I also had an offline discussion with Daniel to confirm shipping as-is in M117, and sanitizing in 118 is acceptable from a security POV (trust but verify, etc).

LGTM1 to ship.

On 8/31/23 7:55 AM, 'Garrett Tanzer' via blink-dev wrote:
After some discussion offline, we're going to sanitize the macro keys and values with EscapeQueryParamValue <https://source.chromium.org/chromium/chromium/src/+/main:base/strings/escape.h;l=29?q=EscapeQueryParamValue%20-f:out> so that macro substitution always stays within the original query parameter field. This should prevent XSS-y substitutions while keeping the API surface exactly the same for regular use.

This fix will be implemented in M118, and we've decided that it need not block launch in M117 due to the low severity (especially because the existing reportEvent surface grants the buyer reporting worklet complete control over the destination URL, so the ad creative is already known to trust the buyer wrt where its reports get sent).

On Monday, August 28, 2023 at 11:55:39 AM UTC-4 Garrett Tanzer wrote:

    Hi Daniel,

      * There are a few relevant call sites in the overall reporting
        flow:
          o Declare allowlist of reporting destination origins
              + This happens in navigator.joinAdInterestGroup(), by an
                ad auction buyer
          o Declare macros (key:value correspondences)
              + This happens in the buyer reporting worklet, by the
                same ad auction buyer that declared the reporting
                destination origins
          o Perform report to custom url
              + This happens under the auction's winning ad creative's
                origin, which isn't necessarily the same as the ad
                auction buyer, but it is chosen by the ad auction buyer
      * Here is the sequence of events:

    1.
         1. The browser ingests and validates the allowlist when the
            interest group is declared.
         2. The browser ingests the macro key:value mapping when the
            auction happens. The key/value strings have no structure
            that is validated. The browser adds "${" and "}" around
            the user-provided keys in the macro mapping.
         3. In the ad that results from the auction:
             1. The ad sends a URL to the browser, including macros
                like ${KEY}. The URL has to be a valid HTTPS url even
                with the macros unsubstituted. (See impl:
                
https://source.chromium.org/chromium/chromium/src/+/main:content/browser/renderer_host/render_frame_host_impl.cc;l=8644?q=content%2Fbrowser%2Frenderer_host%2Frender_frame_host_impl.cc%20-f:out)
             2. The browser performs a simple string substitution to
                replace the keys with the values. The implementation
                is reused from navigator.deprecatedReplaceInURN,
                another part of Protected Audience. It doesn't
                substitute macros recursively, so you can't get an
                infinite loop. deprecatedReplaceInURN has apparently
                not been spec'd, which is was unable to quickly reuse
                that spec and left it as a TODO for now.
             3. The origin of the resulting URL is checked against the
                allowlist. If it doesn't match any of them, no action
                is performed. If the URL is invalid, this will create
                an opaque origin and therefore always fail the check.
                (See impl:
                
https://source.chromium.org/chromium/chromium/src/+/main:content/browser/fenced_frame/fenced_frame_reporter.cc;l=454?q=fenced_frame_reporter.cc%20-f:out)
                We may add an explicit URL valid/HTTPS check after
                substitution for neatness/to be robust to any future
                changes to the allowlist checking.
             4. If the URL does pass the check, a GET fetch will be
                performed on it (the result is unused). There's no
                other parsing/usage of the URL other than performing a
                fetch.

    So in summary, we were not very concerned about XSS because the
    entity choosing the macro pairings is the one we want to protect,
    and the entity providing the base URLs is semi-trusted. The less
    trusted entity is the site of the destination URL, which just
    receives a GET and doesn't get to control anything. Since the URL
    has to be valid both before and after the macro substitution, this
    limits how wacky the substitutions can get even if you don't trust
    one of those entities. The weirdest stuff you could do would be
    like "FOO}&key=value1&${BAR" -> "key=value2", so that it becomes
    "${FOO}&key=value1&${BAR}" -> "key=value2". We can add a check to
    exclude $,{,} characters in the macro key so even this isn't possible.

