feat(premium): cryptographic Mojang session verification for premium bypass

Author: Xephi

README.md

@@ -58,6 +58,7 @@ You can also create your own translation file and, if you want, you can share it
   <li>Graphical login/register dialogs, with optional Paper/Folia pre-join dialogs</li>
   <li>Restricted users (associate a username with an IP)</li>
   <li>Protect player's inventory until correct authentication (requires PacketEvents)</li>
+  <li><strong>Premium bypass: Mojang-account holders skip password auth via cryptographic session verification (requires PacketEvents)</strong></li>
   <li>Saves the quit location of the player</li>
   <li>Automatic database backup</li>
   <li>Available languages: <a href="https://github.com/AuthMe/AuthMeReloaded/blob/master/docs/translations.md">translations</a></li>
@@ -75,6 +76,18 @@ AuthMe can display graphical login/register dialogs instead of chat-based prompt
 - `settings.registration.usePreJoinDialogUi` enables the **pre-join** dialog flow on **Paper/Folia**.
 - Both options are independent: you can enable either one, both, or neither.
 - Pre-join dialogs currently require modern dialog-capable server versions such as **Paper/Folia 1.21.11+**.
+- Verified premium players skip the pre-join dialog entirely when premium bypass is enabled.
+
+#### Premium bypass
+AuthMe can let players with a legitimate Mojang account skip password authentication entirely.
+Identity is verified via a cryptographic handshake with Mojang's session server during the
+Minecraft login phase — no password prompt is ever shown.
+
+- **Requires the [PacketEvents](https://github.com/retrooper/packetevents) plugin** (installed separately). Without it, the feature is disabled at startup (fail-closed).
+- Enable with `settings.premium.enabled: true` in `config.yml`.
+- Players opt in with `/premium` and out with `/freemium` (must be logged in). Admins can enrol or remove players with `/authme premium <player>` / `/authme freemium <player>`.
+- Designed for **direct-connection offline-mode servers**. Behind an online-mode proxy (Velocity, BungeeCord in online mode), the backend cannot intercept the client's raw encryption response — disable premium bypass on the backend in that case and rely on the proxy's own Mojang auth.
+- Full documentation: [docs/premium.md](docs/premium.md)
 
 #### Commands
 [Command list and usage](https://github.com/AuthMe/AuthMeReloaded/blob/master/docs/commands.md)
@@ -145,7 +158,7 @@ AuthMe can display graphical login/register dialogs instead of chat-based prompt
 >  - `AuthMe-*-Spigot-1.21.jar` (Spigot 1.20.x – 1.21.x)
 >  - `AuthMe-*-Paper.jar` (Paper 1.21+)
 >  - `AuthMe-*-Folia.jar` (Folia 1.21+)
->- PacketEvents (optional, required by some features)
+>- [PacketEvents](https://github.com/retrooper/packetevents) 2.x (optional plugin; required for inventory protection, tab-complete blocking, and premium bypass)
 
 ## Credits
 

authme-bungee/src/main/java/fr/xephi/authme/bungee/AuthMeBungeePlugin.java

@@ -10,6 +10,7 @@ public void onEnable() {
         configManager = new BungeeConfigManager(getDataFolder().toPath());
         BungeeAuthenticationStore authenticationStore = new BungeeAuthenticationStore();
         proxyBridge = new BungeeProxyBridge(getProxy(), getLogger(), configManager.getConfiguration(), authenticationStore);
+
         getProxy().getPluginManager().registerListener(this, proxyBridge);
         getProxy().getPluginManager().registerCommand(this, new BungeeReloadCommand(configManager, proxyBridge));
         proxyBridge.logConfigurationDetails();

authme-bungee/src/main/java/fr/xephi/authme/bungee/BungeeProxyBridge.java

@@ -10,8 +10,11 @@
 import net.md_5.bungee.api.connection.ProxiedPlayer;
 import net.md_5.bungee.api.connection.Server;
 import net.md_5.bungee.api.event.ChatEvent;
+import net.md_5.bungee.api.event.LoginEvent;
 import net.md_5.bungee.api.event.PlayerDisconnectEvent;
 import net.md_5.bungee.api.event.PluginMessageEvent;
+import net.md_5.bungee.api.event.PostLoginEvent;
+import net.md_5.bungee.api.event.PreLoginEvent;
 import net.md_5.bungee.api.event.ServerConnectEvent;
 import net.md_5.bungee.api.event.ServerSwitchEvent;
 import net.md_5.bungee.api.plugin.Listener;
@@ -37,6 +40,9 @@ public final class BungeeProxyBridge implements Listener {
     private static final String PERFORM_LOGIN_MESSAGE = "perform.login";
     private static final String PERFORM_LOGIN_ACK_MESSAGE = "perform.login.ack";
     private static final String PROXY_STARTED_MESSAGE = "proxy.started";
+    private static final String PREMIUM_SET_MESSAGE = "premium.set";
+    private static final String PREMIUM_UNSET_MESSAGE = "premium.unset";
+    private static final String PREMIUM_LIST_MESSAGE = "premium.list";
     private static final String PROXY_IDENTITY = "bungee";
     private static final int MAX_RETRIES = 3;
 
