matrix-spec/specification/modules/content_repo.rst

.. Copyright 2016 OpenMarket Ltd
.. Copyright 2019 The Matrix.org Foundation C.I.C.
..
.. Licensed under the Apache License, Version 2.0 (the "License");
.. you may not use this file except in compliance with the License.
.. You may obtain a copy of the License at
..
..     http://www.apache.org/licenses/LICENSE-2.0
..
.. Unless required by applicable law or agreed to in writing, software
.. distributed under the License is distributed on an "AS IS" BASIS,
.. WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
.. See the License for the specific language governing permissions and
.. limitations under the License.

Content repository
==================

.. _module:content:

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
-------------------------

.. _`MXC URI`:

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.

{{content_repo_cs_http_api}}

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 <https://github.com/matrix-org/matrix-doc/issues/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.