8353440: Disable FTP fallback for non-local file URLs by default

Reviewed-by: dfuchs

src/java.base/share/classes/java/net/doc-files/net-properties.html

@@ -169,6 +169,17 @@ to determine the proxy that should be used for connecting to a given URI.</P>
 	globally through their user interface). Note that this property is
 	checked only once at startup.</P>
 </UL>
+<a id="FileHandler"></a>
+<H2>File URL stream protocol handler properties</H2>
+<P>The following properties are used to configure the handler for URLs with the {@code file://} scheme:</P>
+<UL>
+	<LI><P><B>{@systemProperty jdk.net.file.ftpfallback}</B> (default: &lt;false&gt;)<BR>
+		The {@code file://} handler by default rejects any non-local file URL (as defined by RFC 8089)
+		as invalid. Setting this property to <B>true</B> enables a legacy feature where
+		the handler instead opens an FTP connection for such non-local URLs.</P>
+		<P>Any modern code should use explicit {@code ftp://} URLs instead and not rely on
+			enabling this legacy FTP fallback feature.</P>
+</UL>
 <a id="MiscHTTP"></a>
 <H2>Misc HTTP URL stream protocol handler properties</H2>
 <UL>

src/java.base/share/classes/sun/net/www/protocol/file/FileURLConnection.java

@@ -25,6 +25,7 @@
 
 package sun.net.www.protocol.file;
 
+import java.net.MalformedURLException;
 import java.net.URL;
 import java.net.FileNameMap;
 import java.io.*;
@@ -46,6 +47,11 @@ public class FileURLConnection extends URLConnection {
     private static final String TEXT_PLAIN = "text/plain";
     private static final String LAST_MODIFIED = "last-modified";
 
+    // The feature of falling back to FTP for non-local file URLs is disabled
+    // by default and can be re-enabled by setting a system property
+    private static final boolean FTP_FALLBACK_ENABLED =
+            Boolean.getBoolean("jdk.net.file.ftpfallback");
+
     private final File file;
     private InputStream is;
     private List<String> directoryListing;
@@ -222,4 +228,17 @@ public class FileURLConnection extends URLConnection {
         }
         return permission;
     }
+
+    /**
+     * Throw {@link MalformedURLException} if the FTP fallback feature for non-local
+     * file URLs is not explicitly enabled via system property.
+     *
+     * @see #FTP_FALLBACK_ENABLED
+     * @throws MalformedURLException if FTP fallback is not enabled
+     */
+     static void requireFtpFallbackEnabled() throws MalformedURLException {
+        if (!FTP_FALLBACK_ENABLED) {
+            throw new MalformedURLException("Unsupported non-local file URL");
+        }
+    }
 }

src/java.base/unix/classes/sun/net/www/protocol/file/Handler.java

@@ -72,6 +72,7 @@ public class Handler extends URLStreamHandler {
         /* If you reach here, it implies that you have a hostname
            so attempt an ftp connection.
          */
+        FileURLConnection.requireFtpFallbackEnabled();
         URLConnection uc;
         URL ru;
 

src/java.base/windows/classes/sun/net/www/protocol/file/Handler.java

@@ -88,6 +88,7 @@ public class Handler extends URLStreamHandler {
         /*
          * Now attempt an ftp connection.
          */
+        FileURLConnection.requireFtpFallbackEnabled();
         URLConnection uc;
         URL newurl;
 

test/jdk/java/net/URL/OpenStream.java

@@ -25,6 +25,8 @@
  * @bug 4064962 8202708
  * @summary openStream should work even when not using proxies and
  *          UnknownHostException is thrown as expected.
+ * @comment For testing of non-local file URLs with the legacy FTP
+ *          fallback feature enabled, see NonLocalFtpFallback.
  */
 
 import java.io.*;
@@ -34,24 +36,15 @@ import java.net.*;
 public class OpenStream {
 
     private static final String badHttp = "http://foo.bar.baz/";
-    private static final String badUnc = "file://h7qbp368oix47/not-exist.txt";
 
     public static void main(String[] args) throws IOException {
         testHttp();
-        testUnc();
     }
 
     static void testHttp() throws IOException {
         checkThrows(badHttp);
     }
 
-    static void testUnc() throws IOException {
-        boolean isWindows = System.getProperty("os.name").startsWith("Windows");
-        if (isWindows) {
-            checkThrows(badUnc);
-        }
-    }
-
     static void checkThrows(String url) throws IOException {
         URL u = new URL(url);
         try {
@@ -62,7 +55,6 @@ public class OpenStream {
         }
         throw new RuntimeException("Expected UnknownHostException to be " +
                 "thrown for " + url);
-
     }
 }
 

test/jdk/sun/net/www/protocol/file/NonLocalFtpFallback.java

@@ -40,14 +40,16 @@ import java.util.Set;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
 
