Add code flow authorisation type, with or without a client secret and with or without pkce

extensions/guacamole-auth-sso/modules/guacamole-auth-sso-openid/src/main/java/org/apache/guacamole/auth/openid/AuthenticationProviderService.java

@@ -24,11 +24,14 @@ import com.google.inject.Provider;
 import com.google.inject.Singleton;
 import java.net.URI;
 import java.util.Arrays;
+import java.util.Collection;
 import java.util.Collections;
 import java.util.Map;
 import java.util.Set;
 import javax.ws.rs.core.UriBuilder;
 import org.apache.guacamole.auth.openid.conf.ConfigurationService;
+import org.apache.guacamole.auth.openid.OpenIDAuthenticationSessionManager;
+import org.apache.guacamole.auth.openid.OpenIDRedirectResource;
 import org.apache.guacamole.auth.openid.token.TokenValidationService;
 import org.apache.guacamole.GuacamoleException;
 import org.apache.guacamole.auth.sso.NonceService;
@@ -50,9 +53,28 @@ public class AuthenticationProviderService implements SSOAuthenticationProviderS
 
     /**
      * The standard HTTP parameter which will be included within the URL by all
-     * OpenID services upon successful authentication and redirect.
+     * OpenID services upon successful implicit flow authentication.
      */
-    public static final String TOKEN_PARAMETER_NAME = "id_token";
+    public static final String IMPLICIT_TOKEN_PARAMETER_NAME = "id_token";
+
+    /**
+     * The standard HTTP parameter which will be included within the URL by all
+     * OpenID services upon successful code flow authentication. Used to recover
+     * the stored user state.
+     */
+    public static final String CODE_TOKEN_PARAMETER_NAME = "code";
+    
+    /**
+     * The name of the query parameter that identifies an active authentication
+     * session (in-progress OpenID authentication attempt).
+     */
+    public static final String AUTH_SESSION_QUERY_PARAM = "state";
+
+    /**
+     * The REST API used for a local rediect and callback for the IdP to
+     * return a code and access stored PKCE verifiers
+     */
+    public static final String CODE_REDIRECT_API = "/api/ext/openid/redirect";
 
     /**
      * Service for retrieving OpenID configuration information.
@@ -60,6 +82,12 @@ public class AuthenticationProviderService implements SSOAuthenticationProviderS
     @Inject
     private ConfigurationService confService;
 
+    /**
+     * Manager of active SAML authentication attempts.
+     */
+    @Inject
+    private OpenIDAuthenticationSessionManager sessionManager;
+
     /**
      * Service for validating and generating unique nonce values.
      */
@@ -81,15 +109,24 @@ public class AuthenticationProviderService implements SSOAuthenticationProviderS
     @Override
     public SSOAuthenticatedUser authenticateUser(Credentials credentials)
             throws GuacamoleException {
+        if (isImplicit()) {
+            return authenticateUserImplicit(credentials);
+        } else {
+            return authenticateUserCode(credentials);
+        }
+    }
+
+    private SSOAuthenticatedUser authenticateUserImplicit(Credentials credentials)
+            throws GuacamoleException {
 
         String username = null;
         Set<String> groups = null;
         Map<String,String> tokens = Collections.emptyMap();
 
         // Validate OpenID token in request, if present, and derive username
-        String token = credentials.getParameter(TOKEN_PARAMETER_NAME);
+        String token = credentials.getParameter(IMPLICIT_TOKEN_PARAMETER_NAME);
         if (token != null) {
-            JwtClaims claims = tokenService.validateToken(token);
+            JwtClaims claims = tokenService.validateTokenImplicit(token);
             if (claims != null) {
                 username = tokenService.processUsername(claims);
                 groups = tokenService.processGroups(claims);
@@ -112,27 +149,114 @@ public class AuthenticationProviderService implements SSOAuthenticationProviderS
         // OpenID authorization page via JavaScript)
         throw new GuacamoleInvalidCredentialsException("Invalid login.",
             new CredentialsInfo(Arrays.asList(new Field[] {
-                new RedirectField(TOKEN_PARAMETER_NAME, getLoginURI(),
+                new RedirectField(IMPLICIT_TOKEN_PARAMETER_NAME, getLoginURI(),
                         new TranslatableMessage("LOGIN.INFO_IDP_REDIRECT_PENDING"))
             }))
         );
 
