Rely on WedIDL's "implements" definition for isHTML/isScript/isScriptURL?

[fred-wang]

cc @koto @lukewarlow @mbrodesser-Igalia

Initially suggested here: web-platform-tests/wpt#49371 (comment)

See https://w3c.github.io/trusted-types/dist/spec/#dom-trustedtypepolicyfactory-ishtml

"Returns true if value is an instance of TrustedHTML and has an associated data value set, false otherwise."

and there is a note saying this allows to "check if a given object is truly a legitimate Trusted Type object (i.e. it was created via one of the configured policies)".

The CSP spec seems to do something similar but relies on implements instead. In that case we don't need to check if "the object has an associated data". So we the definition could be simplified as follows:

"Returns true if value implements TrustedHTML".

Browsers don't seem to check the presence of associated data, and I believe perform something that matches "implements":

Chromium: https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/core/trustedtypes/trusted_type_policy_factory.cc;l=301;drc=117bbc5c974e90bd7b28653649b4a59344bb22f0
Firefox: https://searchfox.org/mozilla-central/rev/fb8a1b0beee7dbc1b0a35050ec5c0345f0b09173/dom/security/trusted-types/TrustedTypePolicyFactory.cpp#159
WebKit: https://searchfox.org/wubkat/rev/c3d6184dbe8d3daa73c7c9201ce556cf81e35d3e/Source/WebCore/dom/TrustedTypePolicyFactory.cpp#85

BTW, I also believe these methods still return true for Trusted Type objects with forget toString mentioned on w3c/webappsec-csp#697 that is Object.assign(policy.createScript("safe"), { toString: () => "unsafe" }). Maybe the note should mention that, maybe we should have tests too...

BTW, I also believe these methods still return true for Trusted Type objects with forget toString mentioned on w3c/webappsec-csp#697 that is Object.assign(policy.createScript("safe"), { toString: () => "unsafe" }). Maybe the note should mention that, maybe we should have tests too...

They do, test e.g. https://jsbin.com/fiwewuqote/edit?html,console,output with Chrome.

A potential security hole for this could be
somePolicy.createScript(trustedtypes.isScript(someForgedObject) ? someForgedObject.toString() : "fallback");.

[fred-wang]

@smaug---- @annevk @otherdaniel I wonder what you think about this? If that's ok, I guess I can send a spec PR.

[annevk]

Yeah, "implements" or "is a" is the way to check a platform object's brand identity. (And these are platform objects by the time you are looking at them. IDL takes care of that.)

[fred-wang]

We have a couple of instances of "instance of":

grep 'instance of' spec/index.bs 
:: Returns true if value is an instance of {{TrustedHTML}} and has an associated [=TrustedHTML/data=] value set, false otherwise.
:: Returns true if value is an instance of {{TrustedScript}} and has an associated [=TrustedScript/data=] value set, false otherwise.
:: Returns true if value is an instance of {{TrustedScriptURL}} and has an associated [=TrustedScriptURL/data=] value set, false otherwise.
1.  Return a new instance of an interface with a type
1.  If |input| is an instance of |expectedType|, return stringified
1. Assert: |convertedInput| is an instance of |expectedType|.
1.  Return a new instance of an interface with a type

I guess they can be replaced with https://webidl.spec.whatwg.org/#implements and https://webidl.spec.whatwg.org/#internally-create-a-new-object-implementing-the-interface