Re: [OAUTH-WG] DPoP followup I: freshness and coverage of signature

Vladimir Dzhuvinov <vladimir@connect2id.com> Sat, 12 December 2020 08:22 UTC

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From: Vladimir Dzhuvinov <vladimir@connect2id.com>
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Subject: Re: [OAUTH-WG] DPoP followup I: freshness and coverage of signature
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If the current DPoP has code complexity "X", the relative additional
complexity to include access token hashes doesn't seem like very much.
An app choosing DPoP means accepting the code complexity that comes with
dealing with keys, composing the signing inputs for the proofs, signing,
the necessary changes to the token and RS requests. On the other hand,
for some people that additional access token hash may become the straw
that breaks the camel's back, causing them to quit their jobs developing
web apps and never look back :)

Have you thought about letting deployments decide about the access token
hash? To say look, there is also the option to bind an access token to
the DPoP proof, the security benefits can be such an such, and this is
how it can be done.

What I don't like about that proposal:

  * It will complicate the spec

  * The current spec doesn't require implementers / deployments to make
    any decisions, apart from adopt / not DPoP (okay, also choose a JWS
    alg) - which is actually a great feature to have


Vladimir


On 12/12/2020 01:58, Brian Campbell wrote:
> Any type of client could use DPoP and (presumably) benefit from
> sender-constrained access tokens. So yeah, adding complexity
> specifically for browser-based applications (that only mitigates one
> variation of the attacks possible with XSS anyway)  has 'cost' impact
> to those clients as well. And should be considered in the
> cost/benefit. Including the AT hash isn't terribly complicated but
> it's not trivial either. I'm honestly still unsure but am leaning
> towards it not being worth adding.
>
> On Fri, Dec 11, 2020 at 2:14 AM Philippe De Ryck
> <philippe@pragmaticwebsecurity.com
> <mailto:philippe@pragmaticwebsecurity.com>> wrote:
>
>     The scenario you describe here is realistic in browser-based apps
>     with XSS vulnerabilities, but it is pretty complex. Since there
>     are worse problems when XSS happens, it’s hard to say whether DPoP
>     should mitigate this. 
>
>     I’m wondering what other types of clients would benefit from using
>     DPoP for access tokens? Mobile apps? Clients using a Client
>     Credentials grant?
>
>     How are they impacted by any change made specifically for
>     browser-based applications?
>
>     Philippe
>
>
>>     On 9 Dec 2020, at 23:57, Brian Campbell
>>     <bcampbell@pingidentity.com <mailto:bcampbell@pingidentity.com>>
>>     wrote:
>>
>>     Thanks Philippe, I very much concur with your line of reasoning
>>     and the important considerations. The scenario I was thinking of
>>     is: browser based client where XSS is used to exfiltrate the
>>     refresh token along with pre-computed proofs that would allow for
>>     the RT to be exchanged for new access tokens and also
>>     pre-computed proofs that would work with those access tokens for
>>     resource access. With the pre-computed proofs that would allow
>>     prolonged (as long as the RT is valid) access to protected
>>     resources even when the victim is offline. Is that a concrete
>>     attack scenario? I mean, kind of. It's pretty convoluted/complex.
>>     And while an access token hash would reign it in somewhat (ATs
>>     obtained from the stolen RT wouldn't be usable) it's hard to say
>>     if the cost is worth the benefit.
>>
>>
>>
>>     On Tue, Dec 8, 2020 at 11:47 PM Philippe De Ryck
>>     <philippe@pragmaticwebsecurity.com
>>     <mailto:philippe@pragmaticwebsecurity.com>> wrote:
>>
>>         Yeah, browser-based apps are pure fun, aren’t they? :)
>>
>>         The reason I covered a couple of (pessimistic) XSS scenarios
>>         is that the discussion started with an assumption that the
>>         attacker already successfully exploited an XSS vulnerability.
>>         I pointed out how, at that point, finetuning DPoP proof
>>         contents will have little to no effect to stop an attack. I
>>         believe it is important to make this very clear, to avoid
>>         people turning to DPoP as a security mechanism for
>>         browser-based applications.
>>
>>
>>         Specifically to your question on including the hash in the
>>         proof, I think these considerations are important:
>>
>>         1. Does the inclusion of the AT hash stop a concrete attack
>>         scenario?
>>         2. Is the “cost” (implementation, getting it right, …) worth
>>         the benefits?
>>
>>
>>         Here’s my view on these considerations (*/specifically for
>>         browser-based apps, not for other types of applications/*):
>>