     }
 
+    /**
+     * Return the value of the session identifier associated with the given
+     * credentials, or null if no session identifier is found in the
+     * credentials.
+     *
+     * @param credentials
+     *     The credentials from which to extract the session identifier.
+     *
+     * @return
+     *     The session identifier associated with the given credentials, or
+     *     null if no identifier is found.
+     */
+    public static String getSessionIdentifier(Credentials credentials) {
+
+        // Return the session identifier from the request params, if set, or
+        // null otherwise
+        return credentials != null ? credentials.getParameter(AUTH_SESSION_QUERY_PARAM) : null;
+    }
+
+    private SSOAuthenticatedUser authenticateUserCode(Credentials credentials)
+            throws GuacamoleException {
+
+        String username = null;
+        Set<String> groups = null;
+        Map<String,String> tokens = Collections.emptyMap();
+
+        // Recover session
+        String identifier = getSessionIdentifier(credentials);
+        OpenIDAuthenticationSession session = sessionManager.resume(identifier);
+        
+        // Validate OIDC token in request, if present, and derive username
+        if (session != null) {
+            String code = (String) session.getCode();
+            if (code != null) {
+                JwtClaims claims = tokenService.validateTokenCode(code, session);
+                if (claims != null) {
+                    username = tokenService.processUsername(claims);
+                    groups = tokenService.processGroups(claims);
+                    tokens = tokenService.processAttributes(claims);
+                }
+            }
+        }
+
+        // If the username was successfully retrieved from the token, produce
+        // authenticated user
+        if (username != null) {
+            // Create corresponding authenticated user
+            SSOAuthenticatedUser authenticatedUser = authenticatedUserProvider.get();
+            authenticatedUser.init(username, credentials, groups, tokens);
+            return authenticatedUser;
+
+        }
+
+        // Request OpenID token (will automatically redirect the user to the
+        // OpenID authorization page via JavaScript)
+        throw new GuacamoleInvalidCredentialsException("Invalid login.",
+            new CredentialsInfo(Arrays.asList(new Field[] {
+                new RedirectField(AUTH_SESSION_QUERY_PARAM, getLoginURI(),
+                        new TranslatableMessage("LOGIN.INFO_IDP_REDIRECT_PENDING"))
+            }))
+        );
+    }
+
+    private Boolean isImplicit() throws GuacamoleException {
+        String flowType = confService.getFlowType();
+        return flowType.equals("implicit");
+    }
+
     @Override
     public URI getLoginURI() throws GuacamoleException {
-        return UriBuilder.fromUri(confService.getAuthorizationEndpoint())
-                .queryParam("scope", confService.getScope())
-                .queryParam("response_type", "id_token")
-                .queryParam("client_id", confService.getClientID())
-                .queryParam("redirect_uri", confService.getRedirectURI())
-                .queryParam("nonce", nonceService.generate(confService.getMaxNonceValidity() * 60000L))
-                .build();
+        if (isImplicit()) {
+            return UriBuilder.fromUri(confService.getAuthorizationEndpoint())
+                    .queryParam("scope", confService.getScope())
+                    .queryParam("response_type", IMPLICIT_TOKEN_PARAMETER_NAME)
+                    .queryParam("client_id", confService.getClientID())
+                    .queryParam("redirect_uri", confService.getRedirectURI())
+                    .queryParam("nonce", nonceService.generate(confService.getMaxNonceValidity() * 60000L))
+                    .build();
+        } else {
+           /**
+            * For Code flow and PKCE support we need access to the users session to
+            * store the PKCE verifier and be able to pass the returned code form the
+            * IdP to authenticateUser. Guacamole doesn't expose the user session here.
+            * We get around that by creating a redirect to a local REST api that then
+            * does the redirection/callback and stores the code and verifier in the
+            * user session.
+            */
+            URI redirect = UriBuilder.fromUri(confService.getRedirectURI())
+                         .path(CODE_REDIRECT_API).build();
+            return UriBuilder.fromUri(redirect)
+                    .queryParam("scope", confService.getScope())
+                    .queryParam("response_type", CODE_TOKEN_PARAMETER_NAME)
+                    .queryParam("client_id", confService.getClientID())
+                    .queryParam("redirect_uri", redirect)
+                    .build();
+        }
     }
 
     @Override
     public void shutdown() {
-        // Nothing to clean up
+        sessionManager.shutdown();
     }
     
 }

extensions/guacamole-auth-sso/modules/guacamole-auth-sso-openid/src/main/java/org/apache/guacamole/auth/openid/OpenIDAuthenticationProvider.java

@@ -35,7 +35,7 @@ public class OpenIDAuthenticationProvider extends SSOAuthenticationProvider {
      * against an OpenID service.
      */
     public OpenIDAuthenticationProvider() {
-        super(AuthenticationProviderService.class, SSOResource.class,
+        super(AuthenticationProviderService.class, OpenIDRedirectResource.class,
                 new OpenIDAuthenticationProviderModule());
     }
 

extensions/guacamole-auth-sso/modules/guacamole-auth-sso-openid/src/main/java/org/apache/guacamole/auth/openid/OpenIDAuthenticationProviderModule.java

