matrix-media-repo: Unsafe media served inline on download endpoints

Impact

A malicious user can upload an SVG image containing JavaScript to their server. When matrix-media-repo is asked to serve that media via the /_matrix/media/(r0|v3)/download endpoint, it would be served with a Content-Disposition of inline. This can allow JavaScript to run in the browser if a client links to the /download endpoint directly.

Server operators which do not share a domain between matrix-media-repo and other services are not affected, but are encouraged to upgrade regardless.

Patches

https://github.com/turt2live/matrix-media-repo/commit/77ec2354e8f46d5ef149d1dcaf25f51c04149137 and https://github.com/turt2live/matrix-media-repo/commit/bf8abdd7a5371118e280c65a8e0ec2b2e9bdaf59 fix the issue. Operators should upgrade to v1.3.0 as soon as possible.

Workarounds

The Content-Disposition header can be overridden by the reverse proxy in front of matrix-media-repo to always use attachment, defeating this issue at the cost of “worse” user experience when clicking download links.

References

https://developer.mozilla.org/en-US/docs/Web/SVG/Element/script