--- a/src/share/classes/sun/rmi/transport/tcp/TCPEndpoint.java	Tue Oct 08 15:30:46 2019 +0200
+++ b/src/share/classes/sun/rmi/transport/tcp/TCPEndpoint.java	Fri Jan 24 13:33:31 2020 -0500
@@ -554,7 +554,7 @@
             host = in.readUTF();
             port = in.readInt();
             csf = (RMIClientSocketFactory) in.readObject();
-            if (Proxy.isProxyClass(csf.getClass())) {
+            if (csf != null && Proxy.isProxyClass(csf.getClass())) {
                 throw new IOException("Invalid SocketFactory");
             }
           break;

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/test/java/rmi/server/RMISocketFactory/useSocketFactory/unicast/TCPEndpointReadBug.java	Fri Jan 24 13:33:31 2020 -0500
@@ -0,0 +1,113 @@
+/*
+ * Copyright (c) 2020, 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.Serializable;
+import java.io.ObjectInputStream;
+import java.net.Socket;
+import java.rmi.Remote;
+import java.rmi.RemoteException;
+import java.rmi.server.RMIClientSocketFactory;
+import java.rmi.server.RMISocketFactory;
+import java.rmi.server.UnicastRemoteObject;
+
+/* @test
+ * @bug 8237368
+ * @summary Allow custom socket factory to be null in TCPEndpoint.
+ * @run main/othervm TCPEndpointReadBug
+ */
+public class TCPEndpointReadBug {
+
+    public static void main(String[] args) throws Exception {
+        final I implC = new C();
+        final I remoteC = (I)UnicastRemoteObject.exportObject(
+                implC, 0, new CSF(), RMISocketFactory.getDefaultSocketFactory());
+
+        // Pass a remote object with a custom socket factory as an argument
+        remoteC.echo(remoteC);
+
+        // Pass nothing and get an object with a custom socket factory in return
+        remoteC.echo(null);
+    }
+
+    interface I extends Remote {
+        I echo(I intf) throws RemoteException;
+    }
+
+    static class C implements I {
+        @Override
+        public I echo(I intf) {
+            try {
+                return  (I)UnicastRemoteObject
+                    .exportObject(new C(),0, new CSF(), RMISocketFactory.getDefaultSocketFactory());
+            } catch (RemoteException e) {
+                e.printStackTrace();
+            }
+            return null;
+        }
+    }
+
+    /**
+     * A configurable socket factory in which for test purposes supplies null.
+     */
+    static class CSF implements Serializable, RMIClientSocketFactory {
+        private static final long serialVersionUID = 1;
+
+        @Override
+        public boolean equals(Object object) {
+            return object instanceof CSF;
+        }
+
+        @Override
+        public int hashCode() {
+            return 424242;
+        }
+
+        @Override
+        public Socket createSocket(String host, int port)
+            throws IOException {
+
+            final RMIClientSocketFactory defaultFactory =
+                RMISocketFactory.getDefaultSocketFactory();
+            return defaultFactory.createSocket(host, port);
+        }
+
+        /**
+         * Use writeReplace to use a different client socket factory. In the
+         * problematic case, the replacement is null.
+         */
+        private Object writeReplace() {
+            return null;
+        }
+
+        /**
+         * Instances of this class should never be deserialized because they
+         * are always replaced during serialization.
+         */
+        @SuppressWarnings("unused")
+        private void readObject(ObjectInputStream in) {
+            throw new AssertionError();
+        }
+    }
+}