@@ -46,6 +52,9 @@ public final class BungeeProxyBridge implements Listener {
     private final BungeeAuthenticationStore authenticationStore;
     private final Map<String, AtomicInteger> pendingAutoLogins = new ConcurrentHashMap<>();
     private final Set<String> notifiedAuthServers = ConcurrentHashMap.newKeySet();
+    private final Set<String> premiumUsernames = ConcurrentHashMap.newKeySet();
+    // Players whose Mojang UUID was confirmed by the proxy during the login phase (LoginSuccess with UUID v4)
+    private final Set<String> proxyVerifiedPremium = ConcurrentHashMap.newKeySet();
     private final ScheduledExecutorService retryScheduler = Executors.newSingleThreadScheduledExecutor(r -> {
         Thread t = new Thread(r, "authme-bungee-retry");
         t.setDaemon(true);
@@ -60,6 +69,11 @@ public final class BungeeProxyBridge implements Listener {
         this.authenticationStore = authenticationStore;
     }
 
+    private void markProxyVerifiedPremium(String normalizedName) {
+        proxyVerifiedPremium.add(normalizedName);
+        logger.info("Proxy-verified premium: '" + normalizedName + "' authenticated online-mode with Mojang");
+    }
+
     void reload(BungeeProxyConfiguration configuration) {
         this.configuration = configuration;
         logger.info("Configuration reloaded");
@@ -145,6 +159,7 @@ public void onPluginMessage(PluginMessageEvent event) {
                     + server.getInfo().getName() + "'");
                 authenticationStore.markAuthenticated(parsedMessage.playerName());
                 sendAutoLoginIfAlreadySwitched(parsedMessage.playerName(), server.getInfo());
+                redirectToLoginServer(parsedMessage.playerName());
             } else if (pendingAutoLogins.containsKey(parsedMessage.playerName())) {
                 // Implicit ACK: login from non-auth server confirms perform.login was processed
                 logger.info("Auto-login confirmed for " + parsedMessage.playerName()
@@ -158,6 +173,22 @@ public void onPluginMessage(PluginMessageEvent event) {
             logger.info("Auto-login ACK received for " + parsedMessage.playerName()
                 + " from server '" + server.getInfo().getName() + "'");
             cancelPendingLogin(parsedMessage.playerName());
+        } else if (PREMIUM_SET_MESSAGE.equals(parsedMessage.typeId())) {
+            premiumUsernames.add(parsedMessage.playerName());
+            logger.fine("Premium enabled for '" + parsedMessage.playerName() + "' (proxy cache updated)");
+        } else if (PREMIUM_UNSET_MESSAGE.equals(parsedMessage.typeId())) {
+            premiumUsernames.remove(parsedMessage.playerName());
+            logger.fine("Premium disabled for '" + parsedMessage.playerName() + "' (proxy cache updated)");
+        } else if (PREMIUM_LIST_MESSAGE.equals(parsedMessage.typeId())) {
+            premiumUsernames.clear();
+            if (!parsedMessage.playerName().isEmpty()) {
+                for (String name : parsedMessage.playerName().split(",")) {
+                    if (!name.isEmpty()) {
+                        premiumUsernames.add(name.trim());
+                    }
+                }
+            }
+            logger.info("Premium list received from backend: " + premiumUsernames.size() + " premium player(s)");
         }
     }
 
@@ -173,7 +204,7 @@ public void onServerSwitch(ServerSwitchEvent event) {
             return;
         }
 
-        if (currentServer == null || !authenticationStore.isAuthenticated(player)) {
+        if (currentServer == null) {
             return;
         }
 
@@ -184,6 +215,16 @@ public void onServerSwitch(ServerSwitchEvent event) {
         }
 
