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On Nov 2, 2020, at 8:56 AM, Yoav Weiss <yoav@yoav.ws> = wrote:

Thanks for re-reviewing, Maciej!

Adding Mike Taylor, = who's likely to take a closer look at this.

On Mon, Nov = 2, 2020 at 2:17 AM Maciej Stachowiak <mjs@apple.com> wrote:

I just did = a fresh review of that spec and explainer. Thanks for addressing many of = the previous issues. This addresses many of the potential = objections.

Here=E2=80= =99s the new issues I filed:

https://github.com/WICG/ua-client-hints/issues/141
https://github.com/WICG/ua-client-hints/issues/142
https://github.com/WICG/ua-client-hints/issues/143
https://github.com/WICG/ua-client-hints/issues/144
https://github.com/WICG/ua-client-hints/issues/145
https://github.com/WICG/ua-client-hints/issues/146
https://github.com/WICG/ua-client-hints/issues/147
https://github.com/WICG/ua-client-hints/issues/148
https://github.com/WICG/ua-client-hints/issues/149
https://github.com/WICG/ua-client-hints/issues/150
https://github.com/WICG/ua-client-hints/issues/151

Thanks for filing those! We'll take a = look and respond shortly.

Most of these are minor/editorial, but I = think 151 is potentially a deal-breaker. I may be misreading the spec, = but as written getHighEntropyValues seems to give access to all of = the high entropy client hints to third-party scripts in the first party = context, and scripts running in third-party iframes, regardless of which = ones the site has opted into via the relevant HTTP header.

That's = indeed the case, as we didn't consider the Client Hints opt-in to be = something that impacts the availability of the JS API. (as it doesn't do = that for other hints)

We=E2=80=99re currently deeply skeptical of = implementing any of the other client hints due to their expansion of = fingerprinting surface, so I don=E2=80=99t feel particularly compelled = by that precedent. That said, it=E2=80=99s likely the other client hints = have this same problem, where they expose fingerprinting surface way = more widely than they may be intending to.

That would be a huge problem, as it would = grant a lot of active fingerprinting surface unnecessarily

We = did discuss adding a Feature Policy (now = Permission Policy) to that effect. Would that help with your = concerns?

My understanding is that feature policy applies at = the frame level, and therefore could not be used to control what happens = when a third-party script in a first party context calls the API. Even = for third-party iframes, it seems like Feature Policy could only = default-deny this JS API entirely, and would not be able to filter the = results down to the set delegated via HTTP headers (or otherwise). Maybe = you intend a feature policy per individual high entropy hint, but first = of all that seems like overkill, and second, the spec is clearly not = written to support such filtering.

So = no, it would not address the concerns.

I think the best approach is to limit the hints to = those opted into (or, in case of a third-party frame, delegated). That = or remove the script API entirely. The origin-based delegation model = that works well at the HTTP level is not well aligned with the = widespread practice of including third-party scripts in the top = frame.

The spec does not eve allow = denying the request entirely as written. A non-normative Note suggests = that is allowed, but I can=E2=80=99t find any step in the algorithm that = would ever reject the promise.

(perhaps even expanding beyond what is = currently possible with the UA = string).

Can you expand on that last = point?

I mean that the client hints might include info = that is not in the UA sting (possibly not at all, or possibly frozen in = UA string but could be unfrozen in the client hints).

Regards,
Maciej

On Oct 27, 2020, at 12:35 AM, Yoav Weiss = <yoav@yoav.ws> wrote:

Yet-another ping! :)

On Wed, Oct 7, 2020 at 8:23 AM Yoav Weiss <yoav@yoav.ws> wrote:
Friendly ping! :)

On Wed, Sep = 30, 2020 at 9:29 AM Yoav Weiss <yoav@yoav.ws> wrote:
Hi WebKit folks,

Circling back on the previous discussion about User-Agent = ClientHint. The feature was implemented in Chromium and is being = rolled out in Chrome.

There were some = concerns mentioned in the previous thread, that we believe were since = addressed. Would the feature be something that WebKit would = consider shipping?

Cheers :)
Yoav
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