>>         1. The proof precomputation attack is already quite complex,
>>         and short access token lifetimes already reduce the window of
>>         attack. If the attacker can steal a future AT, they could
>>         also precompute new proofs then. 
>>         2. For browser-based apps, it seems that doing this
>>         complicates the implementation, without adding much benefit.
>>         Of course, libraries could handle this, which significantly
>>         reduces the cost. 
>>
>>
>>         Note that these comments are specifically to complicating the
>>         spec and implementation. DPoP’s capabilities of using
>>         sender-constrained access tokens are still useful to counter
>>         various other scenarios (e.g., middleboxes or APIs abusing
>>         access tokens). If other applications would significantly
>>         benefit from having the hash in the proof, I’m all for it.
>>
>>         On a final note, I would be happy to help clear up the
>>         details on web-based threats and defenses if necessary.
>>
>>         —
>>         *Pragmatic Web Security*
>>         /Security for developers/
>>         https://pragmaticwebsecurity.com/
>>
>>
>>>         On 8 Dec 2020, at 22:47, Brian Campbell
>>>         <bcampbell@pingidentity.com
>>>         <mailto:bcampbell@pingidentity.com>> wrote:
>>>
>>>         Danial recently added some text to the working copy of the
>>>         draft with
>>>         https://github.com/danielfett/draft-dpop/commit/f4b42058
>>>         that I think aims to better convey the "nutshell: XSS = Game
>>>         over" sentiment and maybe dissuade folks from looking to
>>>         DPoP as a cure-all for browser based applications.
>>>         Admittedly a lot of the initial impetus behind producing the
>>>         draft in the first place was born out of discussions around
>>>         browser based apps. But it's neither specific to browser
>>>         based apps nor a panacea for them. I hope the language in
>>>         the document and how it's recently been presented is
>>>         reflective of that reality.
>>>
>>>         The more specific discussions/recommendations around
>>>         in-browser apps are valuable (if somewhat over my head) but
>>>         might be more appropriate in the OAuth 2.0 for Browser-Based
>>>         Apps
>>>         <https://datatracker.ietf.org/doc/draft-ietf-oauth-browser-based-apps/>
>>>         draft.
>>>
>>>         With respect to the contents of the DPoP draft, I am still
>>>         keen to try and flush out some consensus around the question
>>>         posed in the start of this thread, which is effectively
>>>         whether or not to include a hash of the access token in the
>>>         proof.  Acknowledging that "XSS = Game over" does sort of
>>>         evoke a tendency to not even bother with such incremental
>>>         protections (what I've tried to humorously coin as "XSS
>>>         Nihilism" with no success). And as such, I do think that
>>>         leaving it how it is (no AT hash in the proof) is not
>>>         unreasonable. But, as Filip previously articulated,
>>>         including the AT hash in the proof would prevent potentially
>>>         prolonged access to protected resources even when the victim
>>>         is offline. And that seems maybe worthwhile to have in the
>>>         protocol, given that it's not a huge change to the spec. But
>>>         it's a trade-off either way and I'm personally on the fence
>>>         about it.
>>>
>>>         Including an RT hash in the proof seems more niche. Best I
>>>         can tell, it would guard against prolonged offline access to
>>>         protected resources when access tokens are bearer and the RT
>>>         was DPoP-bound and also gets rotated. The trade-off there
>>>         seems less worth it (I think an RT hash would be more
>>>         awkward in the protocol too).
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>         On Fri, Dec 4, 2020 at 5:40 AM Philippe De Ryck
>>>         <philippe@pragmaticwebsecurity.com
>>>         <mailto:philippe@pragmaticwebsecurity.com>> wrote:
>>>
>>>
>>>>             The suggestion to use a web worker to ensure that
>>>>             proofs cannot be pre-computed is a good one I think.
>>>>             (You could also use a sandboxed iframe for a separate
>>>>             sub/sibling-domain - dpop.example.com
>>>>             <http://dpop.example.com/>).
>>>
>>>             An iframe with a different origin would also work (not
>>>             really sandboxing, as that implies the use of the
>>>             sandbox attribute to enforce behavioral restrictions).
>>>             The downside of an iframe is the need to host additional
>>>             HTML, vs a script file for the worker, but the effect is
>>>             indeed the same.
>>>
>>>>             For scenario 4, I think this only works if the attacker
>>>>             can trick/spoof the AS into using their redirect_uri?
