Re: [OAUTH-WG] [EXTERNAL] Re: dpop_jkt Authorization Request Parameter

[Brian Campbell <[e-mail]>]

I can't help but wonder if defining this E2E binding allowance as a
composite s256 + dpop_jkt PKCE method might be worth (re)considering here?
As a PCKE method it'd be decoupled from the main flow and could be used in
a more targeted way for situations that call for it while not imposing the
extra cost/complexity on situations that don't need it.

On Fri, Dec 3, 2021 at 9:23 AM Warren Parad <wparad=
[e-mail]> wrote:

> I think the allowed keys would have to be pre-registered in the AS.
>
> Warren Parad
>
> Founder, CTO
> Secure your user data with IAM authorization as a service. Implement
> Authress <https://authress.io/>.
>
>
> On Fri, Dec 3, 2021 at 5:01 PM Warren Parad <[e-mail]> wrote:
>
>> While I agree this is a problem, adding the thumbprint to the
>> authorization request only increases the difficulty for the malicious MITM
>> attack to need to also intercept the authorization request in addition to
>> the token request to swap out the dpop_jkt as well. If I'm right, then it
>> doesn't make sense to implement this as the solution.
>>
>> While I agree, we could move the dpop_jwk determination to the
>> authorization request, instead of the token, it doesn't solve the problem.
>> What it does say is that the authorization request user-agent is the one
>> that has the key, and not the one doing the code exchange. Well this is
>> actually weird in the case of non-public clients, because it doesn't make
>> sense from that client perspective, as the "front-end" would need to now
>> have the constructed dpop_jkt even though it doesn't have the dpop key.
>>
>> Warren Parad
>>
>> Founder, CTO
>> Secure your user data with IAM authorization as a service. Implement
>> Authress <https://authress.io/>.
>>
>>
>> On Fri, Dec 3, 2021 at 12:22 AM Mike Jones <Michael.Jones=
>> [e-mail]> wrote:
>>
>>> Thanks for this thoughtful analysis, Aaron.  I believe you’re spot on
>>> that these attacks can occur “when the attacker has access to both the
>>> authorization code as well as the PKCE code verifier.”
>>>
>>>
>>>
>>>                                                        -- Mike
>>>
>>>
>>>
>>> *From:* OAuth <[e-mail]> *On Behalf Of * Aaron Parecki
>>> *Sent:* Thursday, December 2, 2021 2:58 PM
>>> *To:* Warren Parad <wparad=[e-mail]>
>>> *Cc:* Pieter Kasselman <pieter.kasselman=[e-mail]>;
>>> [e-mail]
>>> *Subject:* Re: [OAUTH-WG] [EXTERNAL] Re: dpop_jkt Authorization Request
>>> Parameter
>>>
>>>
>>>
>>> Hi all, I've been giving this some more thought.
>>>
>>>
>>>
>>> The problem occurs when the attacker has access to both the
>>> authorization code as well as the PKCE code verifier. The assumption being
>>> made with PKCE is that the first time the PKCE code verifier and
>>> authorization code are seen together is in the POST request to the token
>>> endpoint. That means if there is a way to observe this request, the
>>> attacker can complete the exchange and get an access token.
>>>
>>>
>>>
>>> Pieter's writeup in the PDF describes one way that can happen, by
>>> exfiltrating log files. While I still agree with the sentiment in this
>>> thread that this is a relatively obscure condition, I also agree that it is
>>> actually something that can happen in the wild, especially since there are
>>> entire companies built around the idea of real-time log file analysis.
>>>
>>>
>>>
>>> That said, there are other ways an attacker could get access to these
>>> two pieces of information. What are the different points in a request
>>> lifecycle that could be attacked? The beginning, the middle and the end.
>>> We've talked about attacking the end, which is the log file example.
>>> Attacking the middle involves being in the middle of the TLS connection,
>>> which we also know is possible with corporate network proxies and such. We
>>> haven't yet talked about the beginning of the request. What can observe the
>>> beginning of a request? Here's a concrete example:
>>>
>>>
>>>
>>> Assume the OAuth client is a single-page app in a browser. The user is
>>> using an ad blocker installed as a browser extension. The ad blocker can be
>>> configured to observe and block network requests before they are made. If
>>> the extension is configured to attack a particular OAuth server, the JS
>>> client would make the token request containing the authorization code and
>>> PKCE code verifier, then the extension would be able to observe that
>>> request, block it, and ship the two values to the attacker's server where
>>> they can be redeemed and associated with the attacker's own DPoP key. Even
>>> perfectly single-use authorization codes don't help here either, because
>>> the original request was completely blocked.
