MSC3828: Content Repository CORP Headers (#3828)

proposals/3828-content-repository-corp-headers.md

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+# MSC3828: Content Repository Cross Origin Resource Policy (CORP) Headers
+
+In 2018 two side-channel hardware vulnerabilities,
+[Meltdown](https://en.wikipedia.org/wiki/Meltdown_(security_vulnerability))
+and [Spectre](https://en.wikipedia.org/wiki/Spectre_(security_vulnerability)) were disclosed.
+In web browsers this meant that features such as high resolution timers and SharedArrayBuffer
+could be used to expose sensitive data across origins. In response, browser vendors have
+required documents that wish to continue using features such as SharedArrayBuffer to be
+served with the `Cross-Origin-Embedder-Policy: require-corp` header. This header prevents a
+document from loading any cross-origin resources that don't explicitly grant the document
+permission via the `Cross-Origin-Resource-Policy` header.
+
+Currently Matrix homeservers are expected to serve all routes with the
+`Access-Control-Allow-Origin: *` header. This allows web clients on one origin to fetch
+content from homeservers on other origins. However, when the web client uses SharedArrayBuffer
+and the required `Cross-Origin-Embedder-Policy: require-corp` header, all embedded documents
+must set the required CORP header.
+
+## Proposal
+
+The content repository should serve assets with the `Cross-Origin-Resource-Policy: cross-origin`
+header. This allows web clients to set the `Cross-Origin-Embedder-Policy: require-corp`
+header and enable access to APIs like SharedArrayBuffer.
+
+This header should be set on responses from the following endpoints:
+
+- `/_matrix/media/v3/download`
+- `/_matrix/media/v3/thumbnail`
+
+## Potential issues
+
+Chrome 73-75 have problems downloading files with this header, see [bug 952834](https://crbug.com/952834).
+Chrome 80-85 has a [bug](https://crbug.com/1074261) with viewing multi-page PDF documents with CORP
+headers set to `same-origin`. This proposal is for setting the header to `cross-origin` which should
+not have an issue but I was not able to verify this. However,
+[MDN](https://developer.mozilla.org/en-US/docs/Web/HTTP/Cross-Origin_Resource_Policy_(CORP)#browser_compatibility)
+suggests this bug was fixed in Chrome 86 by disabling partial PDF loading. I was able to verify that
+multi-page PDFs are viewable in the latest versions of Chrome, Safari, and Firefox with either header
+value set. This should only be an issue if you require supporting Chrome 73-75.
+
+Also see [Security considerations](#Security-considerations)
+
+## Alternatives
+
+Clients using features like SharedArrayBuffer cannot fetch media from the Matrix media
+repositories without these headers.
+
+
+## Security considerations
+
+I don't believe this poses any additional risks to private data from Matrix homeservers.
+We are not exposing iframes with personal data or any data that could not already be
+gathered from Matrix's existing APIs. However, I believe this proposal deserves thorough review.
+
+## Unstable prefix
+
+None
+
+## Dependencies
+
+None