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MSC3828: Content Repository Cross Origin Resource Policy (CORP) Headers

In 2018 two side-channel hardware vulnerabilities, Meltdown and Spectre 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. Chrome 80-85 has a bug 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 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

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