Re: [OAUTH-WG] Standard URL parameter for mitigating RFC6819's threat 4.6.4?

[Josh Mandel <[e-mail]>]

Hi Phil,

I was curious how you secure the first step.  If your client app is started
> by invoking a custom URL, what's to stop the attacker using that and
> passing in https://attacker.com as the desired RS and audience?


An attacker can do just as you've said -- but you didn't say which AS is
involved. The goal is that:

1. If the attacker.com RS asks the app to authorize with a *legitimate* AS,
that AS will reject the request upon seeing the "aud=https://attacker.com"
parameter in the request. This was the main threat I wanted to address with
this thread.

2. If the attacker.com RS asks the app to authorize against a *phony* AS,
then the end-user is looking at a "classic" phishing attack where the
browser is open to "https://attacker.com" and displaying a web page that
says "please enter your hospital credentials here". Here, the key
mitigation is training end-users not to enter their electronic health
record credentials into any site other than the legitimate EHR's
authorization server.

Again, if you have stronger ideas about how to mitigate #2 we'd love to
discuss. But the reason I started this thread was to address #1. One
alternative we recognzie is just to ask each client to maintain an explicit
whitelist of RS's that it will talk to. We're happy to have this as an
option, but 1) this puts a burden on the client, and a client can get it
wrong, and 2) we wanted to describe a protocol that allows more dynamic
connections. It's certainly a trade-off.

  -Josh

On Mon, Mar 16, 2015 at 11:16 AM, Phil Hunt <[e-mail]> wrote:

> Josh,
>
> I was curious how you secure the first step.  If your client app is
> started by invoking a custom URL, what's to stop the attacker using that
> and passing in https://attacker.com as the desired RS and audience?
>
> Phil
>
> @independentid
> www.independentid.com
> [e-mail]
>
> On Mar 16, 2015, at 11:12 AM, Josh Mandel <[e-mail]> wrote:
>
> Hi Phil,
>
> It might help me if you spelled out the phishing scenario you have in
> mind. The key threat we're attempting to mitigate against with an "aud"
> param is:
>
> 1. Phishing link tells app to launch against RS at https://attacker.com
> 2. App queries https://attacker.com/metadata (this is a healthcare API
> called FHIR, which exposes server metadata) to discover which authorization
> server to talk to. Attacker lies and tells app to authorize at "
> https://my-real-hospital.com/authorize".
> 3. App attempts to authorize against "
> https://my-real-hospital.com/authorize?client_id=...&state=&...&aud=https://attacker.com
> "
> 4. The hospital's authorization server rejects the authorization request
> because https://attacker.com is not a legitimate resource server.
>
> Now you may be describing a different attacker where in step #2 the
> attacker tells the app to authorize against "
> https://attacker.com/authorize", and this page asks a user to sign in
> with her hospital credentials. That's indeed a phishing attack that we need
> separate mitigation against. Happy to think through other mitigations on
> this front, but it's a separate issue than what I was trying to address
> with this thread. (Today our primary mitigation for this kind of phishing
> threat, where attacker.com asks the user to enter her EHR credentials, is
> user training.)
>
>   -J
>
> On Mon, Mar 16, 2015 at 10:59 AM, Phil Hunt <[e-mail]> wrote:
>
>> Josh,
>>
>> I'm not sure this helps. It seems to me, a phishing link could tell your
>> app to launch and pass in any RS value could it not?
>>
>> Phil
>>
>> @independentid
>> www.independentid.com
>> [e-mail]
>>
>> On Mar 16, 2015, at 10:47 AM, Josh Mandel <[e-mail]> wrote:
>>
>> Hi Torsten,
>>
>> You're absolutely correct. The implementation we're contemplating for
>> SMART Platforms does indeed derive the "aud" parameter from the actual URL
>> that the client used to communicate with the RS. Very briefly, the flow is:
>>
>> 1. Electronic Health Record system tells an app to launch, passing the
>> app its RS endpoint as an issuer (iss) URL parameter.
>> 2. App queries the RS's issuer URL to learn what AS to talk to
>> 3. App contacts AS's authorize endpoint, passing an "aud" param that was
>> the same as the issuer from step 1.
>> 4. After obtaining a token, app talks to the RS using that token.
>>
>>   -Josh
>>
>>
>> On Mon, Mar 16, 2015 at 10:40 AM, Torsten Lodderstedt <
>> [e-mail]> wrote:
>>
>>> I don't think putting an aud into an AT will help to prevent counterfeit
>>> RSs (as long as the aud is nit directly derived from the original URL used
>>> by the client to invoke the counterfeit RS. as long as the aud is a
>>> symbolic name of any kind, the counterfeit RS will accept ATs for the
>>> legitimate RS and just (ab)use it.
>>>
>>> POP on the contrary helps since the counterfeit RS, in order to send a
>>> message to the legitimate RS, needs to produce a new digitally signed
>>> message using the correct secret, which it doesn't know.
>>>
>>> kind regards,
>>> Torsten.
>>>
>>>
>>>
>>> Am 16.03.2015 um 17:40 schrieb Dixie Baker <
>>> [e-mail]>:
>>>
>>> Using the "aud" parameter makes sense to me.  Good suggestion.
>>>
>>> Authenticating the client to the RS would not address the counterfeit RS
>>> threat.
>>>
>>> -Dixie
>>>
>>>
>>> Dixie B. Baker, Ph.D.
>>> Senior Partner
>>> Martin, Blanck and Associates
>>> Office (Redondo Beach, CA):  310-791-9671
>>> Mobile:  [phone]
>>>
>>> On Mar 16, 2015, at 6:43 AM, Brian Campbell <[e-mail]>
>>> wrote:
>>>
>>> We've used "aud" (optionally) with OAuth 2 and bearer tokens to help
>>> identify the RS to whom the AT should be issued. It is useful but it's
>>> mostly about getting format/content/etc of the AT correct for the RS rather
>>> than it is about preventing possible AT leaks.
>>>
>>> I do think an "aud(iance)" parameter at both token and authorization
>>> endpoints would have utility beyond the POP work. So defining it
>>> independently might make sense.
>>>
>>> On Sun, Mar 15, 2015 at 11:34 AM, John Bradley <[e-mail]>
>>> wrote:
>>>
>>>> In POP key distribution we do introduce a "audiance" parameter to the
>>>> token_endpoint.
>>>> https://tools.ietf.org/html/draft-ietf-oauth-pop-key-distribution-01#section-3.1
>>>>
>>>> It would be possible to have a small spec to define using "aud" with
>>>> bearer tokens, however that would be undefined behaviour at this point.
>>>>
>>>> I don't know of any clients that would try to access a RS server and
>>>> then besed on the error response try and get a access token from the AS
>>>> specified in the error.
>>>>
>>>> In POP we are trying to both protect agains that attack and more common
>>>> ones like doing a MiM to intercept the AT or the RS being hacked and
>>>> leaking the token.
>>>>
>>>> Using "aud" with bearer tokens would be useful, but probably won't stop
>>>> the majority of possible AT leaks.
>>>>
>>>> John B.
>>>>
>>>>
>>>> On Mar 15, 2015, at 2:18 PM, Torsten Lodderstedt <
>>>> [e-mail]> wrote:
>>>>
>>>>  Hi Josh,
>>>>
>>>> I'm not aware of a common practice to use such a parameter. The WG is
>>>> instead heading towards authenticated requests to the resource server (see
>>>> https://tools.ietf.org/html/rfc6819#section-5.4.2).
>>>>
>>>> Please take a look onto
>>>> http://tools.ietf.org/html/draft-ietf-oauth-pop-architecture and
>>>> further drafts on this topic.
>>>>
>>>> kind regards,
>>>> Torsten.
>>>>
>>>> Am 03.03.2015 um 18:27 schrieb Josh Mandel:
>>>>
>>>>  Hi All,
>>>>
>>>>  In section 4.6.4 ("Threat: Access Token Phishing by Counterfeit
>>>> Resource Server"), RFC6819 describes a threat where a counterfeit resource
>>>> server tricks a client into obtaining and sharing an access token from a
>>>> legitimate authorization server. One of the proposed mitigations involves:
>>>> "telling the authorization server about the resource server endpoint URL in
>>>> the authorization process."
>>>>
>>>>  In other words, this mitigation would ask the client to pass an
>>>> additional parameter when redirecting to the Authorization server's
>>>> "authorize" URL, effectively something like:
>>>>
>>>>  https://auth-server/authorize?
>>>>  response_type=code&
>>>> client_id=123&
>>>> state=456&
>>>>  scope=read-all&
>>>>  redirect_uri=https://app-server/after-auth&
>>>> *resource_server_that_told_me_to_authorize_here=https://attacker.com
>>>> <https://attacker.com/>*
>>>>
>>>>  (And if the authorization server saw a value it didn't like in the
>>>> final parameter, it would reject the request.)
>>>>
>>>>  This is obviously not appropriate in every authorization scenario,
>>>> but it is useful anytime there's a discovery process by which apps learn
>>>> about authorization servers from resource servers. Since it's something of
>>>> a common need, I wanted to see if there was any common practice in how to
>>>> name this parameter, or whether it's worth registering a standard extension
>>>> at
>>>> http://www.iana.org/assignments/oauth-parameters/oauth-parameters.xhtml
>>>> . (I don't see one there now -- possibly I'm just missing it.)
>>>>
>>>>  If so, what should it be called? The name I used in the example above
>>>> is a bit verbose :-)
>>>>
>>>>  Best,
>>>>
>>>>    Josh
>>>>
>>>>
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