         String normalizedName = normalizeName(player.getName());
+
+        boolean isPremiumJoin = connectingToAuthServer && proxyVerifiedPremium.contains(normalizedName);
+        if (!authenticationStore.isAuthenticated(player) && !isPremiumJoin) {
+            return;
+        }
+        if (isPremiumJoin) {
+            logger.fine("Proxy-verified premium player " + normalizedName
+                + " joining auth server — sending perform.login immediately");
+        }
+
         String serverName = currentServer.getInfo().getName();
         logger.info("Sending auto-login request to server '" + serverName + "' for player " + normalizedName);
         currentServer.getInfo().sendData(AUTHME_CHANNEL, createPerformLoginMessage(normalizedName), false);
@@ -254,6 +295,52 @@ public void onPlayerDisconnect(PlayerDisconnectEvent event) {
         }
         cancelPendingLogin(normalizedName);
         authenticationStore.clear(event.getPlayer());
+        proxyVerifiedPremium.remove(normalizedName);
+    }
+
+    @EventHandler
+    public void onPreLogin(PreLoginEvent event) {
+        String normalizedName = normalizeName(event.getConnection().getName());
+        if (premiumUsernames.contains(normalizedName)) {
+            event.getConnection().setOnlineMode(true);
+            logger.fine("Forcing online-mode for premium player '" + normalizedName + "'");
+        }
+    }
+
+    /**
+     * Fires after the proxy has finished the Mojang authentication phase for a connecting player.
+     * If the connection ended up in online mode (real Mojang account verified at the proxy), the
+     * player is recorded as proxy-verified premium so the auto-login bypass on the auth server
+     * will fire on {@link ServerSwitchEvent}.
+     */
+    @EventHandler
+    public void onLogin(LoginEvent event) {
+        if (event.isCancelled()) {
+            return;
+        }
+        if (!event.getConnection().isOnlineMode()) {
+            return;
+        }
+        String normalizedName = normalizeName(event.getConnection().getName());
+        markProxyVerifiedPremium(normalizedName);
+    }
+
+    /**
+     * Fallback: if for any reason the {@link LoginEvent} hook did not flag the player (e.g. the
+     * proxy is in global online mode and {@code isOnlineMode()} on PendingConnection is reported
+     * after {@code LoginEvent}), {@link PostLoginEvent} still gives us the verified UUID from the
+     * proxy. A version-4 UUID means Mojang verified the identity.
+     */
+    @EventHandler
+    public void onPostLogin(PostLoginEvent event) {
+        ProxiedPlayer player = event.getPlayer();
+        if (player.getUniqueId() != null && player.getUniqueId().version() == 4) {
+            String normalizedName = normalizeName(player.getName());
+            if (proxyVerifiedPremium.add(normalizedName)) {
+                logger.info("Proxy-verified premium (PostLogin fallback): '" + normalizedName
+                    + "' has a Mojang UUID");
+            }
+        }
     }
 
     void shutdown() {
@@ -350,16 +437,42 @@ private ParsedPluginMessage parsePluginMessage(byte[] data) {
         try {
             String typeId = input.readUTF();
             if (!LOGIN_MESSAGE.equals(typeId) && !LOGOUT_MESSAGE.equals(typeId)
-                    && !PERFORM_LOGIN_ACK_MESSAGE.equals(typeId)) {
+                    && !PERFORM_LOGIN_ACK_MESSAGE.equals(typeId)
+                    && !PREMIUM_SET_MESSAGE.equals(typeId)
+                    && !PREMIUM_UNSET_MESSAGE.equals(typeId)
+                    && !PREMIUM_LIST_MESSAGE.equals(typeId)) {
                 return ParsedPluginMessage.ignored();
             }
-            return new ParsedPluginMessage(typeId, normalizeName(input.readUTF()));
+            // premium.list carries a CSV in the second field, not a player name; read as-is
+            String argument = input.readUTF();
+            return new ParsedPluginMessage(typeId,
+                PREMIUM_LIST_MESSAGE.equals(typeId) ? argument : normalizeName(argument));
         } catch (IllegalStateException e) {
             logger.warning("Received malformed AuthMe plugin message on the authme:main channel");
             return ParsedPluginMessage.ignored();
         }
     }
 
+    private void redirectToLoginServer(String normalizedPlayerName) {
+        if (configuration.loginServer().isEmpty()) {
+            return;
+        }
+        ProxiedPlayer player = proxyServer.getPlayer(normalizedPlayerName);
+        if (player == null) {
+            logger.fine("Cannot redirect " + normalizedPlayerName + " to loginServer: player no longer on proxy");
+            return;
+        }
+        ServerInfo targetServer = proxyServer.getServerInfo(configuration.loginServer());
+        if (targetServer == null) {
+            logger.warning("loginServer '" + configuration.loginServer()
+                + "' is not registered on the proxy; cannot redirect " + normalizedPlayerName);
+            return;
+        }
+        logger.info("Redirecting " + normalizedPlayerName + " to login server '"
+            + configuration.loginServer() + "' after authentication");
+        player.connect(targetServer);
+    }
+
     private void redirectLoggedOutPlayer(String normalizedPlayerName) {
         if (!configuration.sendOnLogoutEnabled()) {
             return;

authme-bungee/src/main/java/fr/xephi/authme/bungee/BungeeProxyConfiguration.java

@@ -21,14 +21,15 @@ final class BungeeProxyConfiguration {
     private final boolean autoLoginEnabled;
     private final boolean sendOnLogoutEnabled;
     private final String sendOnLogoutTarget;
+    private final String loginServer;
     private final String sharedSecret;
 
