Added agent for redirecting the auth server url

Author: Lenni0451

.github/dependabot.yml

@@ -4,6 +4,10 @@ updates:
     directory: "/"
     schedule:
       interval: "daily"
+  - package-ecosystem: "gradle"
+    directory: "/Agent/"
+    schedule:
+      interval: "daily"
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule:

.gitignore

@@ -15,3 +15,4 @@ hs_err_pid*
 /build/
 /out/
 /run/
+/Agent/build/

Agent/build.gradle

@@ -0,0 +1,16 @@
+dependencies {
+    include "org.ow2.asm:asm:9.7.1"
+    include "org.ow2.asm:asm-tree:9.7.1"
+}
+
+jar {
+    manifest {
+        attributes(
+                "Multi-Release": "true",
+                "Premain-Class": "net.lenni0451.authhook.Agent",
+                "PreMain-Class": "net.lenni0451.authhook.Agent",
+                "Can-Redefine-Classes": "true",
+                "Can-Retransform-Classes": "true"
+        )
+    }
+}

Agent/src/main/java/net/lenni0451/authhook/Agent.java

@@ -0,0 +1,27 @@
+package net.lenni0451.authhook;
+
+import java.lang.instrument.Instrumentation;
+import java.util.Map;
+
+public class Agent {
+
+    public static void agentmain(final String args, final Instrumentation instrumentation) {
+        hook(instrumentation);
+    }
+
+    public static void premain(final String args, final Instrumentation instrumentation) {
+        hook(instrumentation);
+    }
+
+    private static void hook(final Instrumentation instrumentation) {
+        try {
+            Map<String, String> config = Config.load();
+            instrumentation.addTransformer(new URLRedirector(config), true);
+        } catch (Throwable t) {
+            System.err.println("An error occurred while starting AuthHook");
+            t.printStackTrace();
+            System.exit(-1);
+        }
+    }
+
+}

Agent/src/main/java/net/lenni0451/authhook/Config.java

@@ -0,0 +1,51 @@
+package net.lenni0451.authhook;
+
+import java.io.File;
+import java.io.FileWriter;
+import java.io.IOException;
+import java.util.LinkedHashMap;
+import java.util.Map;
+import java.util.Scanner;
+
+public class Config {
+
+    private static final File PATH = new File("auth_hook.properties");
+    public static final String TARGET_ADDRESS = "target_address";
+    public static final String SECRET_KEY = "secret_key";
+
+    private static Map<String, String> getDefaults() {
+        Map<String, String> def = new LinkedHashMap<>();
+        def.put(TARGET_ADDRESS, "http://localhost");
+        def.put(SECRET_KEY, "paste secret key from ViaProxy here");
+        return def;
+    }
+
+    public static Map<String, String> load() throws IOException {
+        Map<String, String> config = getDefaults();
+        if (PATH.exists()) {
+            try (Scanner scanner = new Scanner(PATH)) {
+                while (scanner.hasNextLine()) {
+                    String line = scanner.nextLine();
+                    if (line.startsWith("#")) continue;
+                    if (line.contains("=")) {
+                        String[] split = line.split("=", 2);
+                        config.put(split[0], split[1]);
+                    }
+                }
+            }
+        } else {
+            save(config);
+        }
+        return config;
+    }
+
+    public static void save(final Map<String, String> config) throws IOException {
+        try (FileWriter writer = new FileWriter(PATH)) {
+            writer.write("# AuthHook Configuration File\n\n");
+            for (Map.Entry<String, String> entry : config.entrySet()) {
+                writer.write(entry.getKey() + "=" + entry.getValue() + "\n");
+            }
+        }
+    }
+
+}

