[Openid-specs-ab] Request artifact

John Bradley jbradley at mac.com
Fri Apr 30 01:51:36 UTC 2010


OK, I see what you are getting at. 

This attack requires a combination of directed identity and the RP pushing something to the OP via AX. 

So the request reference becomes a authentication in the opposite direction. 

I need to think about that a bit.

One way to deal with that is to include a nonce/seed/sessionID in the request so that it is unique if you are pushing info to the OP in the request.
That way a attacker who learns the request association can't reverse the value.

That also makes the request association effectively unguessable.
Passing in a sessionID so that in the directed identity case the RP can look for session swapping is something some RP will want.
All we need is a parameter the OP returns unmodified.

John B.
On 2010-04-29, at 9:27 PM, Nat Sakimura wrote:

> 
> 
> On Fri, Apr 30, 2010 at 9:42 AM, John Bradley <jbradley at mac.com> wrote:
> The OP MUST always authenticate the user for every transaction, even if it is with a long lived cookie.
> It is no different than a checkid_imediate.
> 
> Yes. But the attack vector cannot be avoided by the fact that there is an authentication before displaying it to the attacker. 
> 
> Flow is like this. 
> 
> Assumption: Attacker knows about Victim and wants to collect some more information about him. 
> Attacker entices the Victim to go to an RP to do some transaction with it. 
> 
> Victim->RP: Check Out with identifier_select
> RP-->Victim: Redirect to OP with Artifact
> Attacker->OP: Guess Artifact and make request. 
> OP->Attacker:  Present Username/Password etc. Authn screen
> Attacker->OP: Input his Username/Password
> OP->Attacker: Present confirmation screen that includes Victim's request. 
> 
> Of course, Attacker cannot do any useful further processing, because it is him who would 
> pay etc. for the transaction. However, some damages are done already. 
> (I know that this is rather esoteric and unlikely case, but still it is a possibility.) 
> 
> 
> 
> I am missing the double auth part of the flow.  
> 
> In the double auth case, the flow would be like this: (In case of POST binding. No difference in 
> fundamental flow for Artifact Binding.) 
> 
> User->RP: Sign-in Request
> RP-->User: Redirect to OP
> User->OP: Redir 
> OP->User: Authn req
> User->OP:  Authn res 
> OP-->User: Positive Assertion
> User->RP: Positive Assertion - Now RP knows User's Identifier. 
> User->RP: Use Shopping Cart
> User->RP: Check out
> RP-->User: Authz req for payment 
> User->OP: Authz req for payment
> OP->User: Show Invoice form RP
> User->OP: Authorization
> OP-->User: Positive Assertion (payment authorization)
> User->RP: Positive Assertion (payment authorization)
> 
> In this case, since the "check-out" process specifies the user identifier in its request, 
> the attacker cannot gain access to it. 
> 
> This is the proper way to deal with this attack, but for a lower value things, 
> sufficient entropy which is larger than the authenticator the user uses may prove to be enough. 
> 
> 
> The user gets redirected to there OP,  if they don't already have a session cookie for the OP they must authenticate before confirming any attributes or OAuth tokens.    
> 
> If they have a valid session cookie and have previously granted persistent permission for the RP to access the attributes, then there is no user dialog.
> 
> If the RP asks for a attribute when the user logs in and the user defaults to allowing the RP the attribute the RP can ask for it again later in the flow without popping a dialog.
> 
> I however don't know that I would agree to making a payment before I purchased something on a e-commerce site.
> 
> Am I missing something?
> 
> As explained above, for this "request reveal attack", the fact that there has to be some authentication 
> does not act as an effective counter measure.  
> 
> John B.
> On 2010-04-29, at 8:31 PM, Nat wrote:
> 
>> The problem I would have then is the directed identity flow. For something that is sufficiently high value, it would usually be the case that the user has been authenticated before checkout, so we do not have problems. The question is whether we require this "double auth flow" for a lower vale transactions. I suspect that there would be cases where one wants to combine the initial authentication (use identification) and the subsequent authorization in one, like much if the OAuth 1.0a flow. Or us it too much a niche to deal as a spec?  
>> 
>> =nat @ Tokyo via iPhone
>> 
>> On 2010/04/30, at 9:20, John Bradley <jbradley at mac.com> wrote:
>> 
>>> The attacker would need to authenticate to the OP before they are presented with a confirmation screen.
>>> That may be by a session cookie and transparent to the user.
>>> 
>>> Even if it is an attribute request the OP has to be certain it has the correct user.  The request from a RP in itself proves nothing.
>>> That is why I prefer to think of the request artifact as something that could be public.  
