MSC4189: Allowing guests to access uploaded media (#4189)

proposals/4189-guest-access-media-routes.md

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+# MSC4189: Allowing guests to access uploaded media
+
+[MSC3916](https://github.com/matrix-org/matrix-spec-proposals/blob/main/proposals/3916-authentication-for-media.md)
+introduced new endpoints which require clients to provide a valid access token in order to access
+media. The MSC failed to specify [guest access](https://spec.matrix.org/v1.11/client-server-api/#guest-access)
+requirements for the new endpoints.
+
+This MSC specifies the missing guest access requirements on the new endpoints.
+
+## Proposal
+
+The following endpoints explicitly permit guest access, joining the
+[list of other endpoints](https://spec.matrix.org/v1.11/client-server-api/#client-behaviour-13)
+already in the specification:
+
+* [`GET /_matrix/client/v1/media/download/{serverName}/{mediaId}`](https://spec.matrix.org/v1.11/client-server-api/#get_matrixclientv1mediadownloadservernamemediaid)
+* [`GET /_matrix/client/v1/media/download/{serverName}/{mediaId}/{fileName}`](https://spec.matrix.org/v1.11/client-server-api/#get_matrixclientv1mediadownloadservernamemediaidfilename)
+* [`GET /_matrix/client/v1/media/thumbnail/{serverName}/{mediaId}`](https://spec.matrix.org/v1.11/client-server-api/#get_matrixclientv1mediathumbnailservernamemediaid)
+
+The rationale for the above endpoints is that being able to see events without the associated media
+isn't very useful.
+
+For clarity, the following endpoints are *not* added to the guest access list, as their prior (now
+deprecated) versions are not already included. A future MSC may change this with sufficient rationale.
+Note that guests cannot currently *upload* files, but can send messages/events.
+
+* [`GET /_matrix/client/v1/media/config`](https://spec.matrix.org/v1.11/client-server-api/#get_matrixclientv1mediaconfig)
+* [`GET /_matrix/client/v1/media/preview_url`](https://spec.matrix.org/v1.11/client-server-api/#get_matrixclientv1mediapreview_url)
+
+## Potential issues
+
+This MSC fixes an issue where guests cannot download images/files.
+
+## Alternatives
+
+None applicable.
+
+## Security considerations
+
+This MSC does not materially increase the threat profile for guests: guests could already download
+media using the unauthenticated endpoints.
+
+## Unstable prefix
+
+Prefixed endpoints are excessive for this MSC. Implementations can enable guest access on the existing
+endpoints safely, or continue to respond with "guest access forbidden" errors. No `/versions` flag
+is specified for feature detection: clients with guest access tokens should expect failure until a
+server advertises a specification version containing this MSC. Clients should continue trying to make
+requests for the best user experience.
+
+## Dependencies
+
+This MSC has no dependencies.