Reflect that GET, HEAD, and POST are always allowed methods #37266
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We recently came across the behaviour that even a non-simple POST request is allowed by the browsers, even if POST is not explicitely stated in the A-C-Allow-Methods header. So I looked it up in the specification and found that in fact _all_ GET, HEAD, and POST requests are CORS-safelisted methods (see also fetch-spec https://fetch.spec.whatwg.org/#ref-for-cors-safelisted-method%E2%91%A2). A google search revealed that I am not alone in being confused by this. This MDN page added to the confusion, as it shows an example allowing explicitely GET and POST by using this header, which does not make sense, as these HTTP methods are always allowed either way. So I changed the example to be more meaningful and added a short description to the <method> directive.
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(comment last updated: 2024-12-19 11:26:28) |
bsmth
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Dec 19, 2024
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Dec 25, 2024
a non-simple POST request is allowed by the browsers, even if POST is not explicitly stated in the A-C-Allow-Methods header. changed the example to be more meaningful and added a short description to the <method> directive. Co-authored-by: Brian Smith <[email protected]>
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Description
I changed the example to be more meaningful using PUT and DELETE and added a short description to the directive.
Motivation
We recently came across the behaviour that even a non-simple POST request is allowed by the browsers, even if POST is not explicitely stated in the A-C-Allow-Methods header. So I looked it up in the specification and found that in fact all GET, HEAD, and POST requests are CORS-safelisted methods (see also fetch-spec https://fetch.spec.whatwg.org/#ref-for-cors-safelisted-method%E2%91%A2).
A google search revealed that I am not alone in being confused by this. This MDN page added to the confusion, as it shows an example allowing explicitely GET and POST by using this header, which does not make sense, as these HTTP methods are always allowed either way.
Additional details
Fetch Spec ("If request’s method is not in methods, request’s method is not a CORS-safelisted method, ..."
https://fetch.spec.whatwg.org/#ref-for-cors-safelisted-method%E2%91%A2
Confusion on the web:
dotnet/aspnetcore#40616
https://stackoverflow.com/questions/74596745/if-the-post-method-is-not-idempotent-and-is-not-a-safe-method-why-does-the-cors
Related issues and pull requests