>>>
>>>
>>>
>>> (Sidenote: To get ahead of any counterarguments here, yes, Chrome is
>>> eventually migrating to their new "manifest v3" which deprecates the
>>> webRequest API that allows this observation in favor of a different API
>>> that lets the browser block requests without making the actual request data
>>> available to the extension, which I am assuming they are doing in no small
>>> part because of the possibility of what I just described. It sounds like
>>> Mozilla is going to follow suit, but I haven't found concrete confirmation
>>> of that. That said, it will be a while before these changes are rolled out
>>> and support for the (dangerous) webRequest API is fully dropped, so this is
>>> likely going to continue to be a potential attack vector for a few years at
>>> least.)
>>>
>>>
>>>
>>> Because of the ease of deployment of a malicious browser extension, I do
>>> believe this is an important attack vector to consider and is worth
>>> solving. I am going to save any judgment on the particular dpop_jwk
>>> parameter proposal for a different thread, but I wanted to at least get on
>>> the same page about the fact that this is something worth solving first.
>>>
>>>
>>>
>>> Aaron
>>>
>>>
>>>
>>>
>>>
>>> On Thu, Dec 2, 2021 at 10:38 AM Warren Parad <wparad=
>>> [e-mail]> wrote:
>>>
>>> The only mention of sophistication is this logical fallacy:
>>>
>>>  If this leading security company had been penetrated, it almost
>>> certainly took an incredibly sophisticated attack.
>>>
>>>
>>>
>>> But it leaves out exactly what that was. And it doesn't give any insight
>>> into how this attack at MS would have been prevented despite the supply
>>> chain vulnerability based on the second point that Aaron made. If they are
>>> able to get the auth code, why aren't they able to get the DPoP signature?
>>> And then send both of these?
>>>
>>>
>>> Further in this case, it doesn't even matter if the attacker gets the
>>> access token if that access token is bound to the client, because it's
>>> worthless without the DPoP key. That's a much more secure solution than
>>> issuing non-bound Bearer tokens as a response to the bound authorization
>>> code. And if Bearer tokens are being used instead of bound tokens, then
>>> those could still end up in the logs, and be exfiltrated.
>>>
>>>
>>>
>>> In OAuth, the client already needs to authenticate with the AS, the spec
>>> is SHOULD, and options the client_secret already. Adding in the DPoP
>>> signature into the request is duplicating auth. If we don't like the client
>>> auth mechanisms to the AS, we should directly provide an auth RFC
>>> recommending better alternatives than sending a symmetric client_secret
>>> back to the AS.
>>>
>>>
>>>
>>> *Warren Parad*
>>>
>>> Founder, CTO
>>>
>>> Secure your user data with IAM authorization as a service. Implement
>>> Authress <https://authress.io/>.
>>>
>>>
>>>
>>>
>>>
>>> On Thu, Dec 2, 2021 at 4:42 PM Pieter Kasselman <pieter.kasselman=
>>> [e-mail]> wrote:
>>>
>>> Thanks for the comments and engagement Warren.
>>>
>>>
>>>
>>> The attacks we described and the ideas on mitigations are born out of
>>> attack vectors we are observing in the wild. They are not negligible. We
>>> are seeing a new class of very sophisticated attackers, and if you’re
>>> interested, this article provides good context on capability and
>>> sophistication of the attackers Brad Smith: Inside Microsoft during the
>>> SolarWinds hack (fastcompany.com)
>>> <https://www.fastcompany.com/90672384/microsoft-president-brad-smith-solarwinds-exclusive>.
>>> We are sharing this with the hope that the industry will benefit from our
>>> experiences and incorporate it into standards and products. Attacks that
>>> seemed impossibly complex are not only possible, but have become probable.
>>>
>>>
>>>
>>> The proposed changes for DPoP are not meant to replace the need for
>>> one-time use tokens (single use tokens are preferable and we should
>>> continue to expect them), but instead address the limitations around
>>> implementing one-time use, especially at scale. The 60s window you mention
>>> below is sufficiently long to be exploited by these sophisticated attackers.
>>>
>>>
>>>
>>> Cheers
>>>
>>>
>>>
>>> Pieter
>>>
>>>
>>>
>>> *From:* OAuth <[e-mail]> *On Behalf Of *Warren Parad
>>> *Sent:* Wednesday 1 December 2021 15:29
>>> *To:* Pieter Kasselman <pieter.kasselman=[e-mail]>
>>> *Cc:* Mike Jones <Michael.Jones=[e-mail]>;
>>> [e-mail]
>>> *Subject:* Re: [OAUTH-WG] [EXTERNAL] Re: dpop_jkt Authorization Request
>>> Parameter
>>>
>>>
>>>
>>> (e.g. one-time use in a certain timeframe etc).