@@ -23,6 +23,8 @@ import com.google.inject.AbstractModule;
 import com.google.inject.Scopes;
 import org.apache.guacamole.auth.openid.conf.ConfigurationService;
 import org.apache.guacamole.auth.openid.conf.OpenIDEnvironment;
+import org.apache.guacamole.auth.openid.OpenIDAuthenticationSessionManager;
+import org.apache.guacamole.auth.openid.OpenIDRedirectResource;
 import org.apache.guacamole.auth.sso.NonceService;
 import org.apache.guacamole.auth.openid.token.TokenValidationService;
 import org.apache.guacamole.environment.Environment;
@@ -42,7 +44,7 @@ public class OpenIDAuthenticationProviderModule extends AbstractModule {
         bind(ConfigurationService.class);
         bind(NonceService.class).in(Scopes.SINGLETON);
         bind(TokenValidationService.class);
-        
+        bind(OpenIDAuthenticationSessionManager.class);
         bind(Environment.class).toInstance(environment);
     }
 

extensions/guacamole-auth-sso/modules/guacamole-auth-sso-openid/src/main/java/org/apache/guacamole/auth/openid/OpenIDAuthenticationSession.java

@@ -0,0 +1,116 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.guacamole.auth.openid;
+
+import org.apache.guacamole.net.auth.AuthenticationSession;
+
+/**
+ * Representation of an in-progress OpenID authentication attempt.
+ */
+public class OpenIDAuthenticationSession extends AuthenticationSession {
+    /**
+     * The PKCE challenge verifier.
+     */
+    private String verifier = null;
+
+    /**
+     * The redirect URI used by the identity provide.
+     */
+    private String redirect_uri = null;
+    
+    /**
+     * The code returned by the identity provider use to exchange for a token
+     */
+    private String code = null;
+
+    /**
+     * Creates a new AuthenticationSession representing an in-progress OpenID
+     * authentication attempt.
+     *
+     * @param expires
+     *     The number of milliseconds that may elapse before this session must
+     *     be considered invalid.
+     */
+    public OpenIDAuthenticationSession(long expires) {
+        super(expires);
+    }
+
+    /**
+     * Set the pkce_verifier
+     *
+     * @param verifier
+     *     The verifier to be stored
+     */
+    public void setVerifier(String verifier) {
+        this.verifier = verifier;
+    }
+
+    /**
+     * Returns the stored PKCE verifier
+     *
+     * @return
+     *     The PKCE verifier
+     */
+    public String getVerifier() {
+        return verifier;
+    }
+
+    /**
+     * Set the redirect URI sent to the identity provider
+     *
+     * @param redirect_uri
+     *     The redirect UTI to be stored
+     */
+    public void setRedirectURI(String redirect_uri) {
+        this.redirect_uri = redirect_uri;
+    }
+
+    /**
+     * Returns the stored redirect URI sent to the identity provider
+     *
+     * @return
+     *     The redirect URI
+     */
+    public String getRedirectURI() {
+        return redirect_uri;
+    }
+
+    /**
+     * Set the code returned by the identity provider to exchange for a token
+     *
+     * @param code
+     *     The code to be stored
+     */
+    public void setCode(String code) {
+        this.code = code;
+    }
+
+    /**
+     * Returns the stored code returned by the identity provider to exchange for a token
+     *
+     * @return
+     *     The stored code
+     */
+    public String getCode() {
+        return code;
+    }
+    
+}
+

extensions/guacamole-auth-sso/modules/guacamole-auth-sso-openid/src/main/java/org/apache/guacamole/auth/openid/OpenIDAuthenticationSessionManager.java