    Hope this answers your questions,
    Garrett

    On Monday, August 28, 2023 at 9:35:34 AM UTC-4 Daniel Vogelheim wrote:

        Hi Liam,

        This intent has come up in the OWP security triage, and I'm
        trying to figure out whether there's XSS potential in the 3rd
        sub-feature, "Creative macros in FFAR". This looks like a
        string-based pattern replacement where the result string will
        then be parsed by the browser. Similar things have lent
        themselves to XSS, e.g. when a string value contains meta
        characters that code downstream will then parse in unexpected
        ways. Unfortunately, I can't seem to find enough information
        about how exactly the replacement & subsequent usage works to
        make up my mind whether there's a concern or not. Could you
        help me out with a bit more information?

        What I've found is this:
        - In "<dfn>send a beacon</dfn>": "TODO: Substitute macros from
        |macro map| into |destination url|." (source
        <https://github.com/WICG/fenced-frame/pull/113/files>)
        - In "<dfn>asynchronously finish reporting</dfn>" I find where
        macro map being set, but then it says: "TODO: Pass |macroMap|
        and |allowedReportingOrigins| to [=Finalize a reporting
        destination=] when it is updated to take the parameters."
        (source <https://github.com/WICG/turtledove/pull/762/files>) I
        can't find that usage.

        Questions I have:
        - Am I reading the right docs? Where else should I look?
        - Is this meant as a simple string-based substitution? Is
        there any filtering of allowed characters, or so?
        - What happens with the result values?
        - Is the entity that sets the macro values always the same
        that has set the patterns the values are being used for, or
        could those be different entities?

        Thanks!


        On Fri, Aug 25, 2023 at 9:34 PM 'Liam Brady' via blink-dev
        <blin...@chromium.org> wrote:

            Contact emails

            shiva...@chromium.org, d...@chromium.org,
            jka...@chromium.org, lbr...@google.com

            Explainer(s)

            Send Automatic Beacons Once

            https://github.com/WICG/turtledove/pull/718
            <https://github.com/WICG/turtledove/pull/718>


            Serializable Fenced Frames Configs - Minor Change, No
            explainer available.

            Note: With this change, FencedFrameConfig objects will be
            serializable and can be sent through "postMessage()" and
            other similar calls.


            Creative Macros in Fenced Frames Ads Reporting (FFAR)

            https://github.com/WICG/turtledove/pull/763
            <https://github.com/WICG/turtledove/pull/763>


            Spec(s)

            Send Automatic Beacons Once

            https://github.com/WICG/fenced-frame/pull/109
            <https://github.com/WICG/fenced-frame/pull/109>


            Serializable Fenced Frames Configs

            https://github.com/WICG/fenced-frame/pull/111
            <https://github.com/WICG/fenced-frame/pull/111>


            Creative Macros in Fenced Frames Ads Reporting (FFAR)

            Protected Audience:
            https://github.com/WICG/turtledove/pull/762/files
            <https://github.com/WICG/turtledove/pull/762/files>

            Fenced Frames:
            https://github.com/WICG/fenced-frame/pull/113
            <https://github.com/WICG/fenced-frame/pull/113>

            Summary


            We launched Fenced Frames as a part of Chrome 115. We
            would like to add the following three functionalities to
            Fenced Frames.


            1. Send Automatic Beacons Once


            A common feature in ad frames is the "why this ad?" link.
            Since that link is separate from the ad itself, clicking
            "why this ad?", and its subsequent navigation, should be
            considered different from clicking directly on the
            advertisement itself. With our current automatic beacon
            design, however, once an automatic beacon is set (usually
            in the click handler for the ad link), that beacon will
            send out for any subsequent top-level navigations,
            including if "why this ad?" is clicked. This can result in
            erroneous impressions being sent out.


