Support ServerExchangeRejectedHandler @Bean

Closes gh-15975

docs/modules/ROOT/pages/reactive/exploits/firewall.adoc

@@ -200,3 +200,35 @@ firewall.setAllowedHeaderValues {
 }
 ----
 ======
+
+The `ServerExchangeRejectedHandler` interface is used to handle `ServerExchangeRejectedException` throw by Spring Security's `ServerWebExchangeFirewall`.
+By default `HttpStatusExchangeRejectedHandler` is used to send an HTTP 400 response to clients when a request is rejected.
+To customize the behavior, users can expose a `ServerExchangeRejectedHandler` Bean.
+For example, the following will send an HTTP 404 when the request is rejected:
+
+
+.Send 404 on Request Rejected
+[tabs]
+======
+Java::
++
+[source,java,role="primary"]
+----
+@Bean
+ServerExchangeRejectedHandler rejectedHandler() {
+	return new HttpStatusExchangeRejectedHandler(HttpStatus.NOT_FOUND);
+}
+----
+
+Kotlin::
++
+[source,kotlin,role="secondary"]
+----
+@Bean
+fun rejectedHandler(): ServerExchangeRejectedHandler {
+    return HttpStatusExchangeRejectedHandler(HttpStatus.NOT_FOUND)
+}
+----
+======
+
+Handling can be completely customized by creating a custom `ServerExchangeRejectedHandler` implementation.