Refine CORS documentation for wildcard processing

This commit adds a reference documentation section dedicated
to CORS credentialed requests and related wildcard processing.

Closes gh-31143

Author: sdeleuze

framework-docs/modules/ROOT/pages/web/webflux-cors.adoc

@@ -75,6 +75,35 @@ To learn more from the source or to make advanced customizations, see:
 
 
 
+[[webflux-cors-credentialed-requests]]
+== Credentialed Requests
+[.small]#xref:web/webmvc-cors.adoc#mvc-cors-credentialed-requests[See equivalent in the Servlet stack]#
+
+Using CORS with credentialed requests requires enabling `allowedCredentials`. Be aware that
+this option establishes a high level of trust with the configured domains and also increases
+the surface of attack of the web application by exposing sensitive user-specific information
+such as cookies and CSRF tokens.
+
+Enabling credentials also impacts how the configured `"*"` CORS wildcards are processed:
+
+* Wildcards are not authorized in `allowOrigins`, but alternatively
+the `allowOriginPatterns` property may be used to match to a dynamic set of origins.
+* When set on `allowedHeaders` or `allowedMethods`, the `Access-Control-Allow-Headers`
+and `Access-Control-Allow-Methods` response headers are handled by copying the related
+headers and method specified in the CORS preflight request.
+* When set on `exposedHeaders`, `Access-Control-Expose-Headers` response header is set
+either to the configured list of headers or to the wildcard character. While the CORS spec
+does not allow the wildcard character when `Access-Control-Allow-Credentials` is set to
+`true`, most browsers support it and the response headers are not all available during the
+CORS processing, so as a consequence the wildcard character is the header value used when
+specified regardless of the value of the `allowCredentials` property.
+
+WARNING: While such wildcard configuration can be handy, it is recommended when possible to configure
+a finite set of values instead to provide a higher level of security.
+
+
+
+
 [[webflux-cors-controller]]
 == `@CrossOrigin`
 [.small]#xref:web/webmvc-cors.adoc#mvc-cors-controller[See equivalent in the Servlet stack]#

framework-docs/modules/ROOT/pages/web/webmvc-cors.adoc

@@ -25,6 +25,35 @@ powerful workarounds based on IFRAME or JSONP.
 
 
 
+[[mvc-cors-credentialed-requests]]
+== Credentialed Requests
+[.small]#xref:web/webflux-cors.adoc#webflux-cors-credentialed-requests[See equivalent in the Reactive stack]#
+
+Using CORS with credentialed requests requires enabling `allowedCredentials`. Be aware that
+this option establishes a high level of trust with the configured domains and also increases
+the surface of attack of the web application by exposing sensitive user-specific information
+such as cookies and CSRF tokens.
+
+Enabling credentials also impacts how the configured `"*"` CORS wildcards are processed:
+
+* Wildcards are not authorized in `allowOrigins`, but alternatively
+the `allowOriginPatterns` property may be used to match to a dynamic set of origins.
+* When set on `allowedHeaders` or `allowedMethods`, the `Access-Control-Allow-Headers`
+and `Access-Control-Allow-Methods` response headers are handled by copying the related
+headers and method specified in the CORS preflight request.
+* When set on `exposedHeaders`, `Access-Control-Expose-Headers` response header is set
+either to the configured list of headers or to the wildcard character. While the CORS spec
+does not allow the wildcard character when `Access-Control-Allow-Credentials` is set to
+`true`, most browsers support it and the response headers are not all available during the
+CORS processing, so as a consequence the wildcard character is the header value used when
+specified regardless of the value of the `allowCredentials` property.
+
+WARNING: While such wildcard configuration can be handy, it is recommended when possible to configure
+a finite set of values instead to provide a higher level of security.
+
+
+
+
 [[mvc-cors-processing]]
 == Processing
 [.small]#xref:web/webflux-cors.adoc#webflux-cors-processing[See equivalent in the Reactive stack]#