Reading the post a few times, I'm still unclear on a few things, so would appreciate clarification. Here's what I understand from the post:
On the user's machine, there is some entropy, i.e. the set of schemes for which an external protocol handler is registered. This entropy is currently retrievable via various mechanisms. For webcompat reasons, we want to make a change to one of those mechanisms, but the entropy will remain accessible after this change. What's less clear to me is how the proposed change impacts the entropy exposure by the mechanism it targets. It seems to me that using about:blank for both the handled and unhandled case would not expose this entropy, but the post implies otherwise. In other words: does this change put us on a path to eventually making this entropy non-observable, or would we need to undo this change if/when we pursue that? Thanks! On Sun, Mar 29, 2020 at 12:29 AM Emilio Cobos Álvarez <emi...@crisal.io> wrote: > On 3/29/20 9:11 AM, Emilio Cobos Álvarez wrote: > > Doing a bit of digging, > > https://bugzilla.mozilla.org/show_bug.cgi?id=680300 contains some more > > interesting context... > > > > We apparently used to sync-throw when assigning location.href to an > > unknown protocol URI in the past, there we changed it to silently fail, > > and now it is navigating to an error page... > > > > I'll try to look at when behavior changed around here... Though the > > sync-throwing clearly makes no sense as it doesn't account for redirects > > or what not. > > Apparently the pre-firefox63 behavior was to both navigate and throw... > > So not all that much to dig in, that was just obviously wrong, but > didn't address the fingerprinting angle other than patching one of the > holes. > > > On 3/29/20 8:23 AM, Emilio Cobos Álvarez wrote: > >> Hey, a quick web-observable change that may deserve a bit more > >> visibility / an intent. > >> > >> I'd welcome some feedback, specially from the fingerprinting / privacy > >> angle (where I'm clearly not an expert). > >> > >> Summary: A page redirecting / navigating to an unknown protocol will > >> be silently ignored (and logged to the DevTools console) instead of > >> showing an error page. This is not amazing, but it's needed because it > >> causes a bunch of compat issues (see the list of issues in the bug > >> below). > >> > >> There are a few subtle problems here. Part of the issue is that > >> authors don't have a cross-browser way of detecting whether a protocol > >> will be handled externally (that is, whether an app is installed to > >> handle a given protocol). > >> > >> They technically can in Firefox (see example below). It's unclear > >> whether we want to expose more than that, or that at all. > >> > >> Given Chrome just ignores the navigation if they can't handle it, some > >> authors (who don't seem to test on other browsers ;)) just spam the > >> redirect (using various methods like the location href setter / meta > >> refresh / whatevs). This works just fine in Chromium-based browsers, > >> but not in Firefox or Safari, which will just show an error, which > >> causes a very frustrating experience for users. > >> > >> As far as I know, this behavior doesn't really expose more information > >> than we were exposing before. In a test-case like: > >> > >> <iframe src="unhandled-protocol://foo"></iframe> > >> <iframe src="handled-protocol://foo"></iframe> > >> > >> we have different behavior also before this patch: the contentDocument > >> for the handled-protocol iframe will be accessible to content, and be > >> about:blank. The contentDocument for the unhandled protocol will be > >> the error page, which will not be accessible to content and thus be > null. > >> > >> Other browsers seem to do about:blank for both iframes, but you can > >> also detect this in all browsers via window.open instead (there may be > >> other ways around it too, haven't dug all that much). Chrome > >> insta-closes the handled protocol window (so you can check > >> window.closed). Firefox / Safari will leave about:blank in that > >> window, and show an error document for the second (which will throw a > >> security error on access). > >> > >> So I don't think this change introduces a new privacy hole. That being > >> said, this all seems a bit sad, and a somewhat-serious potential > >> fingerprinting factor. I was in fact a bit nervous about this change > >> before realizing we were already exposing this information in other > >> (even simpler to test) ways... But I'm clearly not an expert on this > >> matter. > >> > >> There are mitigations possible both for the ignored-navigation case > >> (maybe only ignore the navigation to an unknown protocol once per > >> document or something, and only for browsing contexts that can't be > >> accessed by any ancestor JS? not sure) and the iframe case (probably > >> just not show error pages in frames?). Please let me know if I you > >> think I should file this and follow-up on it. > >> > >> Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1528305 > >> > >> Standard: https://html.spec.whatwg.org/#process-a-navigate-url-scheme > >> allows for both the old and new behavior, as far as I (and Anne) can > >> tell. This uses urls after redirect and so on so beforeunload and such > >> will fire. This matches the behavior of Chromium browsers. > >> > >> Preference: dom.no_unknown_protocol_error.enabled (I could've sweated > >> it a bit more). > >> > >> Devtools bug: N/A, ignored navigations get logged to the console. > >> > >> Other browsers: Chromium browsers do this, but WebKit doesn't seem to. > >> This was causing compat issues, specially on Android, but also desktop > >> (see above). > >> > >> web-platform-tests: Spec allows both behaviors, so I've added an > >> internal test for now. Furthermore it's not clear if we want to do > >> this for sub-windows, which my patch does, so I may need to tweak the > >> test or add an internal switch depending on the outcome of the > >> discussion here. > >> > >> Secure contexts: not restricted to secure contexts, this is a compat > >> hack, of sorts... > >> > >> Is this feature enabled by default in sandboxed iframes? Not sure if > >> the question quite applies to this, but we don't special-case > >> sandboxed iframes here. This doesn't expose anything you couldn't do > >> before, as discussed above. > >> > >> Thanks for reading all the way until here > >> > >> -- Emilio > >> _______________________________________________ > >> dev-platform mailing list > >> dev-platform@lists.mozilla.org > >> https://lists.mozilla.org/listinfo/dev-platform > > _______________________________________________ > > dev-platform mailing list > > dev-platform@lists.mozilla.org > > https://lists.mozilla.org/listinfo/dev-platform > _______________________________________________ > dev-platform mailing list > dev-platform@lists.mozilla.org > https://lists.mozilla.org/listinfo/dev-platform > _______________________________________________ dev-platform mailing list dev-platform@lists.mozilla.org https://lists.mozilla.org/listinfo/dev-platform