     BungeeProxyConfiguration(Set<String> authServers, boolean allServersAreAuthServers,
                              boolean commandsRequireAuth, Set<String> commandWhitelist,
                              boolean chatRequiresAuth, boolean serverSwitchRequiresAuth,
                              String serverSwitchKickMessage, boolean autoLoginEnabled,
                              boolean sendOnLogoutEnabled, String sendOnLogoutTarget,
-                             String sharedSecret) {
+                             String loginServer, String sharedSecret) {
         this.authServers = authServers;
         this.allServersAreAuthServers = allServersAreAuthServers;
         this.commandsRequireAuth = commandsRequireAuth;
@@ -39,6 +40,7 @@ final class BungeeProxyConfiguration {
         this.autoLoginEnabled = autoLoginEnabled;
         this.sendOnLogoutEnabled = sendOnLogoutEnabled;
         this.sendOnLogoutTarget = normalizeServerName(sendOnLogoutTarget);
+        this.loginServer = normalizeServerName(loginServer);
         this.sharedSecret = sharedSecret;
     }
 
@@ -54,6 +56,7 @@ static BungeeProxyConfiguration from(SettingsManager settingsManager) {
             settingsManager.getProperty(BungeeConfigProperties.AUTOLOGIN),
             settingsManager.getProperty(BungeeConfigProperties.ENABLE_SEND_ON_LOGOUT),
             settingsManager.getProperty(BungeeConfigProperties.SEND_ON_LOGOUT_TARGET),
+            settingsManager.getProperty(BungeeConfigProperties.LOGIN_SERVER),
             settingsManager.getProperty(BungeeConfigProperties.PROXY_SHARED_SECRET));
     }
 
@@ -93,6 +96,10 @@ String sendOnLogoutTarget() {
         return sendOnLogoutTarget;
     }
 
+    String loginServer() {
+        return loginServer;
+    }
+
     String sharedSecret() {
         return sharedSecret;
     }

authme-bungee/src/main/java/fr/xephi/authme/bungee/config/BungeeConfigProperties.java

@@ -52,6 +52,13 @@ public final class BungeeConfigProperties implements SettingsHolder {
     public static final Property<String> SEND_ON_LOGOUT_TARGET =
         newProperty("unloggedUserServer", "");
 
+    @Comment({
+        "Server to redirect players to after successful authentication on an auth server.",
+        "Leave empty to disable proxy-side login redirect (backend handles it via BUNGEECORD_SERVER)."
+    })
+    public static final Property<String> LOGIN_SERVER =
+        newProperty("loginServer", "");
+
     @Comment({
         "Shared secret used to sign perform.login messages sent to backend servers.",
         "Generated automatically on first start — copy this value to the Hooks.proxySharedSecret",

authme-bungee/src/test/java/fr/xephi/authme/bungee/BungeeProxyBridgeTest.java

@@ -155,7 +155,7 @@ void shouldRedirectPlayerOnLogoutWhenConfigured() {
         BungeeProxyBridge bridge = new BungeeProxyBridge(
             proxyServer, logger, new BungeeProxyConfiguration(
                 Set.of("lobby"), false, true, Set.of("/login"), true, true,
-                "Authentication required.", true, true, "limbo", ""),
+                "Authentication required.", true, true, "limbo", "", ""),
             new BungeeAuthenticationStore());
         bridge.onPluginMessage(pluginMessageEvent);
 
@@ -441,7 +441,7 @@ void shouldSendAutoLoginImmediatelyWhenPlayerAlreadySwitchedBeforeLoginMessage()
     private static BungeeProxyConfiguration createConfiguration() {
         return new BungeeProxyConfiguration(
             Set.of("lobby"), false, true, Set.of("/login", "/register", "/l", "/reg", "/email", "/captcha", "/2fa", "/totp", "/log"),
-            true, true, "Authentication required.", true, false, "", "test-secret");
+            true, true, "Authentication required.", true, false, "", "", "test-secret");
     }
 
     private static byte[] createAuthMePayload(String typeId, String playerName) {

authme-bungee/src/test/java/fr/xephi/authme/bungee/BungeeReloadCommandTest.java

@@ -28,7 +28,7 @@ class BungeeReloadCommandTest {
     void shouldReloadConfigAndProxyBridge() {
         BungeeProxyConfiguration configuration = new BungeeProxyConfiguration(
             Set.of("lobby"), false, true, Set.of("/login"), true, true,
-            "Authentication required.", true, false, "", "");
+            "Authentication required.", true, false, "", "", "");
         given(configManager.reload()).willReturn(configuration);
 
         BungeeReloadCommand command = new BungeeReloadCommand(configManager, proxyBridge);