@@ -0,0 +1,83 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.guacamole.auth.openid;
+
+import com.google.inject.Singleton;
+import org.apache.guacamole.net.auth.AuthenticationSessionManager;
+
+/**
+ * Manager service that temporarily stores OpenID authentication attempts while
+ * the authentication flow is underway.
+ */
+@Singleton
+public class OpenIDAuthenticationSessionManager
+        extends AuthenticationSessionManager<OpenIDAuthenticationSession> {
+   /**
+     * Returns the stored code returned by the identity provider to exchange for a token
+     *
+     * @param identifier
+     *     The unique string returned by the call to defer(). For convenience,
+     *     this value may safely be null.
+     *
+     * @return
+     *     The code returned by the identity provider
+     */
+    public String getCode(String identifier) {
+        OpenIDAuthenticationSession session = resume(identifier);
+        if (session != null)
+            return session.getCode();
+        return null;
+    }
+
+   /**
+     * Returns the stored PKCE verifier used with the identity provider
+     *
+     * @param identifier
+     *     The unique string returned by the call to defer(). For convenience,
+     *     this value may safely be null.
+     *
+     * @return
+     *     The PKCE verifier used with the identity provider
+     */
+    public String getVerifier(String identifier) {
+        OpenIDAuthenticationSession session = resume(identifier);
+        if (session != null)
+            return session.getVerifier();
+        return null;
+    }
+    
+   /**
+     * Returns the stored redirect URI used with the identity provider
+     *
+     * @param identifier
+     *     The unique string returned by the call to defer(). For convenience,
+     *     this value may safely be null.
+     *
+     * @return
+     *     The redirect URIused with the identity provider
+     */
+    public String getRedirectURI(String identifier) {
+        OpenIDAuthenticationSession session = resume(identifier);
+        if (session != null)
+            return session.getRedirectURI();
+        return null;
+    }
+}
+

extensions/guacamole-auth-sso/modules/guacamole-auth-sso-openid/src/main/java/org/apache/guacamole/auth/openid/OpenIDRedirectResource.java

@@ -0,0 +1,173 @@
+/*
+
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.guacamole.auth.openid;
+
+import com.google.inject.Inject;
+import com.google.inject.Singleton;
+import java.net.URI;
+import java.util.Map;
+import javax.servlet.http.HttpServletRequest;
+import javax.ws.rs.GET;
+import javax.ws.rs.Path;
+import javax.ws.rs.core.Context;
+import javax.ws.rs.core.Response;
+import javax.ws.rs.core.UriBuilder;
+
+import org.apache.guacamole.GuacamoleException;
+import org.apache.guacamole.auth.openid.AuthenticationProviderService;
+import org.apache.guacamole.auth.openid.conf.ConfigurationService;
+import org.apache.guacamole.auth.openid.OpenIDAuthenticationSessionManager;
+import org.apache.guacamole.auth.openid.util.PKCEUtil;
+import org.apache.guacamole.auth.sso.SSOResource;
+import org.apache.guacamole.net.auth.IdentifierGenerator;
+
+/**
+ * Local REST endpoint used by Guacamole to initiate the OIDC login with code flow and PKCE.
+ * 
+ * This endpoint:
+ *   - receives the request from Guacamole Web UI
+ *   - generates PKCE code_verifier and code_challenge
+ *   - stores code_verifier in an temporary OpenIDAuthenicationSession
+ *   - redirects the browser to the Identity Provider (Keycloak, etc.)
+ *
+ * This endpoint is the place where PKCE MUST be handled because when getLoginURI() in 
+ * AuthenticationProvider is called  the authentication hasn't started yet
+ */
+public class OpenIDRedirectResource extends SSOResource {
+
+    /**
+     * The configuration service for this module.
+     */
+    @Inject
+    private ConfigurationService confService;
+
+    /**
+     * Manager of active OpenID authentication attempts.
+     */
+    @Inject
+    private OpenIDAuthenticationSessionManager sessionManager;
+
+    /**
+     * Local redirect endpoint invoked by Guacamole to pass to the identity provider
+     * for code flow. Used to create and store PKCE challenges and store code values
+     * returned by the identity provider.
+     *
+     * @param request
+     *     The HttpServletRequest from Guacamole or the identity provider
