Re: CORS security hole?

On Mon, Jul 16, 2012 at 11:01 PM, Henry Story <>wrote:

> I first posted this to public-webapps, and was then told the security
> discussions were taking
> place on public-webappsec, so I reposted there.
> On 17 Jul 2012, at 00:39, Adam Barth wrote:
> As I wrote when you first posted this to public-webapps:
> [[
> I'm not sure I fully understand the issue you're worried about, but if I
> understand it correctly, you've installed an HTTP proxy that forcibly adds
> CORS headers to every response.  Such a proxy will indeed lead to security
> problems.  You and I can debate whether the blame for those security
> problem lies with the proxy or with CORS, but I think reasonable people
> would agree that forcibly injecting a security policy into the responses of
> other servers without their consent is a bad practice.
> ]]
> Hmm, I think I just understood where the mistake in my reasoning is: When
> making a request on the CORS proxy (
> on a resource
> intended for, the browser will not send the credentials
> to the proxy - since and are different domains.  So
> the attack I was imagining won't work, since the proxy won't just be able
> to use those to pass itself for the browser user.
> Installing an HTTP proxy locally would of course be a trojan horse attack,
> and then all bets are off.
> The danger might rather be the inverse, namely that if a CORS proxy is
> hosted by a web site used by the browser user for other authenticated
> purposes ( say a social web server running on a freedom box ) that the
> browser pass the authentication information to the proxy in the form of
> cookies, and that this proxy then pass those to any site the initial script
> was connected to.
> Here using only client side certificate authentication helps, as that is
> an authentication mechanism that cannot be forged or accidentally passed on.
> ---
> Having said that, are there plans to get to a point where JavaScript
> agents could be identified in a more fine grained manner?


>  Cryptographically perhaps with a WebID?

That's unlikely to happen soon.

Then the Origin header could be a WebID and it would be possible for a user
> to specify in his foaf profile his trust for a number of such agents? But
> perhaps I should start that discussion in another thread....

I'd recommend getting some implementor interest in your proposals before
starting such a thread.  A standard without implementations is like a fish
without water.


