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

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

> 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&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&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