Re: [websec] Richard Barnes' Discuss on draft-ietf-websec-x-frame-options-09: (with DISCUSS and COMMENT)

[Richard Barnes <[e-mail]>]

On Fri, Aug 16, 2013 at 7:44 PM, Hill, Brad <[e-mail]> wrote:

> Additional comments inline.
> ________________________________________
>
>
> (D3) Shouldn't ALLOW-FROM be followed by an origin, not a URI?  In other
> words, what does it mean to send "X-Frame-Options: ALLOW-FROM
> https://example.com/this/is/a/path?query#fragment"?
>
> [Hill, Brad] Agreed.


Great.



> (D3) In the ALLOW-FROM: what does "top level context" mean?  Do you
> really mean the top level here, as opposed to the next one up?  For
> example, suppose A loads B in an iframe, and B loads C, and then C sends
> an X-Frame-Options header with ALLOW-FROM.  Is the ALLOW-FROM origin
> compared to B or A?  In either case, you should also note the attacks
> that remain.  For example, if the answer is B, then B needs to use
> X-Frame-Options as well, or else, A can maliciously frame A within B.  Or
> if the answer is A, then C is trusting A not to load any malicious
> intermediate frames B.
>
> [Hill, Brad]  This really does mean the top/final origin value in a frame
> ancestor
> chain walk.  Browsers have implemented X-Frame-Options to check the
> Origin context that is topmost in the window or tab.  (the _top target,
> representing the full, original browsing context, not just the immediate
> parent frame)  This could be clarified perhaps, but is not incorrect.
>

OK, that's fine.  Could you please just note the risk that an intermediate
frame in a nested scenario could do bad things?  For example, in the
Security Considerations:
"""
When SAMEORIGIN or ALLOW-FROM values are used, there is some residual risk
in nested framing scenarios.  For example, suppose that A loads B in an
iframe; B loads C; and C sends an X-Frame-Options header with the value
"ALLOW-FROM A".  The browser will allow this setup, because the ALLOW-FROM
origin sent by C matches the top-level origin.  However, the intermediate
framing page B may still be able to perform clickjacking attacks against A.
 Thus, sites using this mechanism should keep in mind that by emitting an
X-Frame-Options header with value SAMEORIGIN or ALLOW-FROM, they are not
only granting permission to the indicated origin (the same origin, or the
ALLOW-FROM origin), but also to any origins included as frames within that
origin.
"""

Thanks,
--Richard