but they  
>> pale in
>> comparison to the kind of deep analysis you can do to a page using  
>> and XPath, for example. This is why filters should always whitelist  
>> only
>> features they consider safe.
> The issue is slightly more subtle than you describe.  Filters aren't
> "faulty" or "safe," they just restrict what kinds of things the
> attacker can inject.  The question is what bad things the attacker can
> do with these injections.  sird's point is that allowing CSS is more
> severe than it used to be (modulo expression() and -moz-binding, which
> are generally considered poor features from a security point of view).

It seems like your filter would have to pass through both CSS and  
seamless iframes to increase the attack surface.

  - Maciej
Received on Monday, 7 December 2009 10:53:59 UTC

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