[security] Danger of Content-Location HTTP response header

[Andrew Arnott]

I'll do that.  Thanks.
--
Andrew Arnott
"I [may] not agree with what you have to say, but I'll defend to the death
your right to say it." - S. G. Tallentyre


On Mon, Nov 16, 2009 at 10:06 AM, Breno de Medeiros <breno at google.com>wrote:

> Andrew,
>
> This is a sufficiently significant risk that should be documented in
> the security wiki. I encourage you to do so.
>
> On Sat, Nov 14, 2009 at 7:21 PM, Andrew Arnott <andrewarnott at gmail.com>
> wrote:
> > That's right, John.
> > For a vulnerable RP, an attacker could set up any URL to impersonate any
> > other user at the RP simply by logging into the RP with his own URL,
> after
> > configuring it to send back the Content-Location header with the victim's
> > claimed_id as its value.
> > I've confirmed that the extremeswankopenid library is vulnerable to this
> > attack, and have contacted the author already.
> > Regarding your question about how this is different than delegating your
> > identifier to a victim's OpenID... I'm not familiar with such an
> approach,
> > or how that would be exploited.
> > --
> > Andrew Arnott
> > "I [may] not agree with what you have to say, but I'll defend to the
> death
> > your right to say it." - S. G. Tallentyre
> >
> >
> > On Sat, Nov 14, 2009 at 10:50 AM, John Bradley <ve7jtb at ve7jtb.com>
> wrote:
> >>
> >> This is a attack on discovery.
> >> If the RP performs discovery on URL A the owner of URL A can return a
> XRDS
> >> with a content-Location header for URL B.  The RP now believes that
> whatever
> >> OP endpoint is in the XRDS is authoritative for URL B without having
> >> retrieved the actual XRDS for it, only the one for URL A claiming to be
> B.
> >> The problem is that .Net "helps" the application by making it think a
> >> redirect has taken place when it hasn't.
> >> There are lots of times when this works just fine however the claimed_id
> >> is tied to the product of the second normalization so is vulnerable to
> this
> >> sort of fake redirect.
> >> Andrew can provide more of the details.
> >> John B.
> >> On 2009-11-14, at 2:24 PM, Allen Tom wrote:
> >>
> >> Hi Andrew,
> >>
> >> Would an attacker be able to exploit this issue by returning the
> >> Content-Location HTTP response header for an URL that he owns, making
> his
> >> URL equivalent to a victim's OpenID? How is this different from having
> the
> >> attacker delegating his URL to the victim's OpenID?
> >>
> >> Can you outline a scenario where the Content-Location HTTP header is
> >> exploited?
> >>
> >> Thanks
> >> Allen
> >>
> >>
> >>
> >> Arnott wrote:
> >>
> >> Just a heads up from something I recently became aware of that impacted
> >> older versions of dotnetopenid.
> >> The HTTP protocol defines a Content-Location HTTP response header that
> >> allows the web server to suggest to the client that another URL would be
> >> equivalent to the one that client actually pulled from.  It is not a
> >> redirect, but merely a suggestion that two URLs are equivalent.  For the
> >> purposes of OpenID claimed identifier discovery, it is imperative that
> an
> >> OpenID RP ignore this header, lest a web server upon which discovery was
> >> performed can spoof an arbitrary claimed_id's identity by fooling the RP
> >> into thinking it discovered an identifier that in fact it did not.
> >> In particular, .NET's "helpful" HTTP stack automatically reads this
> header
> >> and reports it to the client as if it was in fact that actual URL that
> was
> >> pulled from even though it wasn't.  Since .NET follows redirects
> >> automatically by default, a legitimate redirect and this
> Content-Location
> >> header are indiscernable, which is really bad.  This is fixed in the
> >> dotnetopenid and dotnetopenauth libraries.
> >> Other RP library/site authors should verify that the HTTP stack they are
> >> using ignore this header, or workaround the issue.
> >> I've set up a test on test-id.org where an RP can very quickly assess
> >> whether they are vulnerable.  Please take a moment to find out, and fix
> it
> >> ASAP if you are.
> >> http://test-id.org/RP/IgnoresContentLocationHeader.aspx
> >> --
> >> Andrew Arnott
> >> "I [may] not agree with what you have to say, but I'll defend to the
> death
> >> your right to say it." - S. G. Tallentyre
> >>
> >> ________________________________
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> >> security at lists.openid.net
> >> http://lists.openid.net/mailman/listinfo/openid-security
> >>
> >>
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> >>
> >
> >
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> >
>
>
>
> --
> --Breno
>
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