> On Mon, Jul 16, 2012 at 8:13 AM, Henry Story <>wrote:
>> Hi,
>>  Two things came together to make me notice the problem I want to discuss
>> here:
>>  1. On the read-write-web and WebID community groups we were discussion
>> the possibility of delegated authorisation with WebID [1]
>>  2. I was building a CORS proxy for linked data javascript agents (see
>> the forwarded message )
>> Doing this I ended up seeing some very strong parallels between CORS and
>> WebID delegation. One could
>> say that they are nearly the same protocol except that WebID delegation
>> uses a WebID URL to identify an agent, rather than the CORS Origin
>> protocol:hostname:port service id. The use case for WebID authorisation
>> delegation is also different. In CORS the browser is the secretary doing
>> protecting information leakage to an external JS agent, in WebID
>> authorisation delegation a server would be doing work on behalf of a user.
>> Having worked out the parallel I was able to start looking into the
>> security reasoning between CORS. And this is where I found a huge security
>> hole ( I think ). Essentially if you allow authenticated CORS requests,
>> then any javascript agent can use a doggy CORS proxy, and use that to fool
>> any browser into thinking the server was aware that it was not the user
>> directly it was communicating with but the javascript agent.
>> This is possible because CORS authenticated delegation with cookies is
>> hugely insecure. CORS authenticated delegation over TLS would not suffer
>> from this problem, as the proxy could not pass itself off as the browser
>> user.
>>  Another reason to use TLS, another reason to use WebID.
>>  More of the detailed reasoning below...
>>         Henry
>> [1] using
>> Begin forwarded message:
>> > From: Henry Story <>
>> > Subject: Re: CORS Proxy
>> > Date: 7 July 2012 08:37:24 CEST
>> > To: Read-Write-Web <>
>> > Cc: WebID <>, Joe Presbrey <>,
>> Mike Jones <>, Romain BLIN <
>>>, Julien Subercaze <
>> >
>> >
>> > On 7 Jul 2012, at 07:35, Henry Story wrote:
>> >
>> >>
>> >> On 6 Jul 2012, at 23:10, Henry Story wrote:
>> >>
>> >>> Hi,
>> >>>
>> >>> I just quickly put together a CORS Proxy [1], inspired by Joe
>> Presbrey's CORS proxy [2].
>> >>>
>> >>> But first, what is a CORS proxy?
>> >>> --------------------------------
>> >>>
>> >>> A CORS [3] proxy is needed in order to allow read-write-web pages
>> containing javascript agents written with libraries such as rdflib [5] to
>> fetch remote resources. Pages containing such javascript agents are able to
>> fetch and parse RDF on the web, and thus crawl the web by following their
>> robotic nose. A CORS Proxy is needed here because:
>> >>>
>> >>> 1- browsers restrict which sites javascript agents can fetch data
>> from to those from which the javascript came from - the famous "same origin
>> policy" ( javascript can only fetch resources from the same site it
>> originated from)
>> >>> 2- CORS allows an exception to the above restriction, if the resource
>> has the proper headers. For a GET request this is the
>> Access-Control-Allow-Origin header
>> >>> 3- most RDF resources on the web are not served with such headers
>> >>>
>> >>> Hence javascript agents running inside web browsers that need to
>> crawl the web, need a CORS proxy, so that libraries such as rdflib can go
>> forward and make those requests through the proxy. In short: a CORS proxy
>> is a service that can forward the request to the appropriate server and on
>> receiving the answer add the correct headers, if none were found.
>> >>>
>> >>> Security
>> >>> --------
>> >>>
>> >>> So is there a security problem having a CORS proxy make a GET request
>> for some information on behalf of JS Agent? This is an important question,
>> because otherwise we'd be introducing a security hole with such a proxy.
>> >>>
>> >>> In order to answer that question we need to explain why browsers have
>> the same origin restriction.
>> >>>
>> >>> The answer is quite simple I think. A Javascript agent running in a
>> browser is using the credentials of the user when it makes requests for
>> resources on the web. One can therefore think of the browser as acting as a
>> secretary for the javascript agent: the JS agent makes a request, but does
>> not log in to a web site, but instead asks the browser to fetch the
>> information. The browser uses its authentication credentials - the user of
>> the browser's credentials to be precise - to connect to remote sites and
>> request resources. The remote site may be perfectly fine with the browser
>> user/owner having access to the resource, but not like the idea of the
>> agent in the browser doing so. (after all that could be some JS on some
>> random site the user came across) In order to avoid this danger, the
>> browser sends along with its requests an Origin: header and the URL of the
>> host where the javascript was found. The service receiving such a request
>> must respond with an Access-Control-Allow-Origin header to make clear that
>> it is ok with the JS Agent receiving this information.
>> >>> IF the browser finds out that the web site allows the JS Agent to
>> receive the information too, then it will pass the information on to it.
>> >>>
>> >>> This is a bit like what we discussed about a secretary agent on a web
>> site requesting a resource On-Behalf-Of a user in our WebID delegation [5]
>> . Here the Browser is the secretary, and the JS agent is the user being
>> acted on behalf of. The difference is that the Secretary/Browser in this
>> case is well known, and the JS Agent is the unknown; or put another way in
>> the CORS case the server's authorisation policies were geared towards
>> giving the browser owner access and not for the JS it is acting on behalf
>> of, whereas in our delegation use case we were imagining the remote
>> resource not being initially authorised directly to the secretary, but
>> rather for the agent she was acting on behalf of. (small shift in focus)
>> >>>
>> >>> Anonymous Proxy
>> >>>
>> >>> So now it should be clear why using the proxy is not creating a new
>> security issue. This is because the Proxy is not the Browser and so has NOT
>> authenticated as the user when it is making a request. The Proxy is making
>> an anonymous request to a remote resource. This remote resource is
>> therefore public. As such it is fine to allow any JS agent to read it. This
>> is particularly true of GET requests. But it should even be true of POST,
>> PUT and DELETE requests. If those are public and allow anonymous usage,
>> then it should be possible for a CORS proxy to do that on behalf of a
>> javascript. It is quite possible that the CORS proxy might want to