+import static org.junit.Assert.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertArrayEquals;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 
 /**
  * @test
- * @bug 8353662
- * @summary Verify long-standing behavior of resolving non-local file URLs using FTP.
- * @run junit NonLocalFtpFallback
+ * @bug 8353662 8202708
+ * @summary Verify long-standing, disabled by default behavior of resolving non-local
+ *          file URLs using FTP.
+ * @run junit/othervm -Djdk.net.file.ftpfallback=true NonLocalFtpFallback
  */
 
 public class NonLocalFtpFallback {
@@ -112,9 +114,8 @@ public class NonLocalFtpFallback {
      * Verifies the long-standing and unspecified FTP fallback feature where the file
      * URL scheme handler attempts an FTP connection for non-local files.
      *
-     * The non-local file URL used here is of the form file://127.0.0.1/path. Since the
-     * host component here is not equal to "localhost", this is considered a non-local
-     * URL.
+     * The non-local file URL used here is of the form 'file://remotehost/path'. Since the
+     * host component is not equal to 'localhost', this is considered a non-local URL.
      *
      * @throws Exception
      */
@@ -142,4 +143,45 @@ public class NonLocalFtpFallback {
         URL ftpURL = new URL("ftp", hostname, localURL.getFile());
         assertEquals(ftpURL.toURI(), uris.iterator().next());
     }
+
+    /**
+     * Sanity check that a local file URL (with a host component equal to 'localhost')
+     * does not open any FtpURLConnection when the FTP fallback feature is enabled.
+     *
+     * @throws Exception
+     */
+    @Test
+    public void verifyLocalFileURL() throws Exception {
+        URL localURL = file.toUri().toURL();
+        URL nonLocalURL = new URL("file", "localhost", localURL.getFile());
+
+        // Open the local file: URL connection supplying a proxy
+        Proxy proxy = new Proxy(Proxy.Type.HTTP,
+                new InetSocketAddress(proxyServer.getAddress().getAddress(),
+                        proxyServer.getAddress().getPort()));
+        URLConnection con = nonLocalURL.openConnection(proxy);
+
+        // Assert that the expected file content is read
+        try (InputStream in = con.getInputStream()) {
+            byte[] retrived = in.readAllBytes();
+            assertArrayEquals(Files.readAllBytes(file), retrived);
+        }
+
+        // Assert that no FTP URIs were requested in the HTTP proxy
+        assertEquals(0, uris.size());
+    }
+
+    /**
+     * Verify that opening a stream on a non-proxy URLConnection for a non-local
+     * file URL with an unknown host fails with UnknownHostException
+     * when the fallback FtpURLConnection attempts to connect to the non-existing
+     * FTP server.
+     */
+    @Test
+    public void verifyFtpUnknownHost() throws IOException {
+        URL url = new URL("file://nonexistinghost/not-exist.txt");
+        assertThrows(UnknownHostException.class, () -> {
+            InputStream in = url.openConnection(Proxy.NO_PROXY).getInputStream();
+        });
+    }
 }

test/jdk/sun/net/www/protocol/file/NonLocalFtpFallbackDisabled.java

@@ -0,0 +1,73 @@
+/*
+ * 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 org.junit.jupiter.api.Test;
+
+import java.net.MalformedURLException;
+import java.net.URI;
+import java.net.URISyntaxException;
+import java.net.URL;
+import java.nio.file.Path;
+
+import static org.junit.Assert.assertThrows;
+
+/**
+ * @test
+ * @bug 8353440
+ * @summary Verify that non-local file URLs are rejected by default
+ * @run junit/othervm NonLocalFtpFallbackDisabled
+ * @run junit/othervm -Djdk.net.file.ftpfallback=false NonLocalFtpFallbackDisabled
+ * @run junit/othervm -Djdk.net.file.ftpfallback NonLocalFtpFallbackDisabled
+ */
+public class NonLocalFtpFallbackDisabled {
+
+    // The file requested in this test
+    private Path file = Path.of("ftp-file.txt");
+
+    /**
+     * Verifies that the long-standing and unspecified FTP fallback feature
+     * where the file URL scheme handler attempts an FTP connection for non-local
+     * files is disabled by default and that opening connections for such URLs
+     * is rejected with a MalformedURLException.
+     *
+     * @throws MalformedURLException if an unexpected URL exception occurs
+     * @throws URISyntaxException if an unexpected URI exception occurs
+     */
+    @Test
+    public void verifyNonLocalFileURLRejected() throws MalformedURLException, URISyntaxException {
+        // We can use a fake host name here, no actual FTP request will be made
+        String hostname = "remotehost";
+
+        URL local = file.toUri().toURL();
+
+        URL nonLocal = new URI("file", hostname, local.getFile(), "").toURL();
+        assertThrows(MalformedURLException.class, () -> {
+            nonLocal.openConnection();
+        });
+
+        URL nonLocalEmptyPath = new URI("file", hostname, "", "").toURL();
+        assertThrows(MalformedURLException.class, () -> {
+            nonLocalEmptyPath.openConnection();
+        });
+    }
+}