diff --git a/jetty-security/src/main/java/org/eclipse/jetty/security/SecurityHandler.java b/jetty-security/src/main/java/org/eclipse/jetty/security/SecurityHandler.java
index 6fb32155cb6d..3a8a0f9cde2c 100644
--- a/jetty-security/src/main/java/org/eclipse/jetty/security/SecurityHandler.java
+++ b/jetty-security/src/main/java/org/eclipse/jetty/security/SecurityHandler.java
@@ -572,6 +572,11 @@ else if (authentication instanceof Authentication.Deferred)
                             authenticator.secureResponse(request, response, isAuthMandatory, null);
                     }
                 }
+                else if (isAuthMandatory)
+                {
+                    response.sendError(HttpServletResponse.SC_UNAUTHORIZED, "unauthenticated");
+                    baseRequest.setHandled(true);
+                }
                 else
                 {
                     baseRequest.setAuthentication(authentication);
diff --git a/jetty-security/src/test/java/org/eclipse/jetty/security/UnauthenticatedTest.java b/jetty-security/src/test/java/org/eclipse/jetty/security/UnauthenticatedTest.java
new file mode 100644
index 000000000000..3d51575015ca
--- /dev/null
+++ b/jetty-security/src/test/java/org/eclipse/jetty/security/UnauthenticatedTest.java
@@ -0,0 +1,145 @@
+//
+// ========================================================================
+// Copyright (c) 1995-2021 Mort Bay Consulting Pty Ltd and others.
+//
+// This program and the accompanying materials are made available under the
+// terms of the Eclipse Public License v. 2.0 which is available at
+// https://www.eclipse.org/legal/epl-2.0, or the Apache License, Version 2.0
+// which is available at https://www.apache.org/licenses/LICENSE-2.0.
+//
+// SPDX-License-Identifier: EPL-2.0 OR Apache-2.0
+// ========================================================================
+//
+
+package org.eclipse.jetty.security;
+
+import java.io.IOException;
+import java.util.concurrent.atomic.AtomicReference;
+import javax.servlet.ServletException;
+import javax.servlet.ServletRequest;
+import javax.servlet.ServletResponse;
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+
+import org.eclipse.jetty.http.HttpStatus;
+import org.eclipse.jetty.security.authentication.DeferredAuthentication;
+import org.eclipse.jetty.security.authentication.LoginAuthenticator;
+import org.eclipse.jetty.server.Authentication;
+import org.eclipse.jetty.server.LocalConnector;
+import org.eclipse.jetty.server.Request;
+import org.eclipse.jetty.server.Server;
+import org.eclipse.jetty.server.handler.AbstractHandler;
+import org.eclipse.jetty.util.security.Constraint;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.containsString;
+
+public class UnauthenticatedTest
+{
+    private LocalConnector connector;
+    private TestAuthenticator authenticator;
+
+    @BeforeEach
+    public void beforeEach() throws Exception
+    {
+        Server server = new Server();
+        connector = new LocalConnector(server);
+        server.addConnector(connector);
+
+        // Authenticator that always returns UNAUTHENTICATED.
+        authenticator = new TestAuthenticator();
+
+        // Add a security handler which requires paths under /requireAuth to be authenticated.
+        ConstraintSecurityHandler securityHandler = new ConstraintSecurityHandler();
+        Constraint requireAuthentication = new Constraint();
+        requireAuthentication.setRoles(new String[]{"**"});
+        requireAuthentication.setAuthenticate(true);
+        ConstraintMapping constraintMapping = new ConstraintMapping();
+        constraintMapping.setPathSpec("/requireAuth/*");
+        constraintMapping.setConstraint(requireAuthentication);
+        securityHandler.addConstraintMapping(constraintMapping);
+
+        securityHandler.setAuthenticator(authenticator);
+        securityHandler.setHandler(new AbstractHandler()
+        {
+            @Override
+            public void handle(String target, Request baseRequest, HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException
+            {
+                baseRequest.setHandled(true);
+                response.setStatus(HttpStatus.OK_200);
+                response.getWriter().println("authentication: " + baseRequest.getAuthentication());
+            }
+        });
+
+        server.setHandler(securityHandler);
+        server.start();
+    }
+
+    @Test
+    public void testUnauthenticated() throws Exception
+    {
+        TestAuthenticator.AUTHENTICATION.set(Authentication.UNAUTHENTICATED);
+
+        // Request to URI which doesn't require authentication can get through even though auth is UNAUTHENTICATED.
+        String response = connector.getResponse("GET / HTTP/1.1\r\nHost: localhost\r\n\r\n");
+        assertThat(response, containsString("HTTP/1.1 200 OK"));
+        assertThat(response, containsString("authentication: UNAUTHENTICATED"));
+
+        // This URI requires just that the request is authenticated.
+        response = connector.getResponse("GET /requireAuth/test HTTP/1.1\r\nHost: localhost\r\n\r\n");
+        assertThat(response, containsString("HTTP/1.1 401 Unauthorized"));
+    }
+
+    @Test
+    public void testDeferredAuth() throws Exception
+    {
+        TestAuthenticator.AUTHENTICATION.set(new DeferredAuthentication(authenticator));
+
+        // Request to URI which doesn't require authentication can get through even though auth is UNAUTHENTICATED.
+        String response = connector.getResponse("GET / HTTP/1.1\r\nHost: localhost\r\n\r\n");
+        assertThat(response, containsString("HTTP/1.1 200 OK"));
+        assertThat(response, containsString("DeferredAuthentication"));
+
+        // This URI requires just that the request is authenticated. But DeferredAuthentication can bypass this.
+        response = connector.getResponse("GET /requireAuth/test HTTP/1.1\r\nHost: localhost\r\n\r\n");
+        assertThat(response, containsString("HTTP/1.1 200 OK"));
+        assertThat(response, containsString("DeferredAuthentication"));
+    }
+
+    public static class TestAuthenticator extends LoginAuthenticator
+    {
+        static AtomicReference<Authentication> AUTHENTICATION = new AtomicReference<>();
+
+        @Override
+        public void setConfiguration(AuthConfiguration configuration)
+        {
+            // Do nothing.
+        }
+
+        @Override
+        public String getAuthMethod()
+        {
+            return this.getClass().getSimpleName();
+        }
+
+        @Override
+        public void prepareRequest(ServletRequest request)
+        {
+            // Do nothing.
+        }
+
+        @Override
+        public Authentication validateRequest(ServletRequest request, ServletResponse response, boolean mandatory) throws ServerAuthException
+        {
+            return AUTHENTICATION.get();
+        }
+
+        @Override
+        public boolean secureResponse(ServletRequest request, ServletResponse response, boolean mandatory, Authentication.User validatedUser) throws ServerAuthException
+        {
+            return true;
+        }
+    }
+}