[security] Danger of Content-Location HTTP response header

[John Bradley]

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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>>   
> 
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