>>>
>>>
>>>
>>> Sure but couldn't we just reduce the lifetime? Even if the token isn't
>>> one time use, surely the reuse time is trivially short which would prevent
>>> against exfiltration of the necessary security tokens to issue the attack?
>>>
>>>
>>>
>>> I feel like the simpler solution will always win, which in this case is
>>> one-time use tokens, then the problem is moot, right? So this only comes
>>> into play if you want to allow token reuse in a time window. The previously
>>> suggested max allowed time window from OAuth 2.1 was 60s for auth codes. So
>>> we are saying that the attack surface is still too large, for the .01% of
>>> implementations that have multi-use tokens, and the .01% of implementations
>>> that use the maximum 60s reuse, and then the subset of those that aren't
>>> correctly scrubing their logs, and then the subset of those that have a
>>> vulnerability which allows for exfiltration of both those logged tokens and
>>> the logged PKCE verifier?
>>>
>>>
>>>
>>> Why are we making this more complicated for a majority of cases, which:
>>>
>>>    - Only have single use tokens
>>>    - Or Only have a very short lifetime
>>>    - Or Are already correctly sanitizing their logs
>>>    - Or Have defense in depth for their deployments.
>>>
>>> If the implementation is so insecure that none of those are happening,
>>> wouldn't the implementation for this functionality also be suspect for an
>>> opportunity for attack?
>>>
>>>
>>>
>>> I feel like we are justifying here that multi-use tokens are wrong, but
>>> still want a solution to use them. Once we've proven that an deployment is
>>> not okay with using multi-use tokens, then the conclusion should be "don't
>>> have multi-use tokens", not: "let's still have multi-use tokens, but come
>>> up with a complex way to prevent their multi-use from accidentally being
>>> abused".
>>>
>>>
>>>
>>> Or am I missing something that would actually make this a
>>> non-negligible attack vector?
>>>
>>>
>>>
>>> - Warren
>>>
>>>
>>>
>>> *Warren Parad*
>>>
>>> Founder, CTO
>>>
>>> Secure your user data with IAM authorization as a service. Implement
>>> Authress
>>> <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fauthress.io%2F&data=04%7C01%7Cpieter.kasselman%40microsoft.com%7Cb5c71bfcbfbb48fd641508d9b4df5fcf%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637739693847580905%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=%2FBkvuWZ3FVTcdTtfe%2FoLurIGxcsJHCz6zXmW1PROTSc%3D&reserved=0>
>>> .
>>>
>>>
>>>
>>>
>>>
>>> On Wed, Dec 1, 2021 at 4:14 PM Pieter Kasselman <pieter.kasselman=
>>> [e-mail]> wrote:
>>>
>>> Hi Aaron, Neil
>>>
>>>
>>>
>>> Thanks for the questions.
>>>
>>>
>>>
>>> We agree that ideally authorization codes and PKCE proofs would never
>>> end up in log files and one-time use would be perfectly implemented.
>>>
>>>
>>>
>>> However, in practice these artefacts do find their way into log files in
>>> various places and one-time use may not always be practical (e.g. one-time
>>> use in a certain timeframe etc).
>>>
>>>
>>>
>>> The addition of these mitigations is not meant to replace the need for
>>> one-time use or good logging hygiene. Instead they provide pragmatic
>>> defence in depth against real attacks rather than assuming perfect
>>> implementations. We are deploying these mitigations and are sharing them
>>> for inclusion in DPoP to enable others to do the same.
>>>
>>>
>>>
>>> Regarding the question about interrupting/intercepting the HTTPS
>>> connection, the attacker don’t need to intercept the HTTPS connection or
>>> modify the content in the TLS tunnel, rather they just need to prevent the
>>> authorization code from being presented to the Authorization Server. It may
>>> even happen due to a poor network connection. The poor connection may be
>>> engineered by an attacker, or they may opportunistically benefit from it.
>>> The networks are not perfect either.
>>>
>>>
>>>
>>> Cheers
>>>
>>>
>>>
>>> Pieter
>>>
>>>
>>>
>>>
>>>
>>> *From:* OAuth <[e-mail]> *On Behalf Of *Aaron Parecki
>>> *Sent:* Wednesday 1 December 2021 00:05
>>> *To:* Neil Madden <[e-mail]>
>>> *Cc:* Mike Jones <Michael.Jones=[e-mail]>;
>>> [e-mail]
>>> *Subject:* [EXTERNAL] Re: [OAUTH-WG] dpop_jkt Authorization Request
>>> Parameter
>>>
>>>
>>>
>>> I tend to agree with Neil here. I'm struggling to see the relevance of
>>> this attack.