+     *
+     * @return
+     *     A redirect to the identity provider if the query parameter 'code' doesn't
+     *     exist or a redirect to Guacamole to continue the authentication process
+     *     with the values necessary to code flow in authenticateUser
+     *
+     *  @throws GuacamoleException
+     *     If the PKCE challenge can not be generated or the verifier recovered
+     */
+    @GET
+    @Path("/redirect")
+    public Response redirectToFromIdentityProvider(@Context HttpServletRequest request)
+            throws GuacamoleException {
+        String code = request.getParameter("code");
+
+        if (code == null) {
+            UriBuilder builder = UriBuilder.fromUri(confService.getAuthorizationEndpoint());
+
+            // Copy inbound request params
+            @SuppressWarnings("unchecked")
+            Map<String, String[]> params = (Map<String, String[]>) (Map) request.getParameterMap();
+            for (Map.Entry<String, String[]> entry : params.entrySet()) {
+                for (String value : entry.getValue()) {
+                    builder = builder.queryParam(entry.getKey(), value);
+                }
+            }
+            
+            // Create a new authentication session to represent this attempt while
+            // it is in progress, using the request ID that was just issued
+            OpenIDAuthenticationSession session = new OpenIDAuthenticationSession(
+                        confService.getAuthenticationTimeout() * 60000L);
+
+            // PKCE support
+            if (confService.isPKCERequired()) {
+
+                String codeVerifier = PKCEUtil.generateCodeVerifier();
+                String codeChallenge;
+
+                try {
+                    codeChallenge = PKCEUtil.generateCodeChallenge(codeVerifier);
+                } 
+                catch (Exception e) {
+                    throw new GuacamoleException("Unable to compute PKCE challenge", e);
+                }
+
+                // Store verifier for authenticateUser
+                session.setVerifier(codeVerifier);
+
+                builder.queryParam("code_challenge", codeChallenge)
+                       .queryParam("code_challenge_method", "S256");
+            }
+
+            // Store redirect_uri for exchange of code for token, requires exact same uri
+            String redirectURI = request.getParameter("redirect_uri");
+            session.setRedirectURI(redirectURI);
+            
+            // Generate a unique ID to use to identify stored values
+            String identifier = IdentifierGenerator.generateIdentifier();    
+            builder.queryParam(AuthenticationProviderService.AUTH_SESSION_QUERY_PARAM, identifier);
+            
+            // Save the session with the stored values
+            sessionManager.defer(session, identifier);
+
+            return Response.seeOther(builder.build()).build();
+        }
+
+        // Retrieve the stored session
+        String identifier = request.getParameter(AuthenticationProviderService.AUTH_SESSION_QUERY_PARAM);        
+        OpenIDAuthenticationSession session = sessionManager.resume(identifier);
+        
+        if (confService.isPKCERequired()) {
+            // Retrieve stored PKCE verifier
+            String verifier = session.getVerifier();
+
+            if (verifier == null)
+                throw new GuacamoleException("Missing PKCE verifier from session.");
+        }
+
+        // Retrieve stored redirect URI
+        String redirectURI = session.getRedirectURI();
+
+        if (redirectURI == null)
+            throw new GuacamoleException("Missing redirect URI from session.");        
+
+        // Store the authorization code for authenticateUser()
+        session.setCode(code);
+        
+        // Save the session with the stored values. Need to reactivate so it is
+        // available for the next resume
+        sessionManager.defer(session, identifier);   
+        sessionManager.reactivateSession(identifier);
+
+        // Redirect browser back to Guacamole UI to continue login
+        URI resume = UriBuilder.fromUri(confService.getRedirectURI())
+                .queryParam(AuthenticationProviderService.AUTH_SESSION_QUERY_PARAM, identifier)
+                .build();
+        return Response.seeOther(resume).build();
+    }  
+}