>>>>             Otherwise the AC will go to the legitimate app which
>>>>             will reject it due to mismatched state/PKCE. Or are you
>>>>             thinking of XSS on the redirect_uri itself? I think
>>>>             probably a good practice is that the target of a
>>>>             redirect_uri should be a very minimal and locked down
>>>>             page to avoid this kind of possibility. (Again, using a
>>>>             separate sub-domain to handle tokens and DPoP seems
>>>>             like a good idea).
>>>
>>>             My original thought was to use a silent flow with Web
>>>             Messaging. The scenario would go as follows:
>>>
>>>             1. Setup a Web Messaging listener to receive the
>>>             incoming code
>>>             2. Create a hidden iframe with the DOM APIs
>>>             3. Create an authorization request such as
>>>             “//authorize?response_type=code&client_id=...&redirect_uri=https%3A%2F%example.com
>>>             <http://example.com/>&state=...&code_challenge=7-ffnU1EzHtMfxOAdlkp_WixnAM_z9tMh3JxgjazXAk&code_challenge_method=S256&prompt=none&response_mode=web_message/”
>>>             4. Load this URL in the iframe, and wait for the result
>>>             5. Retrieve code in the listener, and use PKCE (+ DPoP
>>>             if needed) to exchange it for tokens
>>>
>>>             This puts the attacker in full control over every aspect
>>>             of the flow, so no need to manipulate any of the parameters.
>>>
>>>
>>>             After your comment, I also believe an attacker can run
>>>             the same scenario without the
>>>             “/response_mode=web_message/”. This would go as follows:
>>>
>>>             1. Create a hidden iframe with the DOM APIs
>>>             2. Setup polling to read the URL (this will be possible
>>>             for same-origin pages, not for cross-origin pages)
>>>             3. Create an authorization request such as
>>>             “//authorize?response_type=code&client_id=...&redirect_uri=https%3A%2F%example.com
>>>             <http://example.com/>&state=...&code_challenge=7-ffnU1EzHtMfxOAdlkp_WixnAM_z9tMh3JxgjazXAk&code_challenge_method=S256/”
>>>             4. Load this URL in the iframe, and keep polling
>>>             5. Detect the redirect back to the application with the
>>>             code in the URL, retrieve code, and use PKCE (+ DPoP if
>>>             needed) to exchange it for tokens
>>>
>>>             In step 5, the application is likely to also try to
>>>             exchange the code. This will fail due to a mismatching
>>>             PKCE verifier. While noisy, I don’t think it affects the
>>>             scenario. 
>>>
>>>
>>>>             IMO, the online attack scenario (i.e., proxying
>>>>             malicious requests through the victim’s browser) is
>>>>             quite appealing to an attacker, despite the apparent
>>>>             inconvenience:
>>>>
>>>>              - the victim’s browser may be inside a corporate
>>>>             firewall or VPN, allowing the attacker to effectively
>>>>             bypass these restrictions
>>>>              - the attacker’s traffic is mixed in with the user’s
>>>>             own requests, making them harder to distinguish or to block
>>>>
>>>>             Overall, DPoP can only protect against XSS to the same
>>>>             level as HttpOnly cookies. This is not nothing, but it
>>>>             means it only prevents relatively naive attacks. Given
>>>>             the association of public key signatures with strong
>>>>             authentication, people may have overinflated
>>>>             expectations if DPoP is pitched as an XSS defence.
>>>
>>>             Yes, in the cookie world this is known as “Session
>>>             Riding”. Having the worker for token isolation would
>>>             make it possible to enforce a coarse-grained policy on
>>>             outgoing requests to prevent total abuse of the AT.
>>>
>>>             My main concern here is the effort of doing DPoP in a
>>>             browser versus the limited gains. It may also give a
>>>             false sense of security. 
>>>
>>>
>>>
>>>             With all this said, I believe that the AS can lock down
>>>             its configuration to reduce these attack vectors. A few
>>>             initial ideas:
>>>
>>>             1. Disable silent flows for SPAs using RT rotation
>>>             2. Use the sec-fetch headers to detect and reject
>>>             non-silent iframe-based flows
>>>
>>>             For example,  an OAuth 2.0 flow in an iframe in
>>>             Brave/Chrome carries these headers:
>>>             /
>>>             sec-fetch-dest: iframe
>>>             sec-fetch-mode: navigate
>>>             sec-fetch-site: cross-site
>>>             sec-fetch-user: ?1
>>>             /
>>>
>>>
>>>             Philippe
>>>
>>>
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>>
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-- 
Vladimir Dzhuvinov