>>>
>>>
>>>
>>> It seems like the PDF writeup describes two possible reasons an attacker
>>> could get access to the authorization code and PKCE code verifier.
>>>
>>>
>>>
>>> 1. The attacker has access to the logs of the token endpoint.
>>>
>>> 2. The attacker can intercept HTTPS connections between the client and
>>> AS (VPN, corporate network proxy, etc)
>>>
>>>
>>>
>>> For 1, the solution is to stop logging the contents of the POST body,
>>> and secure your infrastructure. I don't think making the client jump
>>> through extra hoops is a good solution if you are already logging more than
>>> you should be or you don't trust the people who have access to the
>>> infrastructure. If this really is a concern, I suspect there are a lot more
>>> places in the flow that would need to be patched up if you don't trust your
>>> own token endpoint.
>>>
>>>
>>>
>>> For 2, if the attacker can intercept the HTTPS connection, then the
>>> proposed solution doesn't add anything because the attacker could modify
>>> the requests before it hits the authorization server anyway, and change
>>> which DPoP key the token gets bound to in the first place. Plus, the
>>> attacker would also have access to anything else the client is sending to
>>> the AS, such as the user's password when they authenticate at the AS.
>>>
>>>
>>>
>>> Are there other attack vectors I'm missing that might actually be solved
>>> by this mechanism?
>>>
>>>
>>>
>>> Aaron
>>>
>>>
>>>
>>>
>>>
>>> On Tue, Nov 30, 2021 at 12:40 PM Neil Madden <[e-mail]>
>>> wrote:
>>>
>>> Sadly I couldn’t make the DPoP session, but I’m not convinced the attack
>>> described in the earlier message really needs to be prevented at all. The
>>> attack largely hinges on auth codes not being one-time use, which is not a
>>> good idea, or otherwise on poor network security on the token endpoint. I’m
>>> not convinced DPoP needs to protect against these things. Is there more to
>>> this?
>>>
>>>
>>>
>>> The proposed solutions also seem susceptible to the same problems they
>>> attempt to solve - if an attacker is somehow able to interrupt the client’s
>>> (TLS-protected) token request, why are they somehow not able to
>>> interrupt/modify the (far less protected) redirect to the authorization
>>> endpoint?
>>>
>>>
>>>
>>> — Neil
>>>
>>>
>>>
>>> On 30 Nov 2021, at 20:15, Mike Jones <
>>> Michael.Jones=[e-mail]> wrote:
>>>
>>>
>>>
>>> As described during the OAuth Security Workshop session on DPoP, I
>>> created a pull request adding the dpop_jkt authorization request parameter
>>> to use for binding the authorization code to the client’s DPoP key.  See
>>> https://github.com/danielfett/draft-dpop/pull/89
>>> <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fdanielfett%2Fdraft-dpop%2Fpull%2F89&data=04%7C01%7Cpieter.kasselman%40microsoft.com%7Cb5c71bfcbfbb48fd641508d9b4df5fcf%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637739693847580905%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=ASCRFFPMA7qIItkxpVTrVaJtC53R2niWOzB0l0GQKrw%3D&reserved=0>
>>> .
>>>
>>>
>>>
>>> This is an alternative to
>>> https://github.com/danielfett/draft-dpop/pull/86
>>> <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fdanielfett%2Fdraft-dpop%2Fpull%2F86&data=04%7C01%7Cpieter.kasselman%40microsoft.com%7Cb5c71bfcbfbb48fd641508d9b4df5fcf%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637739693847580905%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=lgb9WmnOwWVtNIFsZ1mQG4jSBQYLZv%2BETe6HIKFeerg%3D&reserved=0>,
>>> which achieved this binding using a new DPoP PKCE method.  Using this
>>> alternative allows PKCE implementations to be unmodified, while adding DPoP
>>> in new code, which may be an advantage in some deployments.
>>>
>>>
>>>
>>> Please review and comment.  Note that I plan to add more of the attack
>>> description written by Pieter Kasselman to the security considerations in a
>>> future commit.  This attack description was sent by Pieter yesterday in a
>>> message with the subject “Authorization Code Log File Attack (was DPoP
>>> Interim Meeting Minutes)”.
>>>
>>>
>>>
>>>                                                        -- Mike
>>>
>>>
>>>
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>>>
>>>
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