diff --git a/web/src/main/java/org/springframework/security/web/server/firewall/StrictServerWebExchangeFirewall.java b/web/src/main/java/org/springframework/security/web/server/firewall/StrictServerWebExchangeFirewall.java
index 0916f1ffd9..72871f2b45 100644
--- a/web/src/main/java/org/springframework/security/web/server/firewall/StrictServerWebExchangeFirewall.java
+++ b/web/src/main/java/org/springframework/security/web/server/firewall/StrictServerWebExchangeFirewall.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2024 the original author or authors.
+ * Copyright 2002-2025 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -802,42 +802,42 @@ public class StrictServerWebExchangeFirewall implements ServerWebExchangeFirewal
 
 				@Override
 				public Builder method(HttpMethod httpMethod) {
-					return this.delegate.method(httpMethod);
+					return new StrictFirewallBuilder(this.delegate.method(httpMethod));
 				}
 
 				@Override
 				public Builder uri(URI uri) {
-					return this.delegate.uri(uri);
+					return new StrictFirewallBuilder(this.delegate.uri(uri));
 				}
 
 				@Override
 				public Builder path(String path) {
-					return this.delegate.path(path);
+					return new StrictFirewallBuilder(this.delegate.path(path));
 				}
 
 				@Override
 				public Builder contextPath(String contextPath) {
-					return this.delegate.contextPath(contextPath);
+					return new StrictFirewallBuilder(this.delegate.contextPath(contextPath));
 				}
 
 				@Override
 				public Builder header(String headerName, String... headerValues) {
-					return this.delegate.header(headerName, headerValues);
+					return new StrictFirewallBuilder(this.delegate.header(headerName, headerValues));
 				}
 
 				@Override
 				public Builder headers(Consumer<HttpHeaders> headersConsumer) {
-					return this.delegate.headers(headersConsumer);
+					return new StrictFirewallBuilder(this.delegate.headers(headersConsumer));
 				}
 
 				@Override
 				public Builder sslInfo(SslInfo sslInfo) {
-					return this.delegate.sslInfo(sslInfo);
+					return new StrictFirewallBuilder(this.delegate.sslInfo(sslInfo));
 				}
 
 				@Override
 				public Builder remoteAddress(InetSocketAddress remoteAddress) {
-					return this.delegate.remoteAddress(remoteAddress);
+					return new StrictFirewallBuilder(this.delegate.remoteAddress(remoteAddress));
 				}
 
 				@Override
diff --git a/web/src/test/java/org/springframework/security/web/server/firewall/StrictServerWebExchangeFirewallTests.java b/web/src/test/java/org/springframework/security/web/server/firewall/StrictServerWebExchangeFirewallTests.java
index b8803bc0d1..8f24b0522c 100644
--- a/web/src/test/java/org/springframework/security/web/server/firewall/StrictServerWebExchangeFirewallTests.java
+++ b/web/src/test/java/org/springframework/security/web/server/firewall/StrictServerWebExchangeFirewallTests.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2024 the original author or authors.
+ * Copyright 2002-2025 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -534,4 +534,17 @@ class StrictServerWebExchangeFirewallTests {
 			.isThrownBy(() -> headers.get(invalidHeaderName));
 	}
 
+	// gh-16978
+	@Test
+	void getMutatedFirewalledExchangeHeadersGetHeaderWhenNotAllowedHeaderNameThenException() {
+		String invalidHeaderName = "bad name";
+		this.firewall.setAllowedHeaderNames((name) -> !name.equals(invalidHeaderName));
+		ServerWebExchange exchange = getFirewalledExchange();
+		var mutatedRequest = exchange.getRequest().mutate().method(HttpMethod.POST).build();
+		var mutatedExchange = exchange.mutate().request(mutatedRequest).build();
+		HttpHeaders headers = mutatedExchange.getRequest().getHeaders();
+		assertThatExceptionOfType(ServerExchangeRejectedException.class)
+			.isThrownBy(() -> headers.get(invalidHeaderName));
+	}
+
 }