From bed70b768213f4d71dab80b825dcc70bab16bb97 Mon Sep 17 00:00:00 2001
From: Meyon Soo  Kim <39788337+PreAgile@users.noreply.github.com>
Date: Mon, 4 May 2026 14:10:54 +0900
Subject: [PATCH] Forward incoming headers in WebFlux ProxyExchange body
 requests

The WebFlux ProxyExchange.exchange() helper added incoming request
headers to the outgoing WebClient call only on the body-less path
(GET/HEAD/OPTIONS). The body-carrying branches (Publisher body and
value body) skipped the addHeaders(...) call, so custom headers such
as Authorization or X-Custom set on the original request were not
propagated to the upstream service for POST/PUT/PATCH/DELETE with a
body.

Hoist the incoming-headers propagation to the builder's construction
site so every branch picks it up. addHeaders is idempotent (it skips
already-present keys), so this is a no-op for the body-less branches
that previously called it explicitly. Those duplicate calls are
removed.

Adds a regression test (postWithBodyForwardsIncomingHeaders) that
asserts a custom header on an incoming POST reaches the upstream
service via the existing /proxy/{id} -> /bars test wiring.

The webmvc variant of ProxyExchange is unaffected: its private
headers(BodyBuilder) helper already adds incoming headers on every
method.

Fixes gh-2559

Signed-off-by: Meyon Soo  Kim <39788337+PreAgile@users.noreply.github.com>
---
 .../cloud/gateway/webflux/ProxyExchange.java           |  9 ++++-----
 .../gateway/webflux/ProductionConfigurationTests.java  | 10 ++++++++++
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/spring-cloud-gateway-proxyexchange-webflux/src/main/java/org/springframework/cloud/gateway/webflux/ProxyExchange.java b/spring-cloud-gateway-proxyexchange-webflux/src/main/java/org/springframework/cloud/gateway/webflux/ProxyExchange.java
index da21176ec..88f3bf5a2 100644
--- a/spring-cloud-gateway-proxyexchange-webflux/src/main/java/org/springframework/cloud/gateway/webflux/ProxyExchange.java
+++ b/spring-cloud-gateway-proxyexchange-webflux/src/main/java/org/springframework/cloud/gateway/webflux/ProxyExchange.java
@@ -372,7 +372,8 @@ private Mono<ResponseEntity<T>> exchange(RequestEntity<?> requestEntity) {
 		Objects.requireNonNull(requestEntity.getMethod(), "Method must not be null");
 		RequestBodySpec builder = rest.method(requestEntity.getMethod())
 			.uri(requestEntity.getUrl())
-			.headers(headers -> addHeaders(headers, requestEntity.getHeaders()));
+			.headers(headers -> addHeaders(headers, requestEntity.getHeaders()))
+			.headers(headers -> addHeaders(headers, exchange.getRequest().getHeaders()));
 		WebClient.ResponseSpec result;
 		if (requestEntity.getBody() instanceof Publisher) {
 			@SuppressWarnings("unchecked")
@@ -384,12 +385,10 @@ else if (requestEntity.getBody() != null) {
 		}
 		else {
 			if (hasBody) {
-				result = builder.headers(headers -> addHeaders(headers, exchange.getRequest().getHeaders()))
-					.body(exchange.getRequest().getBody(), DataBuffer.class)
-					.retrieve();
+				result = builder.body(exchange.getRequest().getBody(), DataBuffer.class).retrieve();
 			}
 			else {
-				result = builder.headers(headers -> addHeaders(headers, exchange.getRequest().getHeaders())).retrieve();
+				result = builder.retrieve();
 			}
 		}
 		return result.onStatus(HttpStatusCode::isError, t -> Mono.empty())
diff --git a/spring-cloud-gateway-proxyexchange-webflux/src/test/java/org/springframework/cloud/gateway/webflux/ProductionConfigurationTests.java b/spring-cloud-gateway-proxyexchange-webflux/src/test/java/org/springframework/cloud/gateway/webflux/ProductionConfigurationTests.java
index 73eb60cc8..fb6894821 100644
--- a/spring-cloud-gateway-proxyexchange-webflux/src/test/java/org/springframework/cloud/gateway/webflux/ProductionConfigurationTests.java
+++ b/spring-cloud-gateway-proxyexchange-webflux/src/test/java/org/springframework/cloud/gateway/webflux/ProductionConfigurationTests.java
@@ -123,6 +123,16 @@ public void post() throws Exception {
 			.isEqualTo("host=localhost:" + port + ";foo");
 	}
 
+	@Test
+	public void postWithBodyForwardsIncomingHeaders() throws Exception {
+		ResponseEntity<Bar> response = rest
+			.exchange(RequestEntity.post(rest.getRestTemplate().getUriTemplateHandler().expand("/proxy/0"))
+				.header("X-Custom", "incoming-header-value")
+				.contentType(MediaType.APPLICATION_JSON)
+				.body(Collections.singletonMap("name", "foo")), Bar.class);
+		assertThat(response.getBody().getName()).contains("incoming-header-value");
+	}
+
 	@Test
 	public void postJsonWithWhitespace() {
 		var json = """