extensions/guacamole-auth-sso/modules/guacamole-auth-sso-openid/src/main/java/org/apache/guacamole/auth/openid/conf/ConfigurationService.java

@@ -26,6 +26,7 @@ import java.util.Collections;
 import java.util.List;
 import org.apache.guacamole.GuacamoleException;
 import org.apache.guacamole.environment.Environment;
+import org.apache.guacamole.properties.BooleanGuacamoleProperty;
 import org.apache.guacamole.properties.IntegerGuacamoleProperty;
 import org.apache.guacamole.properties.StringGuacamoleProperty;
 import org.apache.guacamole.properties.URIGuacamoleProperty;
@@ -36,6 +37,11 @@ import org.apache.guacamole.properties.URIGuacamoleProperty;
  */
 public class ConfigurationService {
 
+    /**
+     * The default OICD flow type
+     */
+    private static final String DEFAULT_FLOW_TYPE = "implicit";
+
     /**
      * The default claim type to use to retrieve an authenticated user's
      * username.
@@ -110,6 +116,66 @@ public class ConfigurationService {
 
     };
 
+    /**
+     * The token endpoint (URI) of the OIDC service.
+     */
+    private static final URIGuacamoleProperty OPENID_TOKEN_ENDPOINT =
+            new URIGuacamoleProperty() {
+        @Override
+        public String getName() {
+            return "openid-token-endpoint";
+        }
+    };
+
+    /**
+     * The flow type of the OpenID service.
+     */
+    private static final StringGuacamoleProperty OPENID_FLOW_TYPE =
+            new StringGuacamoleProperty() {
+
+        @Override
+        public String getName() { return "openid-flow-type"; }
+
+    };
+
+    /**
+     * OIDC client secret which should be submitted to the OIDC service when
+     * validating tokens with code flow
+     */
+    private static final StringGuacamoleProperty OPENID_CLIENT_SECRET =
+            new StringGuacamoleProperty() {
+        @Override
+        public String getName() {
+            return "openid-client-secret";
+        }
+    };
+
+    /**
+     * True if "Proof Key for Code Exchange" (PKCE) must be used.
+     */
+    private static final BooleanGuacamoleProperty OPENID_PKCE_REQUIRED =
+            new BooleanGuacamoleProperty() {
+        @Override
+        public String getName() {
+            return "openid-pkce-required";
+        }
+    };
+
+    /**
+     * The maximum amount of time to allow for an in-progress OpenID
+     * authentication attempt to be completed, in minutes. A user that takes
+     * longer than this amount of time to complete authentication with their
+     * identity provider will be redirected back to the identity provider to
+     * try again.
+     */
+    private static final IntegerGuacamoleProperty OPENID_AUTH_TIMEOUT =
+            new IntegerGuacamoleProperty() {
+        @Override
+        public String getName() { return "openid-auth-timeout"; }
+                
+    };
+
+
     /**
      * The claim type which contains the authenticated user's username within
      * any valid JWT.
@@ -312,6 +378,90 @@ public class ConfigurationService {
         return environment.getRequiredProperty(OPENID_JWKS_ENDPOINT);
     }
 
+    /**
+     * Returns the token endpoint (URI) of the OIDC service as
+     * configured with guacamole.properties.
+     *
+     * @return
+     *     The token endpoint of the OIDC service, as configured with
+     *     guacamole.properties.
+     *
+     * @throws GuacamoleException
+     *     If guacamole.properties cannot be parsed, or if the authorization
+     *     endpoint property is missing.
+     */
+    public URI getTokenEndpoint() throws GuacamoleException {
+        return environment.getRequiredProperty(OPENID_TOKEN_ENDPOINT);
+    }
+
+    /**
+     * Returns the flow type of the OpenID service as configured with guacamole.properties.
+     *
+     * @return
+     *     The flow type of the OpenID service, as configured with guacamole.properties. Can
+     *     be either 'implicit' or 'code'.
+     *
+     * @throws GuacamoleException
+     *     If guacamole.properties cannot be parsed.
+     */
+    public String getFlowType() throws GuacamoleException {
+        return environment.getProperty(OPENID_FLOW_TYPE, DEFAULT_FLOW_TYPE);
+    }
+
+    /**
+     * Returns the OIDC client secret used for toen validation, as configured
+     * with guacamole.properties. This value is typically provided by the OIDC
+     * service when OIDC credentials are generated for your application, and
+     * may be null.
+     *
+     * @return
+     *     The client secret to use when communicating with the OIDC service,
+     *     as configured with guacamole.properties.
+     *
+     * @throws GuacamoleException
+     *     If guacamole.properties cannot be parsed, or if the client ID
+     *     property is missing.
+     */
+    public String getClientSecret() throws GuacamoleException {
+       return environment.getProperty(OPENID_CLIENT_SECRET);
+    }
+
+    /**
+     * Returns  a boolean value of whether "Proof Key for Code Exchange (PKCE)"
+     * will be used, as configured with guacamole.properties. The choice of
+     * whether to use PKCE is up to you, but the OIDC service must support
+     * it. By default will be false.
+     *
+     * @return
+     *     The whether to use PKCE or not, as configured with guacamole.properties.
+     *
+     * @throws GuacamoleException
+     *     If guacamole.properties cannot be parsed.
+     */
+    public boolean isPKCERequired() throws GuacamoleException {
+        Boolean enabled = environment.getProperty(OPENID_PKCE_REQUIRED);
+        return enabled != null && enabled;
+    }
+
+    /**
+     * Returns the maximum amount of time to allow for an in-progress OpenID
+     * authentication attempt to be completed, in minutes. A user that takes
+     * longer than this amount of time to complete authentication with their
+     * identity provider will be redirected back to the identity provider to
+     * try again.
+     *
+     * @return
+     *     The maximum amount of time to allow for an in-progress SAML
+     *     authentication attempt to be completed, in minutes.
+     *
+     * @throws GuacamoleException
+     *     If the authentication timeout cannot be parsed.
+     */
+    public int getAuthenticationTimeout() throws GuacamoleException {
+        return environment.getProperty(OPENID_AUTH_TIMEOUT, 5);
+    }
+
+
     /**
      * Returns the claim type which contains the authenticated user's username
      * within any valid JWT, as configured with guacamole.properties. By

extensions/guacamole-auth-sso/modules/guacamole-auth-sso-openid/src/main/java/org/apache/guacamole/auth/openid/token/TokenValidationService.java

