Well it's negotiated, right?
So say you run a SaaS that allows for something like letting people set their Avatar from an image URL which gets downloaded.
As an attacker I setup a new server that serves only brotli and then submit that URL to the SaaS.
There’s both request compression (no negotiation) and response compression (negotiated).
Just send a POST with Content-Encoding:br to any URL and hope it gets decided before the URL route is checked.
> So say you run a SaaS that allows for something like letting people set their Avatar from an image URL which gets downloaded.
That's a clever example, but why would you do that? The avatar should be a static resource. As such, I'd only provide an upload file dialog.
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Lots of apps will, for example, bring in your Google avatar (i.e. they fetch its URL for ingestion) as a default avatar if you use them for SSO. Slack included, IIRC.
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