>> authenticate the user in order to not become a vector for denial of service
>> attacks, and it could even give users a history of requests it made. Unless
>> perhaps some people are placing identification cookies in URLs! (But one
>> could argue that's their problem?!)
>> >>
>> >> Ah, there is an issue here I just realised. Currently my proxy is
>> passing along all the headers sent to
>> >> it by the client.  Well all minus the "Accept" headers, because my
>> Proxy only understands a subset of RDF
>> >> serialisations. ( And the proxy has to interpret the rdf in the
>> serialisation in order to transform relative
>> >> URIs into full URIs.  The proxy we are building is not a real HTTP
>> Proxy, because those would require a browser to be set up correctly and
>> doing that would probably not be possible on a case by case basis )
>> >>
>> >>
>> >> case CORSFetch(url, headers) => {
>> >>   val hdrs = for (key <- (headers - "Accept").keys;
>> >>                  value <- (headers(key))) yield (key,value)
>> >>   val promiseIteratee: Promise[Iteratee[Array[Byte],
>> Either[CORSException, CORSResponse[Rdf]]]] =
>> >>     WS.url(url.toExternalForm).
>> >>       withHeaders(hdrs.toSeq:_*).
>> >>       withHeaders("Accept" ->
>> "application/rdf+xml,text/turtle,application/xhtml+xml;q=0.8,text/html;q=0.7,text/n3;q=0.2").
>> >>       get {
>> >>       response: ResponseHeaders =>
>> >>              ....
>> >>
>> >> So here all the headers - "Accept" are passed along. But that means
>> cookies are passed along too!
>> >> And if cookies are passed along, then the user's identifications are
>> passed along, and that would be
>> >> problematic, for the reasons given above.
>> >>
>> >> Luckily as explained by this CORS article
>> >>
>> >>
>> >> unless the javascript makes the XMLHttpRequest with the
>> withCredentials property set to true, cookies are not sent. Also - even
>> then - unless the server in its response places the header
>> >>
>> >>     Access-Control-Allow-Credentials: true
>> >>
>> >> in the header the browser will not pass the credentials on to the
>> javascript (and it also has to be explicitly about which origin it allows)
>> >>
>> >> But this raises another issue: it would be extremely easy for me to
>> change my proxy to add those headers to the response. If so this means that
>> any evil script, can communicate with such a slightly enhanced proxy that
>> does the transformation described. And so CORS is protecting nothing
>> seriously here.
>> >
>> > An evil proxy could also - come to think of it - change the headers for
>> the Origin of the request to one that any bank would feel comfortable with,
>> so that with this system in place a service that would allow access to some
>> Origin, would allow access to all of them (via evil proxies).
>> >
>> > Interestingly enough, this attack does not work with WebID over TLS
>> authentication. The Proxy cannot know the browser's private key, and so
>> cannot authenticate to the remote service other than by using its own
>> certificate and WebID. Identity here is not passed by a spoofable cookie,
>> but as a cryptographically verified TLS session.
>> >
>> > In a secure WebID world one may still want the proxy to act on behalf
>> of the browser owner. This could be done if the Proxy specified that it was
>> acting on behalf of the user of the browser using WebID Authorization
>> Delegation [5]. This would be something that the resource serving service
>> could both verify and evaluate.
>> >
>> >
>> >>
>> >>
>> >>
>> >>>
>> >>> Authenticated Proxies
>> >>>
>> >>> Now let us assume we have CORS proxies that can also authenticate
>> with WebID. If they did so, then they would have to follow the same rules
>> as the Browser: they should not pass on the information unless the server
>> had allowed them to by setting the correct Access-Control-Allow-Origin
>> headers, and allowing the javascript access, since the information they
>> were given was not meant for this other JS agent.
>> >>>
>> >>> But things could get a little more advanced yet: Imagine that the
>> proxy authenticated itself on behalf of the user who made the request. The
>> server serving the resource could then verify that the proxy was allowed to
>> do act on behalf of the user using the procedure outlined in WebID
>> delegation [5]. Then we would have to deal with WebID delegation plus CORS
>> delegation. The server would know that the proxy was acting on behalf of
>> the user, and that the user was acting on behalf of the JS Agent... This
>> may be the use case we had trouble putting our finger on at this weeks
>> WebID teleconf...
>> >>>
>> >>>
>> >>> Improving CORS with LinkedData
>> >>> ------------------------------
>> >>>
>> >>> Here is an idea to improve CORS: In WebID delegation we found a way
>> to let the server know what relation the secretary had to the agent she
>> acted on behalf of. That Agent she was acting on behalf of could add a
>> >>>
>> >>> :me cert:secretaty myprofile:secr .
>> >>>
>> >>> relation to his profile. This would help the server know what
>> relation the secretary had to the agent she claimed to be working for.
>> >>>
>> >>> With CORS this relationship is nowhere made explicit (as far as I can
>> see): there is no way for me to tell a web site that the javascript I am
>> using ( and that is hosted on my freedom box) is something I would like the
>> server I am connecting to trust, as opposed to some javascript that just
>> started executing itself on some server I came across. This makes me thing
>> that
>> >>> 1. it would be useful if JS could be signed so as to have a better
>> identity than just the identity of a whole site
>> >>> 2. we could create a relation such as the cert:secretary one that
>> would allow me in my profile to say for example
>> >>>
>> >>> :me cert:trustedJS <> .
>> >>>
>> >>> IT's really much to vague, but it would make it easier for people to
>> trust my JS.
>> >>>
>> >>>
>> >>> -----------
>> >>>
>> >>> There are really quite a lot of details questions left open for CORS
>> proxies. Should a CORS proxy return a 203? When? Who should it deal with
>> error messages? .... I think we should organise those on a wiki page, and
>> keep the discussion alive. My code at present was put together really
>> quickly, but it made me ask a lot of questions whilst writing it.
>> >>>
>> >>>
>> >>>     Henry
>> >>>
>> >>>
>> >>> [1] the code is here
>> >>> But I have not placed it online yet.
>> >>> [2] Joe's proxy is online and available by sending GET requests here:
>> >>> and the code is somewhere here:
>> >>> [3]
>> >>> [4]
>> >>> [5]
>   Social Web Architect

Received on Tuesday, 17 July 2012 06:11:08 UTC