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

[Brian Campbell <[e-mail]>]

I think that puts Jim in the XSS Nihilism camp :)

Implicit type flows are being deprecated/discouraged. But keeping tokens
out of browsers doesn't seem likely. There is some menton of CSP in
https://tools.ietf.org/html/draft-ietf-oauth-browser-based-apps-07#section-9.7

On Wed, Dec 9, 2020 at 4:10 PM Jim Manico <[e-mail]> wrote:

> The basic theme from the web attacker community is:
>
> 1) XSS is a game over event to web clients. XSS can steal or abuse
> (request forgery) tokens, and more.
>
> 2) Even if you prevent stolen tokens from being used outside of a web
> client, XSS still allows the attacker to force a user to make any request
> in a fraudulent way, abusing browser based tokens as a form of request
> forgery.
>
> 3) There are advanced measures to stop a token from being stolen from a
> web client, like a HTTPonly cookies and to a lesser degree, JS Closures and
> Webworkers.
>
> 4) However, these measures to protect cookies are mostly moot. Attackers
> can just force clients to make fraudulent requests.
>
> 5) Many recommend the BFF pattern to hide tokens on the back end, but
> still, request forgery via XSS allows all kinds of abuse.
>
> XSS is game over no matter how you slice it.
>
> Crypto solutions do not help. Perhaps the world of OAuth can start
> suggesting that web clients use CSP 3.0 in specific ways, if you still plan
> to support Implicit type flows or tokens in browsers?
>
> Respectfully,
>
> - Jim
>
>
> On 12/9/20 12:57 PM, Brian Campbell 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 <
> [e-mail]> 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 <[e-mail]>
>> 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 <
>> [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).
>>>
>>>
>>> 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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> --
> Jim Manico
> Manicode Securityhttps://www.manicode.com
>
>

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