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

[Philippe De Ryck <[e-mail]>]

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 <[e-mail]> wrote:
> 
> Danial recently added some text to the working copy of the draft with https://github.com/danielfett/draft-dpop/commit/f4b42058 <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). 
> 
> 
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> 
> 
> On Fri, Dec 4, 2020 at 5:40 AM Philippe De Ryck <[e-mail] <mailto:[e-mail]>> 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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