8359709: java.net.HttpURLConnection sends unexpected "Host" request header in some cases after JDK-8344190

src/java.base/share/classes/sun/net/www/protocol/http/HttpURLConnection.java

@@ -621,10 +621,10 @@ private void writeRequests() throws IOException {
             if (port != -1 && port != url.getDefaultPort()) {
                 host += ":" + String.valueOf(port);
             }
-            String reqHost = requests.findValue("Host");
-            if (reqHost == null || !reqHost.equalsIgnoreCase(host)) {
-                requests.set("Host", host);
-            }
+            // if the "Host" header hasn't been explicitly set, then set its
+            // value to the one determined through the request URL
+            requests.setIfNotSet("Host", host);
+
             requests.setIfNotSet("Accept", acceptString);
 
             /*

test/jdk/java/net/HttpURLConnection/HostHeaderTest.java

@@ -0,0 +1,143 @@
+/*
+ * Copyright (c) 2025, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.OutputStream;
+import java.net.HttpURLConnection;
+import java.net.InetAddress;
+import java.net.InetSocketAddress;
+import java.net.Proxy;
+import java.net.URL;
+import java.net.URLConnection;
+import java.util.List;
+
+import com.sun.net.httpserver.HttpExchange;
+import com.sun.net.httpserver.HttpHandler;
+import com.sun.net.httpserver.HttpServer;
+import jdk.test.lib.net.URIBuilder;
+import org.junit.jupiter.api.AfterAll;
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.Test;
+import static java.nio.charset.StandardCharsets.US_ASCII;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertInstanceOf;
+import static org.junit.jupiter.api.Assertions.assertNull;
+
+/*
+ * @test
+ * @bug 8359709
+ * @summary verify that if the Host header is allowed to be set by the application
+ *          then the correct value gets set in a HTTP request issued through
+ *          java.net.HttpURLConnection
+ * @library /test/lib
+ * @run junit HostHeaderTest
+ * @run junit/othervm -Dsun.net.http.allowRestrictedHeaders=true HostHeaderTest
+ * @run junit/othervm -Dsun.net.http.allowRestrictedHeaders=false HostHeaderTest
+ */
+class HostHeaderTest {
+
+    private static final String SERVER_CTX_ROOT = "/8359709/";
+    private static final boolean allowsHostHeader = Boolean.getBoolean("sun.net.http.allowRestrictedHeaders");
+
+    private static HttpServer server;
+
+    @BeforeAll
+    static void beforeAll() throws Exception {
+        final InetSocketAddress addr = new InetSocketAddress(InetAddress.getLoopbackAddress(), 0);
+        server = HttpServer.create(addr, 0);
+        server.createContext(SERVER_CTX_ROOT, new Handler());
+        server.start();
+        System.err.println("started server at " + server.getAddress());
+    }
+
+    @AfterAll
+    static void afterAll() throws Exception {
+        if (server != null) {
+            System.err.println("stopping server " + server.getAddress());
+            server.stop(0);
+        }
+    }
+
+    @Test
+    void testHostHeader() throws Exception {
+        final InetSocketAddress serverAddr = server.getAddress();
+        final URL reqURL = URIBuilder.newBuilder()
+                .scheme("http")
+                .loopback()
+                .port(serverAddr.getPort())
+                .path(SERVER_CTX_ROOT)
+                .build().toURL();
+        final URLConnection conn = reqURL.openConnection(Proxy.NO_PROXY);
+
+        conn.setRequestProperty("Host", "foobar");
+        if (!allowsHostHeader) {
+            // if restricted headers aren't allowed to be set by the user, then
+            // we expect the previous call to setRequestProperty to not set the Host
+            // header
+            assertNull(conn.getRequestProperty("Host"), "Host header unexpectedly set");
+        }
+
+        assertInstanceOf(HttpURLConnection.class, conn);
+        final HttpURLConnection httpURLConn = (HttpURLConnection) conn;
+
+        // send the HTTP request
+        System.err.println("sending request " + reqURL);
+        final int respCode = httpURLConn.getResponseCode();
+        assertEquals(200, respCode, "unexpected response code");
+        // verify that the server side handler received the expected
+        // Host header value in the request
+        try (final InputStream is = httpURLConn.getInputStream()) {
+            final byte[] resp = is.readAllBytes();
+            // if Host header wasn't explicitly set, then we expect it to be
+            // derived from the request URL
+            final String expected = allowsHostHeader
+                    ? "foobar"
+                    : reqURL.getHost() + ":" + reqURL.getPort();
+            final String actual = new String(resp, US_ASCII);
+            assertEquals(expected, actual, "unexpected Host header received on server side");
+        }
+    }
+
+    private static final class Handler implements HttpHandler {
+        private static final int NO_RESPONSE_BODY = -1;
+
+        @Override
+        public void handle(final HttpExchange exchange) throws IOException {
+            final List<String> headerVals = exchange.getRequestHeaders().get("Host");
+            System.err.println("Host header has value(s): " + headerVals);
+            // unexpected Host header value, respond with 400 status code
+            if (headerVals == null || headerVals.size() != 1) {
+                System.err.println("Unexpected header value(s) for Host header: " + headerVals);
+                exchange.sendResponseHeaders(400, NO_RESPONSE_BODY);
+                return;
+            }
+            // respond back with the Host header value that we found in the request
+            final byte[] response = headerVals.getFirst().getBytes(US_ASCII);
+            exchange.sendResponseHeaders(200, response.length);
+            try (final OutputStream os = exchange.getResponseBody()) {
+                os.write(response);
+            }
+        }
+    }
+}