Allow empty token endpoint for implicit flow (#45038)

When using the implicit flow in OpenID Connect, the
op.token_endpoint_url should not be mandatory as there is no need
to contact the token endpoint of the OP.

x-pack/plugin/security/src/main/java/org/elasticsearch/xpack/security/authc/oidc/OpenIdConnectProviderConfiguration.java

@@ -23,9 +23,10 @@ public class OpenIdConnectProviderConfiguration {
     private final String jwkSetPath;
 
     public OpenIdConnectProviderConfiguration(Issuer issuer, String jwkSetPath, URI authorizationEndpoint,
-                                              URI tokenEndpoint, @Nullable URI userinfoEndpoint, @Nullable URI endsessionEndpoint) {
+                                              @Nullable URI tokenEndpoint, @Nullable URI userinfoEndpoint,
+                                              @Nullable URI endsessionEndpoint) {
         this.authorizationEndpoint = Objects.requireNonNull(authorizationEndpoint, "Authorization Endpoint must be provided");
-        this.tokenEndpoint = Objects.requireNonNull(tokenEndpoint, "Token Endpoint must be provided");
+        this.tokenEndpoint = tokenEndpoint;
         this.userinfoEndpoint = userinfoEndpoint;
         this.endsessionEndpoint = endsessionEndpoint;
         this.issuer = Objects.requireNonNull(issuer, "OP Issuer must be provided");

x-pack/plugin/security/src/main/java/org/elasticsearch/xpack/security/authc/oidc/OpenIdConnectRealm.java

@@ -277,25 +277,31 @@ public class OpenIdConnectRealm extends Realm implements Releasable {
             // This should never happen as it's already validated in the settings
             throw new SettingsException("Invalid URI: " + OP_AUTHORIZATION_ENDPOINT.getKey(), e);
         }
+        String responseType = require(config, RP_RESPONSE_TYPE);
+        String tokenEndpointString = config.getSetting(OP_TOKEN_ENDPOINT);
+        if (responseType.equals("code") && tokenEndpointString.isEmpty()) {
+            throw new SettingsException("The configuration setting [" + OP_TOKEN_ENDPOINT.getConcreteSettingForNamespace(name()).getKey()
+                + "] is required when [" + RP_RESPONSE_TYPE.getConcreteSettingForNamespace(name()).getKey() + "] is set to \"code\"");
+        }
         URI tokenEndpoint;
         try {
-            tokenEndpoint = new URI(require(config, OP_TOKEN_ENDPOINT));
+            tokenEndpoint = tokenEndpointString.isEmpty() ? null : new URI(tokenEndpointString);
         } catch (URISyntaxException e) {
             // This should never happen as it's already validated in the settings
             throw new SettingsException("Invalid URL: " + OP_TOKEN_ENDPOINT.getKey(), e);
         }
         URI userinfoEndpoint;
         try {
-            userinfoEndpoint = (config.getSetting(OP_USERINFO_ENDPOINT, () -> null) == null) ? null :
-                new URI(config.getSetting(OP_USERINFO_ENDPOINT, () -> null));
+            userinfoEndpoint = (config.getSetting(OP_USERINFO_ENDPOINT).isEmpty()) ? null :
+                new URI(config.getSetting(OP_USERINFO_ENDPOINT));
         } catch (URISyntaxException e) {
             // This should never happen as it's already validated in the settings
             throw new SettingsException("Invalid URI: " + OP_USERINFO_ENDPOINT.getKey(), e);
         }
         URI endsessionEndpoint;
         try {
-            endsessionEndpoint = (config.getSetting(OP_ENDSESSION_ENDPOINT, () -> null) == null) ? null :
-                new URI(config.getSetting(OP_ENDSESSION_ENDPOINT, () -> null));
+            endsessionEndpoint = (config.getSetting(OP_ENDSESSION_ENDPOINT).isEmpty()) ? null :
+                new URI(config.getSetting(OP_ENDSESSION_ENDPOINT));
         } catch (URISyntaxException e) {
             // This should never happen as it's already validated in the settings
             throw new SettingsException("Invalid URI: " + OP_ENDSESSION_ENDPOINT.getKey(), e);

x-pack/plugin/security/src/test/java/org/elasticsearch/xpack/security/authc/oidc/OpenIdConnectRealmSettingsTests.java

@@ -86,7 +86,7 @@ public class OpenIdConnectRealmSettingsTests extends ESTestCase {
             Matchers.containsString(getFullSettingKey(REALM_NAME, OpenIdConnectRealmSettings.OP_AUTHORIZATION_ENDPOINT)));
     }
 
-    public void testMissingTokenEndpointThrowsError() {
+    public void testMissingTokenEndpointThrowsErrorInCodeFlow() {
         final Settings.Builder settingsBuilder = Settings.builder()
             .put(getFullSettingKey(REALM_NAME, OpenIdConnectRealmSettings.OP_AUTHORIZATION_ENDPOINT), "https://op.example.com/login")
             .put(getFullSettingKey(REALM_NAME, OpenIdConnectRealmSettings.OP_ISSUER), "https://op.example.com")
@@ -103,6 +103,22 @@ public class OpenIdConnectRealmSettingsTests extends ESTestCase {
             Matchers.containsString(getFullSettingKey(REALM_NAME, OpenIdConnectRealmSettings.OP_TOKEN_ENDPOINT)));
     }
 
+    public void testMissingTokenEndpointIsAllowedInImplicitFlow() {
+        final Settings.Builder settingsBuilder = Settings.builder()
+            .put(getFullSettingKey(REALM_NAME, OpenIdConnectRealmSettings.OP_AUTHORIZATION_ENDPOINT), "https://op.example.com/login")
+            .put(getFullSettingKey(REALM_NAME, OpenIdConnectRealmSettings.OP_ISSUER), "https://op.example.com")
+            .put(getFullSettingKey(REALM_NAME, OpenIdConnectRealmSettings.OP_JWKSET_PATH), "https://op.example.com/jwks.json")
+            .put(getFullSettingKey(REALM_NAME, OpenIdConnectRealmSettings.PRINCIPAL_CLAIM.getClaim()), "sub")
+            .put(getFullSettingKey(REALM_NAME, OpenIdConnectRealmSettings.RP_REDIRECT_URI), "https://rp.my.com")
+            .put(getFullSettingKey(REALM_NAME, OpenIdConnectRealmSettings.RP_CLIENT_ID), "rp-my")
+            .put(getFullSettingKey(REALM_NAME, OpenIdConnectRealmSettings.RP_RESPONSE_TYPE), "id_token token");
+        settingsBuilder.setSecureSettings(getSecureSettings());
+        final OpenIdConnectRealm realm = new OpenIdConnectRealm(buildConfig(settingsBuilder.build()), null, null);
+        assertNotNull(realm);
+
+    }
+
+
     public void testInvalidTokenEndpointThrowsError() {
         final Settings.Builder settingsBuilder = Settings.builder()
             .put(getFullSettingKey(REALM_NAME, OpenIdConnectRealmSettings.OP_AUTHORIZATION_ENDPOINT), "https://op.example.com/login")