[keycloak-dev] client requested account linking

[Bill Burke]

Ok, I changed this again.  Any solution we come up with will require 
that the application protect itself from CSRF for this operation.  
There's nothing really the server can do. We can give guidelines on how 
to accomplish this.  So, instead of this handshake, there will only be 
one GET request to a link url that takes client_id, redirect_uri, nonce, 
and hash query parameters.

The nonce will be a random string sent by the client.  The hash will be 
a base64url encoded SHA_256 hash of nonce + clientSessionId.  The server 
will verify this hash by looking for an SSO cookie, matching it up with 
a UserSession, then looping through ClientSessionModels to find a 
matching clientId then creating and matching the hash.  This is combined 
with not allowing CORS preflight requests to this URL  (protects SSO 
cookie) as well as checking that any Origin header matches the client.  
I think this is enough to ensure that this server URL is protected from 
direct CSRF attacks.


On 2/24/17 8:46 PM, Bill Burke wrote:
> Ok, option #3 where I would pass an access token to initiate the link
> won't work as the access token could be leaked by a CSRF attack on the
> client.  My new solution is to do a custom protocol.  The goal of this
> protocol is for the server to verify that the client initiated the
> account link and not some rogue site.
>
> 1. Client does a background bearer-token authenticated to the Keycloak
> operation /initiate-link using the access token it got from login.
>
> 2. /initiate-link gets the ClientSessionModel associated with the
> bearer-token. It generates a server-nonce and stores it within the
> ClientSessionModel.
>
> 3. keycloak /initiate-link returns a response that contains the server-nonce
>
> 4. Client generates its own client-nonce.  This client-nonce is stored
> in its HttpSession.
>
> 5. Client generates a nonce-hash of server-nonce + client-nonce +
> provider-id.
>
> 6. Client redirects the browser to Keycloak /link operation passing the
> client-nonce, nonce-hash, provider-id, client-id, and redirect-uri as
> query parameters.
>
> 7. Keycloak /link operation validates that the browser is logged in.  If
> the user is not logged, an error page is shown.  We DO NOT redirect back
> to the client when there is an error as this is an attack vector.
>
> 8. Keycloak verifies clientid, redirect uri, and provider id.
>
> 9. Keycloak loops through the browser's UserSessionModel looking for
> ClientSessionModels that match the clientId.  Checks to see if this
> ClientSession has a server-nonce within it.  If so, it validates the
> hash.  We DO NOT redirect back to the client when there is an error as
> this is an attack vector.  The server-nonce client session attribute is
> cleared so that it can only be used once.
>
> 10. Keycloak initiates the social provider login
>
> 11. Keycloak redirects back to the client using the redirect_uri
>
> What isn't protected here is the client.  If the client is a traditional
> web app, the client is responsible for protecting itself from CSRF.  The
> Account Service does this with a cookie nonce plus the nonce in the
> rendered page links, and it also rejects any CORS requests.  The client
> app will have to do this too.  I don't think Javascript apps are
> affected unless there is a URL you can redirect to that triggers this flow.
>
> Again, I'm not even sure we have to do this sort of thing.  All a CSRF
> attack could do is request that the user link their Keycloak account.
> The attacker can't get any information about the user.
>
>
>
> On 2/24/17 9:10 AM, Bill Burke wrote:
>> Account linking *MUST* be browser based as you need to delegate
>> authentication to the IDP you are brokering with.
>>
>>
>> On 2/24/17 3:21 AM, Marek Posolda wrote:
>>> Don't we have a plan to rewrite AccountService to use Angular+REST? If
>>> yes, then we are fine though. We will have REST endpoint for link
>>> broker, which can be invoked with the bearer token as long as the
>>> token has "manage-account" scope. We don't need to care about CSRF then.
>>>
>>> I can see the only reason to keep support browser+cookie based flows -
>>> the case when client application doesn't have Keycloak token. But from
>>> what you mentioned in the last paragraph, it looks that their client
>>> has our access token?
>>>
>>> Marek
>>>
>>>
>>> On 23/02/17 23:24, Bill Burke wrote:
>>>> A customer has asked us to implement a feature in which there is a
>>>> browser endpoint on keycloak.  This URL can be told to link to a
>>>> specific identity broker provider (Google, Facebook, etc.) and then the
>>>> browser would be redirected back to the client. Pretty much what exists
>>>> in the Account Service console, but without having to look at the
>>>> Account Service.  The reason for this is that they are doing integration
>>>> with a specific social provider and don't want to have to go through the
>>>> Account Service pages. Seems pretty reasonable and valid use case...
>>>> I'm worried about a couple of things:
>>>>
>>>> * The design of it
>>>>
>>>> * The security implications.
>>>>
>>>> The implementation would be simple enough, it would just extract code
>>>> from the accoutn service.  The endpoint would take a "providerId"
>>>> paramter that would be the idp linking to, a "clientId" for the client
>>>> requesting the link, and a "redirect_uri" to redirect back to after the
>>>> link is successful. Obviously the redirect_uri would be validated
>>>> against the clientId.
>>>>
>>>> Now comes the interesting part, the security implications. Account
>>>> linking in the Account Service is fine and dandy and doesn't have any
>>>> security holes because we guarantee that the Account Service initiated
>>>> the linking via a CSRF check.  For this new Client-Requested-Linking
>>>> feature, if we do nothing, and just model it as above, we cannot
>>>> guarantee that the client initiated the linking request.  What are the
>>>> implications of this?  This feature would be vulnerable to CSRF.  A
>>>> rogue website could initiate account linking to a specific configured
>>>> provider.  Is this bad?  I don't know.  We can guarantee that the
>>>> redirect uri is valid.  The browser would never get back to the rogue
>>>> website. So what can we do to improve this?
>>>>
>>>> * We could do nothing and hope its ok that anybody can initiate a link.
>>>>
>>>> * We could add a consent screen like this: "Application 'clientId' is
>>>> requesting that you link your user account to 'providerId'.  Do you
>>>> agree to this?  You will be redirected to this provider if you click
>>>> ok.".  This of course relies on the user to actually read the page.  Is
>>>> this good enough?
>>>>
>>>> * My last thought would be OIDC specific.  We could use the POST binding
>>>> trick that SAML does to do a browser redirect via a POST call. THe POST
>>>> would contain the access token the client has.  We match this token up
>>>> against the browser cookie to make sure its the same session. We also
>>>> make sure that the access token has permission to request a link.  There
>>>> are 2 downsides to this approach a) This requires some code on the
>>>> client side to obtain the access token and then to package it up into an
>>>> HTML document so the POST binding trick can be done.  b) This will only
>>>> work for OIDC clients.  SAML clients will be left in the dust, although
>>>> I guess we could allow SAML to push a signed assertion back.
>>>>
>>>>
>>>> Thoughts?
>>>>
>>>>
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