            As a solution, this feature introduces a new "once" member
            to the FenceEvent
            <https://wicg.github.io/fenced-frame/#dictdef-fenceevent>dictionary
            passed into
            "window.fence.setReportEventDataForAutomaticBeacons()". If
            set to true, the saved automatic beacon data will be
            cleared out after the next beacon is sent, ensuring that
            automatic beacons with that data are only sent once. This
            means that further clicks to non-ad parts of the frame
            that result in top-level navigations will not send out
            erroneous beacons.


            (This feature already shipped in M116. That was our
            mistake. At the time, we thought we would only need a PSA
            for it, and shipped it without it being behind a flag. We
            apologize for the mistake.)


            2. Serializable FencedFramesConfigs


            With this change, FencedFrameConfig objects will be
            serializable and can be sent through "postMessage()" and
            other similar calls. Serialization allows for a case where
            the frame that runs an ad auction is not the same frame
            that ends up embedding the winning ad in a fenced frame.
            FencedFrameConfigs cannot be serialized to storage, nor
            can they be sent in a message that crosses a fenced frame
            boundary. A FencedFrameConfig object is only valid in the
            traversable navigable
            <https://wicg.github.io/fenced-frame/#traversable-navigables>it
            was originally created in, and, if sent outside to a
            different context, will not be able to navigate, since the
            new traversable navigable
            <https://wicg.github.io/fenced-frame/#traversable-navigables>'s
            fenced frame config mapping
            
<https://wicg.github.io/fenced-frame/#traversable-navigable-fenced-frame-config-mapping>will
            not contain the internal config needed to do the navigation.


            3. Creative macros in Fenced Frames Ads Reporting (FFAR)


            This feature extends the Fenced Frame Ads Reporting (FFAR)
            API to support macro substitution in reporting URLs and
            allows reports to be sent to up to ten other origins that
            have enrolled with the Privacy Sandbox and allow-listed by
            the DSP. Use case: In online ad auctions for ad space,
            advertisers buying through DSPs in several situations use
            other adtech providers to monitor performance and keep
            track of how their advertising dollars are spent. (issue
            link <https://github.com/WICG/turtledove/issues/477>)


            Blink component

            Blink>FencedFrames
            
<https://bugs.chromium.org/p/chromium/issues/list?q=component:Blink%3EFencedFrames>


            TAG reviews and status

            Fenced frames existing TAG review appended with these spec
            changes
            
https://github.com/w3ctag/design-reviews/issues/838#issuecomment-1693631006
            
<https://github.com/w3ctag/design-reviews/issues/838#issuecomment-1693631006>


            Link to Origin Trial feedback summary

            No Origin Trial performed


            Is this feature supported on all six Blink platforms
            (Windows, Mac, Linux, Chrome OS, Android, and Android
            WebView)?

            Supported on all the above platforms except Android WebView.


            Debuggability

            Additional debugging capabilities are not necessary for
            these feature changes.


            Risks


            Compatibility

            There are no compatibility risks, as described below:

            1. Send Automatic Beacons Once: This is backward
            compatible with the existing API since the default value
            of “once” is false which is the same behavior as the
            previous behavior.

            2. Serializable FencedFramesConfigs: This is added
            functionality and backward compatible with the existing
            FencedFramesConfig.

            3. Creative macros in Fenced Frames Ads Reporting (FFAR):
            This is adding a new API and a backward compatible change
            to reportEvent.


            Interoperability

            there are no interoperability risks as no other browsers
            have decided to implement these features yet.


            Is this feature fully tested by web-platform-tests
            
<https://chromium.googlesource.com/chromium/src/+/master/docs/testing/web_platform_tests.md>?
            Link to test suite results from wpt.fyi <https://wpt.fyi>.

