MSC4126: Deprecation of query string auth (#4126)

* MSC: Deprecation of query string auth

* Update proposals/4126-deprecate-query-string-auth.md

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

---------

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

proposals/4126-deprecate-query-string-auth.md

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+# MSC4126: Deprecation of query string auth
+
+Presently, the Client-Server API allows clients to provide their access token via the `Authorization`
+request header or via an `access_token` query string parameter, described [here](https://spec.matrix.org/v1.10/client-server-api/#using-access-tokens).
+Clients are already encouraged to use the header approach, though the query string option exists for
+largely backwards compatibility reasons.
+
+The query string approach is subject a number of security, usability, and practical concerns, discussed
+on [matrix-spec#1780](https://github.com/matrix-org/matrix-spec/issues/1780):
+
+* The query string of an HTTP request is often logged by the client itself, middleware reverse proxy,
+  and application/homeserver as well. Though some of these layers may be aware of this issue, they
+  can trivially accidentally break to log sensitive credentials again. By contrast, headers are not
+  typically logged by default.
+
+* Users often copy and paste URLs from their clients to either get support or provide direct links
+  to content/media. While the media angle is largely expected to be resolved with [MSC3916](https://github.com/matrix-org/matrix-spec-proposals/pull/3916),
+  users are currently able to right click images in their client and copy the URL - if this URL
+  includes authentication in the query string, the user will likely end up disclosing their access
+  token. The same scenario applies when copy/pasting request logs out of a client when getting
+  support.
+
+* Having two ways of doing things could lead to compatibility issues, where a client using the query
+  string approach is tried against a server which only supports the header. The client ends up not
+  working, leading to subpar user experience.
+
+* Most clients have already adopted the header approach, largely forgetting that the query string
+  even exists. Continuing to support the query string option leaves some maintenance burden for what
+  is effectively unused code.
+
+* Matrix has [decided](https://matrix.org/blog/2023/09/matrix-2-0/) to adopt OIDC for authentication,
+  which is based on OAuth 2.0, which [advises against](https://datatracker.ietf.org/doc/html/draft-ietf-oauth-security-topics#section-4.3.2)
+  the query string approach.
+
+With these conditions in mind, this proposal sets the query string approach on a path towards removal
+from the Matrix specification. This affects the Client-Server API and [Identity Service API](https://spec.matrix.org/v1.10/identity-service-api/#authentication)
+as both support the approaches described above.
+
+## Proposal
+
+For both the Client-Server API and Identity Service API, the `access_token` query string authentication
+parameter becomes *deprecated*, and SHOULD NOT be used by clients (as already stated in the specification).
+Deprecation is required for at least 1 spec version before removal under the [deprecation policy](https://spec.matrix.org/v1.10/#deprecation-policy).
+
+Removal from the specification requires a second MSC and at least 1 specification release to pass. This
+is currently described as [MSC4127](https://github.com/matrix-org/matrix-spec-proposals/pull/4127).
+
+## Potential issues
+
+Clients which rely on the query string approach may stop working. This is considered acceptable for
+the purposes of this MSC.
+
+## Alternatives
+
+Most alternatives are not practical as they would maintain the security risk described in the introduction
+for this proposal.
+
+Alterations to the deprecation policy may be discussed in a future MSC to make this sort of removal
+easier.
+
+## Security considerations
+
+Security considerations are described throughout this proposal.
+
+## Unstable prefix
+
+This proposal cannot feasibly have an unstable prefix. Clients are already discouraged from using
+query string authentication and should switch to `Authorization` as soon as possible, regardless of
+this MSC.
+
+## Dependencies
+
+This MSC has no direct dependencies itself. [MSC4127](https://github.com/matrix-org/matrix-spec-proposals/pull/4127)
+requires this MSC to land first.