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MSC4138: Update allowed HTTP methods in CORS responses #4138

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MSC4138: Update allowed HTTP methods in CORS responses #4138

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Update allowed HTTP methods in CORS responses
turt2live May 3, 2024
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turt2live May 3, 2024
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47 changes: 47 additions & 0 deletions proposals/4138-update-cors-methods.md
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Implementation requirements:

  • None, in my opinion. See "unstable prefix" section for rationale.

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# MSC4138: Update allowed HTTP methods in CORS responses

The [specification](https://spec.matrix.org/v1.10/client-server-api/#web-browser-clients) suggests
that servers allow a limited subset of the available [HTTP methods](https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods)
available in [CORS](https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS) responses. However, it's
reasonable to expect the specification to use other methods in the future or as part of feature
detection. To permit these use cases early, this MSC proposes adding a few more allowable values to
the `Access-Control-Allow-Methods` header.

## Proposal

The [`Access-Control-Allow-Methods` header's recommended value](https://spec.matrix.org/v1.10/client-server-api/#web-browser-clients)
is updated to include the following:

* `PATCH` - A plausibly useful HTTP method for future use.
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#4133 now wants to use this.

* `HEAD` - Similar to `PATCH`, `HEAD` is plausibly useful for feature detection and cases like
[MSC4120](https://github.com/matrix-org/matrix-spec-proposals/pull/4120).
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The following methods are *not* included because they don't have foreseeable use in Matrix:

* `CONNECT`
* `TRACE`
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I might suggest that this could be an MSC where we propose a literal update to the text of the spec: the current spec just plucks a recommendation out of thin air with no explanation of why it's recommended, but this MSC contains a bunch of useful context. I know you're trying to write it without including a reference to an in-review MSC in the spec, but I'm finding the result quite obtuse: "future use" just makes me wonder why the server wouldn't add those methods when it has an endpoint that actually uses them.

I would suggest something along the lines of, "if you implement all of the Matrix spec, you'll need this set, however this expanded set would be safe and sensible should you ever implement any MSCs that use them"?

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or maybe something along the lines of "... if you implement any other endpoints that use other HTTP methods, make sure that you also include those methods on CORS".

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#4138 (comment) is in line with this thread.

Will update the MSC.


## Potential issues

None anticipated.

## Alternatives

No significant alternatives.

## Security considerations

CORS is meant to help ensure requests made by the client are properly scoped in the client. If the
client wishes to use an HTTP method not allowed by the server, the web browser will mask the
response with an error before the application can inspect it. Therefore, to increase future
compatibility, we append a few useful HTTP methods while still excluding ones which are (currently)
nonsensical.

## Unstable prefix

This proposal cannot have an unstable prefix due to the nature of CORS. Servers are already able to
go off-spec and serve different headers because the spec is merely a recommendation.

## Dependencies

This proposal has no dependencies.