            Yes

            Tests:
            https://github.com/web-platform-tests/wpt/tree/master/fenced-frame
            <https://github.com/web-platform-tests/wpt/tree/master/fenced-frame>

            Results:
            
https://wpt.fyi/results/fenced-frame?label=experimental&label=master&aligned
            
<https://wpt.fyi/results/fenced-frame?label=experimental&label=master&aligned>


            Specifically, these features correspond to the following
            tests:

            Send Automatic Beacons Once:

             *

                automatic-beacon-two-events-clear.https.html (test
                
<https://github.com/web-platform-tests/wpt/blob/master/fenced-frame/automatic-beacon-two-events-clear.https.html>)
                (result
                
<https://wpt.fyi/results/fenced-frame/automatic-beacon-two-events-clear.https.html?label=experimental&label=master&aligned>)

             *

                automatic-beacon-two-events-persist.https.html (test
                
<https://github.com/web-platform-tests/wpt/blob/master/fenced-frame/automatic-beacon-two-events-persist.https.html>)
                (result
                
<https://wpt.fyi/results/fenced-frame/automatic-beacon-two-events-persist.https.html?label=experimental&label=master&aligned>)

            Serializable FencedFrameConfigs:

             *

                deep-copy-config.https.html (test
                
<https://github.com/web-platform-tests/wpt/blob/master/fenced-frame/deep-copy-config.https.html>)
                (result
                
<https://wpt.fyi/results/fenced-frame/deep-copy-config.https.html?label=experimental&label=master&aligned>)

            Creative macros in Fenced Frames Ads Reporting (FFAR):

             *

                fence-report-event-destination-url.https.html (test
                
<https://github.com/web-platform-tests/wpt/blob/master/fenced-frame/fence-report-event-destination-url.https.html>)
                (result
                
<https://wpt.fyi/results/fenced-frame/fence-report-event-destination-url.https.html?label=experimental&label=master&aligned>)


            Anticipated spec changes

            None


            Link to entry on the Chrome Platform Status

            https://chromestatus.com/feature/5103970808233984
            <https://chromestatus.com/feature/5103970808233984>


            Links to previous Intent discussions

            Intent to prototype:
            
https://groups.google.com/a/chromium.org/g/blink-dev/c/Ko9UXQYPgUE/m/URRsB-qvAAAJ
            
<https://groups.google.com/a/chromium.org/g/blink-dev/c/Ko9UXQYPgUE/m/URRsB-qvAAAJ>


            Intent to experiment:

            
https://groups.google.com/a/chromium.org/g/blink-dev/c/y6G3cvKXjlg/m/Lcpmpi_LAgAJ
            
<https://groups.google.com/a/chromium.org/g/blink-dev/c/y6G3cvKXjlg/m/Lcpmpi_LAgAJ>


            Intent to extend origin trial:

            
https://groups.google.com/a/chromium.org/g/blink-dev/c/SD8Ot2gpz4g/m/A9uA-_cGAwAJ
            
<https://groups.google.com/a/chromium.org/g/blink-dev/c/SD8Ot2gpz4g/m/A9uA-_cGAwAJ>

            
https://groups.google.com/a/chromium.org/g/blink-dev/c/gpmaOi3of_w/m/SyMclFhMAAAJ
            
<https://groups.google.com/a/chromium.org/g/blink-dev/c/gpmaOi3of_w/m/SyMclFhMAAAJ>

            
https://groups.google.com/a/chromium.org/g/blink-dev/c/CBrV-2DrYFI/m/RTojC6kHAgAJ
            
<https://groups.google.com/a/chromium.org/g/blink-dev/c/CBrV-2DrYFI/m/RTojC6kHAgAJ>


            Intent to ship:

            
https://groups.google.com/a/chromium.org/g/blink-dev/c/tpw8wW0VenQ/m/mePLTiHlDQAJ
            
<https://groups.google.com/a/chromium.org/g/blink-dev/c/tpw8wW0VenQ/m/mePLTiHlDQAJ>

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