@@ -20,6 +20,14 @@
 package org.apache.guacamole.auth.openid.token;
 
 import com.google.inject.Inject;
+import java.io.BufferedReader;
+import java.io.OutputStream;
+import java.io.InputStreamReader;
+import java.net.HttpURLConnection;
+import java.net.URL;
+import java.net.URLEncoder;
+import java.net.URI;
+import java.nio.charset.StandardCharsets;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.HashMap;
@@ -27,10 +35,14 @@ import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
+import javax.servlet.http.HttpServletRequest;
+import javax.ws.rs.core.UriBuilder;
 import org.apache.guacamole.GuacamoleException;
 import org.apache.guacamole.auth.openid.conf.ConfigurationService;
+import org.apache.guacamole.auth.openid.OpenIDAuthenticationSession;
 import org.apache.guacamole.auth.sso.NonceService;
 import org.apache.guacamole.token.TokenName;
+import org.jose4j.json.JsonUtil;
 import org.jose4j.jwk.HttpsJwks;
 import org.jose4j.jwt.JwtClaims;
 import org.jose4j.jwt.MalformedClaimException;
@@ -70,8 +82,8 @@ public class TokenValidationService {
     private NonceService nonceService;
 
     /**
-     * Validates the given ID token, returning the JwtClaims contained therein.
-     * If the ID token is invalid, null is returned.
+     * Validates the given ID token, using implicit flow, returning the JwtClaims
+     * contained therein. If the ID token is invalid, null is returned.
      *
      * @param token
      *     The ID token to validate.
@@ -83,7 +95,7 @@ public class TokenValidationService {
      * @throws GuacamoleException
      *     If guacamole.properties could not be parsed.
      */
-    public JwtClaims validateToken(String token) throws GuacamoleException {
+    public JwtClaims validateTokenImplicit(String token) throws GuacamoleException {
         // Validating the token requires a JWKS key resolver
         HttpsJwks jwks = new HttpsJwks(confService.getJWKSEndpoint().toString());
         HttpsJwksVerificationKeyResolver resolver = new HttpsJwksVerificationKeyResolver(jwks);
@@ -133,6 +145,156 @@ public class TokenValidationService {
         return null;
     }
 