>>> Possessing the request artifact proves nothing.   If we say it is secret people will think it proves something.
>>> 
>>> John B.
>>> 
>>> On 2010-04-29, at 8:08 PM, Nat wrote:
>>> 
>>>> 
>>>> 
>>>> =nat @ Tokyo via iPhone
>>>> 
>>>> On 2010/04/30, at 0:17, John Bradley <jbradley at mac.com> wrote:
>>>> 
>>>>> Having the artifact for the artifact response be unguessable is important.
>>>>> 
>>>>> For the request, if an attacker guesses the artifact they can't learn any information.
>>>> 
>>>> If the request was a pure authentication, you probably are right. However, if it were an e-commerce transaction, it would not be. 
>>>> 
>>>> For example, it may be a purchase of a medication which may indicate certain desease. The OP may display that to the user for the consent of the payment etc. If the attacker can view that consent screen, it will be a release of sensitive information. 
>>>> 
>>>> 
>>>>> 
>>>>> The best they can do is use it in a authentication attempt. 
>>>>> 
>>>>> Given that anyone can make a artifact request to the OP for anyone else and get the request artifact that seems like a bunch of work for something that at best could be a denial of service.
>>>>> 
>>>>> If I were a OP I would probably generate the request artifacts as URL 
>>>>> 
>>>>> eg https://myOP.com/server1/openID?HASH=RC5_HASH
>>>>> 
>>>>> That way I can use the URI for a database lookup if I have one server,  or store the request in a file at the URL in rpf format if I have a cluster.
>>>>> 
>>>>> Others with clusters may want to compress and encrypt the request into the artifact,  I suspect normal losses compression won't be enough in some cases.
>>>>> You could do something custom to compress it but that is still non deterministic.
>>>>> Thats is why I would store the request by a hash.
>>>>> 
>>>>> With most OP's using directed identity, I am guessing you are probably only going to store one entry per RP in a lot of cases.
>>>>> 
>>>>> I think those choices are up to the OP to make.
>>>>> 
>>>>> I don't think that there is a real requirement in 7.5 for the artifact to be unique or random.
>>>>> 
>>>>> The most you can say is that it is an opaque reference to the original request less than 400 characters.
>>>>> 
>>>>> We need precise language in 7.6 saying one of openid.artifact | openid.rpfurl is required.
>>>>> 
>>>>> The terminology that we have used in the openID specs where we have multiple optional elements is not super clear if one from a set is required.
>>>>> 
>>>>> John B.
>>>>> 
>>>>> 
>>>>> On 2010-04-29, at 2:23 AM, Nat wrote:
>>>>> 
>>>>>> 
>>>>>> 
>>>>>> On 2010/04/29, at 0:48, John Bradley <jbradley at mac.com> wrote:
>>>>>> 
>>>>>>> Is the randomness requirement different for the request?   I think that we can safely assume that the request can be public.
>>>>>> 
>>>>>> I am not so sure about it. For a static request, it is safe to assume that it is a public request. However, for a dynamic request, there are chances that's request contains personalized data, which may be revealed at the OP. 
>>>>>> 
>>>>>> Under such circumstances, it may be wise to use a randomized reference to the request, IMHO.  
>>>>>> 
>>>>>>>  
>>>>>>> 
>>>>>>> The only randomness requirement would be to prevent an attacker from guessing it.   I think it would be better to only assume it is a reference to the request and may be used across multiple requests.
>>>>>>> 
>>>>>>> Why do you think there is a randomness requirement?
>>>>>>> 
>>>>>>> John B.
>>>>>>> 
>>>>>>> 
>>>>>>> On 2010-04-28, at 10:32 AM, Nat wrote:
>>>>>>> 
>>>>>>>> John, 
>>>>>>>> 
>>>>>>>> I am open to call request artifact as something else, but I do not think it is a good idea to combine the request artifact and rpfurl as the randomness requirement is very different. 
>>>>>>>> 
>>>>>>>> =nat @ Tokyo via iPhone
>>>>>>>> 
>>>>>>>> On 2010/04/28, at 23:25, John Bradley <jbradley at mac.com> wrote:
>>>>>>>> 
>>>>>>>>> Nat,
>>>>>>>>> 
>>>>>>>>> One simplification to consider for 7.6 may be to combine artifact and rpfurl.
>>>>>>>>> 
>>>>>>>>> If the OP has returned artifact that could be:
>>>>>>>>> Some internal refrence ID.
>>>>>>>>> A URL pointing to some internal reference.
>>>>>>>>> Some compressed version of the request.
>>>>>>>>> 
>>>>>>>>> If we think of the value as a reference to the request then the rpfurl is also a reference to the request.
>>>>>>>>> 
>>>>>>>>> The only difference is that one is defined by the OP and the other by the RP.
>>>>>>>>> 
>>>>>>>>> It may be confusing for people to have two things called artifact one for the request and one for the response.
>>>>>>>>> 
>>>>>>>>> The request could be renamed to something like request_refrence 
>>>>>>>>> 
>>>>>>>>> Some people may prefer them separate to make validation easier.
>>>>>>>>> 
>>>>>>>>> It is not a big thing.
>>>>>>>>> 
>>>>>>>>> John B.
>>>>>>> 
>>>>> 
>>> 
> 
> 
> 
> 
> -- 
> Nat Sakimura (=nat)
> http://www.sakimura.org/en/
> http://twitter.com/_nat_en

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