matrix-spec-proposals/content/client-server-api/modules/content_repo.md

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Content repository

The content repository (or "media repository") allows users to upload files to their homeserver for later use. For example, files which the user wants to send to a room would be uploaded here, as would an avatar the user wants to use.

Uploads are POSTed to a resource on the user's local homeserver which returns a MXC URI which can later be used to GET the download. Content is downloaded from the recipient's local homeserver, which must first transfer the content from the origin homeserver using the same API (unless the origin and destination homeservers are the same).

When serving content, the server SHOULD provide a Content-Security-Policy header. The recommended policy is sandbox; default-src 'none'; script-src 'none'; plugin-types application/pdf; style-src 'unsafe-inline'; object-src 'self';.

Matrix Content (MXC) URIs

Content locations are represented as Matrix Content (MXC) URIs. They look like:

mxc://<server-name>/<media-id>

<server-name> : The name of the homeserver where this content originated, e.g. matrix.org
<media-id> : An opaque ID which identifies the content.

Client behaviour

Clients can upload and download content using the following HTTP APIs.

{{% http-api spec="client-server" api="content-repo" %}}

Thumbnails

The homeserver SHOULD be able to supply thumbnails for uploaded images and videos. The exact file types which can be thumbnailed are not currently specified - see Issue #1938 for more information.

The thumbnail methods are "crop" and "scale". "scale" tries to return an image where either the width or the height is smaller than the requested size. The client should then scale and letterbox the image if it needs to fit within a given rectangle. "crop" tries to return an image where the width and height are close to the requested size and the aspect matches the requested size. The client should scale the image if it needs to fit within a given rectangle.

The dimensions given to the thumbnail API are the minimum size the client would prefer. Servers must never return thumbnails smaller than the client's requested dimensions, unless the content being thumbnailed is smaller than the dimensions. When the content is smaller than the requested dimensions, servers should return the original content rather than thumbnail it.

Servers SHOULD produce thumbnails with the following dimensions and methods:

• 32x32, crop
• 96x96, crop
• 320x240, scale
• 640x480, scale
• 800x600, scale

In summary:

• "scale" maintains the original aspect ratio of the image
• "crop" provides an image in the aspect ratio of the sizes given in the request
• The server will return an image larger than or equal to the dimensions requested where possible.

Servers MUST NOT upscale thumbnails under any circumstance. Servers MUST NOT return a smaller thumbnail than requested, unless the original content makes that impossible.

Security considerations

The HTTP GET endpoint does not require any authentication. Knowing the URL of the content is sufficient to retrieve the content, even if the entity isn't in the room.

MXC URIs are vulnerable to directory traversal attacks such as mxc://127.0.0.1/../../../some_service/etc/passwd. This would cause the target homeserver to try to access and return this file. As such, homeservers MUST sanitise MXC URIs by allowing only alphanumeric (A-Za-z0-9), _ and - characters in the server-name and media-id values. This set of whitelisted characters allows URL-safe base64 encodings specified in RFC 4648. Applying this character whitelist is preferable to blacklisting . and / as there are techniques around blacklisted characters (percent-encoded characters, UTF-8 encoded traversals, etc).

Homeservers have additional content-specific concerns:

• Clients may try to upload very large files. Homeservers should not store files that are too large and should not serve them to clients, returning a HTTP 413 error with the M_TOO_LARGE code.
• Clients may try to upload very large images. Homeservers should not attempt to generate thumbnails for images that are too large, returning a HTTP 413 error with the M_TOO_LARGE code.
• Remote homeservers may host very large files or images. Homeservers should not proxy or thumbnail large files or images from remote homeservers, returning a HTTP 502 error with the M_TOO_LARGE code.
• Clients may try to upload a large number of files. Homeservers should limit the number and total size of media that can be uploaded by clients, returning a HTTP 403 error with the M_FORBIDDEN code.
• Clients may try to access a large number of remote files through a homeserver. Homeservers should restrict the number and size of remote files that it caches.
• Clients or remote homeservers may try to upload malicious files targeting vulnerabilities in either the homeserver thumbnailing or the client decoders.