MediaElementAudioSourceNode and cross-origin media resources

HTML media elements can play media resources from any origin. When an
element plays a media resource from an origin different from the page's
origin, we must prevent page script from being able to read the contents of
the media (e.g. extract video frames or audio samples). In particular we
should prevent ScriptProcessorNodes from getting access to the media's
audio samples. We should also information about samples leaking in other
ways (e.g. timing channel attacks). Currently the Web Audio spec says
nothing about this.

Anyone know how Webkit/Blink solves this?

I think we should solve this by preventing any non-same-origin data from
entering Web Audio. That will minimize the attack surface and the impact on
Web Audio.

My proposal is to make MediaElementAudioSourceNode convert data coming from
a non-same origin stream to silence.

Note that it's not possible to confine origin checks to when a
MediaElementAudioSourceNode is created. For example, it's possible for a
media element's src to be set to a new URL after the Web Audio graph is up
and running.

Rob
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