Agent/src/main/java/net/lenni0451/authhook/URLRedirector.java

@@ -0,0 +1,72 @@
+package net.lenni0451.authhook;
+
+import org.objectweb.asm.ClassReader;
+import org.objectweb.asm.ClassWriter;
+import org.objectweb.asm.tree.AbstractInsnNode;
+import org.objectweb.asm.tree.ClassNode;
+import org.objectweb.asm.tree.LdcInsnNode;
+import org.objectweb.asm.tree.MethodNode;
+
+import java.lang.instrument.ClassFileTransformer;
+import java.security.ProtectionDomain;
+import java.util.Map;
+
+public class URLRedirector implements ClassFileTransformer {
+
+    private static final String URL = "https://sessionserver.mojang.com";
+
+    private final String targetAddress;
+    private final String secretKey;
+
+    public URLRedirector(final Map<String, String> config) {
+        this.targetAddress = this.formatURL(config.get(Config.TARGET_ADDRESS));
+        this.secretKey = config.get(Config.SECRET_KEY);
+    }
+
+    private String formatURL(String url) {
+        if (!url.startsWith("http://") && !url.startsWith("https://")) {
+            throw new IllegalArgumentException("Invalid URL (missing protocol): " + url);
+        }
+        while (url.endsWith("/")) url = url.substring(0, url.length() - 1);
+        return url;
+    }
+
+    @Override
+    public byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer) {
+        try {
+            ClassNode node = this.read(classfileBuffer);
+            boolean modified = false;
+            for (MethodNode method : node.methods) {
+                for (AbstractInsnNode insn : method.instructions) {
+                    if (insn instanceof LdcInsnNode ldc && ldc.cst instanceof String str) {
+                        if (str.startsWith(URL)) {
+                            str = str.substring(URL.length());
+                            str = this.targetAddress + "/" + this.secretKey + str;
+                            ldc.cst = str;
+
+                            modified = true;
+                            System.out.println("Redirected Auth URL in class '" + node.name + "' method '" + method.name + "'");
+                        }
+                    }
+                }
+            }
+            return modified ? this.write(node) : null;
+        } catch (Throwable ignored) {
+        }
+        return null;
+    }
+
+    private ClassNode read(final byte[] bytes) {
+        ClassNode node = new ClassNode();
+        ClassReader reader = new ClassReader(bytes);
+        reader.accept(node, ClassReader.EXPAND_FRAMES);
+        return node;
+    }
+
+    private byte[] write(final ClassNode node) {
+        ClassWriter writer = new ClassWriter(ClassWriter.COMPUTE_MAXS);
+        node.accept(writer);
+        return writer.toByteArray();
+    }
+
+}

build.gradle

@@ -1,26 +1,46 @@
-plugins {
-    id "java-library"
-    id "idea"
-}
+allprojects {
+    apply plugin: "java"
 
-base {
-    java.toolchain.languageVersion = JavaLanguageVersion.of(17)
-    compileJava.options.encoding = compileTestJava.options.encoding = javadoc.options.encoding = "UTF-8"
+    base {
+        java.toolchain.languageVersion = JavaLanguageVersion.of(17)
+        compileJava.options.encoding = compileTestJava.options.encoding = javadoc.options.encoding = "UTF-8"
 
-    group = project.maven_group ?: rootProject.maven_group
-    archivesName = project.maven_name ?: rootProject.maven_name
-    version = project.maven_version ?: rootProject.maven_version
-}
+        group = project.maven_group ?: rootProject.maven_group
+        archivesName = project.maven_name ?: rootProject.maven_name
+        version = project.maven_version ?: rootProject.maven_version
+    }
+
+    repositories {
+        mavenCentral()
+    }
 
-configurations {
-    include
+    configurations {
+        include
+
+        implementation.extendsFrom include
+        api.extendsFrom include
+    }
+
+    jar {
+        dependsOn configurations.include
+        from {
+            duplicatesStrategy = DuplicatesStrategy.EXCLUDE
+            configurations.include.collect {
+                zipTree(it)
+            }
+        } {
+            exclude "META-INF/*.RSA", "META-INF/*.SF", "META-INF/*.DSA"
+        }
 
-    implementation.extendsFrom include
-    api.extendsFrom include
+        from("LICENSE") {
+            rename { "${it}_${project.name ?: rootProject.name}" }
+        }
+    }
 }
 
+apply plugin: "idea"
+
 repositories {
-    mavenCentral()
     maven {
         name = "ViaVersion"
         url = "https://repo.viaversion.com"
@@ -39,22 +59,6 @@ processResources {
     }
 }
 
-jar {
-    dependsOn configurations.include
-    from {
-        duplicatesStrategy = DuplicatesStrategy.EXCLUDE
-        configurations.include.collect {
-            zipTree(it)
-        }
-    } {
-        exclude "META-INF/*.RSA", "META-INF/*.SF", "META-INF/*.DSA"
-    }
-
-    from("LICENSE") {
-        rename { "${it}_${project.name ?: rootProject.name}" }
-    }
-}
-
 idea {
     module {
         ["run"].each {

settings.gradle

@@ -10,3 +10,5 @@ plugins {
 }
 
 rootProject.name = "ViaProxyAuthHook"
+
+include(":Agent")