+    /**
+     * Validates the given ID token, using code flow, returning the JwtClaims
+     * contained therein. If the ID token is invalid, null is returned.
+     *
+     * @param code
+     *     The code to validate and receive the id_token.
+     *
+     * @param session
+     *     A OpenIDAuthenicationSession storing the in progress authentication parameters
+     *
+     * @return
+     *     The JWT claims contained within the given ID token if it passes tests,
+     *     or null if the token is not valid.
+     *
+     * @throws GuacamoleException
+     *     If guacamole.properties could not be parsed.
+     */
+    public JwtClaims validateTokenCode(String code, OpenIDAuthenticationSession session) throws GuacamoleException {
+        // Validating the token requires a JWKS key resolver
+        HttpsJwks jwks = new HttpsJwks(confService.getJWKSEndpoint().toString());
+        HttpsJwksVerificationKeyResolver resolver = new HttpsJwksVerificationKeyResolver(jwks);
+
+        /* Exchange code → token */
+        String token = exchangeCode(code, session);
+
+        // Create JWT consumer for validating received token
+        JwtConsumer jwtConsumer = new JwtConsumerBuilder()
+                .setRequireExpirationTime()
+                .setMaxFutureValidityInMinutes(confService.getMaxTokenValidity())
+                .setAllowedClockSkewInSeconds(confService.getAllowedClockSkew())
+                .setRequireSubject()
+                .setExpectedIssuer(confService.getIssuer())
+                .setExpectedAudience(confService.getClientID())
+                .setVerificationKeyResolver(resolver)
+                .build();
+
+        try {
+            // Validate JWT
+            return jwtConsumer.processToClaims(token);
+        }
+        // Log any failures to validate/parse the JWT
+        catch (InvalidJwtException e) {
+            logger.info("Rejected invalid OpenID token: {}", e.getMessage(), e);
+        }
+
+        return null;
+    }
+    
+    /**
+     * URLEncodes a key/value pair
+     *
+     * @param key
+     *     The key to encode
+     *
+     * @param value
+     *     The value to encode
+     *
+     * @return
+     *     The urlencoded kay/value pair
+     */
+     private String urlencode(String key, String value) {
+         StringBuilder builder = new StringBuilder();
+         return builder.append(URLEncoder.encode(key, StandardCharsets.UTF_8))
+                       .append("=")
+                       .append(URLEncoder.encode(value, StandardCharsets.UTF_8))
+                       .toString();
+     }
+
+    /**
+     * Exchanges the authorization code for tokens.
+     *
+     * @param code
+     *     The authorization code received from the IdP.
+     * @param codeVerifier
+     *     The PKCE verifier (or null if PKCE is disabled).
+     *
+     * @return
+     *     The token string returned.
+     *
+     * @throws GuacamoleException
+     *     If a valid token is not returned.
+     */
+    private String exchangeCode(String code, OpenIDAuthenticationSession session) throws GuacamoleException {
+
+        try {
+            StringBuilder bodyBuilder = new StringBuilder();
+            bodyBuilder.append(urlencode("grant_type", "authorization_code")).append("&");
+            bodyBuilder.append(urlencode("code", code)).append("&");
+            bodyBuilder.append(urlencode("redirect_uri", session.getRedirectURI())).append("&");
+            bodyBuilder.append(urlencode("scope", confService.getScope())).append("&");
+            bodyBuilder.append(urlencode("client_id", confService.getClientID()));
+ 
+            String clientSecret = confService.getClientSecret();
+            if (clientSecret != null && !clientSecret.trim().isEmpty()) {
+                bodyBuilder.append("&").append(urlencode("client_secret", clientSecret));
+            }
+
+            if (confService.isPKCERequired()) {
+                String codeVerifier = session.getVerifier();
+                bodyBuilder.append("&").append(urlencode("code_verifier", codeVerifier));
+            }
+            
+            // Build the final URI and convert to a URL
+            URL url = confService.getTokenEndpoint().toURL();
+
+            // Open connection, using HttpURLConnection
+            HttpURLConnection conn = (HttpURLConnection) url.openConnection();
+
+            conn.setRequestMethod("POST");
+            conn.setDoOutput(true);
+            conn.setRequestProperty("Content-Type", "application/x-www-form-urlencoded; charset=UTF-8"
+            );
+
+            try (OutputStream out = conn.getOutputStream()) {
+                byte [] body = bodyBuilder.toString().getBytes(StandardCharsets.UTF_8);
+                out.write(body, 0, body.length);
+            }
+
+            // Read response
+            int status = conn.getResponseCode();
+
+            BufferedReader reader = new BufferedReader(
+                    new InputStreamReader(
+                            status >= 200 && status < 300
+                                    ? conn.getInputStream()
+                                    : conn.getErrorStream(),
+                            StandardCharsets.UTF_8
+                    )
+            );
+
+            StringBuilder responseBody = new StringBuilder();
+            String line;
+            while ((line = reader.readLine()) != null) {
+                responseBody.append(line);
+            }
+            reader.close();
+
+            Map<String,Object> json = JsonUtil.parseJson(responseBody.toString());
+
+            if (status < 200 || status >= 300) {
+                throw new GuacamoleException("Token endpoint error (" + status + "): " + json.toString());
+            }
+
+            return (String) json.get("id_token");
+
+        } catch (Exception e) {
+            throw new GuacamoleException("Token exchange failed.", e);
+        }
+    }
+
     /**
      * Parses the given JwtClaims, returning the username contained
      * therein, as defined by the username claim type given in

extensions/guacamole-auth-sso/modules/guacamole-auth-sso-openid/src/main/java/org/apache/guacamole/auth/openid/util/PKCEUtil.java

@@ -0,0 +1,64 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.guacamole.auth.openid.util;
+
+import java.security.MessageDigest;
+import java.security.SecureRandom;
+
+/**
+ * Utility class for generating PKCE parameters.
+ *
+ * Supports:
+ *   - code_verifier (random Base64URL)
+ *   - code_challenge (S256)
+ */
+public final class PKCEUtil {
+
+    private static final SecureRandom RANDOM = new SecureRandom();
+
+    private PKCEUtil() {}
+
+    /**
+     * Generates a high-entropy PKCE code_verifier.
+     */
+    public static String generateCodeVerifier() {
+        byte[] bytes = new byte[64];
+        RANDOM.nextBytes(bytes);
+        return base64Url(bytes);
+    }
+
+    /**
+     * Computes the PKCE code_challenge = BASE64URL(SHA256(code_verifier)).
+     */
+    public static String generateCodeChallenge(String verifier) throws Exception {
+        MessageDigest sha = MessageDigest.getInstance("SHA-256");
+        byte[] hash = sha.digest(verifier.getBytes("US-ASCII"));
+        return base64Url(hash);
+    }
+
+    /**
+     * Base64URL encoding without padding.
+     */
+    public static String base64Url(byte[] bytes) {
+        return java.util.Base64.getUrlEncoder()
+                .withoutPadding()
+                .encodeToString(bytes);
+    }
+}