Published Jun 18, 2024
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A CORS preflight request is a CORS request that checks to see if the CORS protocol is understood and a server is aware using specific methods and headers.
It is an OPTIONS request, using two or three HTTP request headers:
Access-Control-Request-Method, Origin, and optionally
Access-Control-Request-Headers.
A preflight request is automatically issued by a browser and in normal cases, front-end developers don’t need to craft such requests themselves. It appears when request is qualified as “to be preflighted” and omitted for simple requests.
For example, a client might be asking a server if it would allow a DELETE
request, before sending a DELETE request, by using a preflight request:
OPTIONS /resource/foo
Access-Control-Request-Method: DELETE
Access-Control-Request-Headers: x-requested-with
Origin: https://foo.bar.org
If the server allows it, then it will respond to the preflight request with an
Access-Control-Allow-Methods response header, which lists DELETE:
HTTP/1.1 204 No Content
Connection: keep-alive
Access-Control-Allow-Origin: https://foo.bar.org
Access-Control-Allow-Methods: POST, GET, OPTIONS, DELETE
Access-Control-Allow-Headers: X-Requested-With
Access-Control-Max-Age: 86400
The preflight response can be optionally cached for the requests created in the
same URL using Access-Control-Max-Age header like in the above example. To
cache preflight responses, the browser uses a specific cache that is separate
from the general HTTP cache that the browser manages. Preflight responses are
never cached in the browser’s general HTTP cache.