破解某安卓微信群发APP(dex解密分析)

daojue

破解某安卓微信群发APP(dex解密分析)
jadx载入

寻觅xposed_init文件中定义的xposed程序的入口，发现主体只要如下三个函数，那猜测真正的hook函数被加密存储了，执行时经过dexClassloader动态加载执行

[AppleScript] 纯文本查看 复制代码

public class XposedEntry implements IXposedHookLoadPackage {
        private static final String enDexName = "appcompat_v4.dex";
        private static final String gsonDexName = "gson.dex";
    public static String pkgName = "wechat.simpleforwarder";
    private static final String soName = "libJpush.so";        

        public void copyFileFromAssets(InputStream inputStream, String str) {
          ...
        }

        String getCurProcessName(Context context) {
          ...
        }

        public void handleLoadPackage(LoadPackageParam loadPackageParam) {
          ...
        }
}

在程序的assets下发现了如下几个后缀为dex的文件，直接尝试了运用jadx去反编译，发现反编译不胜利，拖入010Editor

dex被作者停止了加密，那就得去代码中寻觅解密执行代码
直接看ui的入口并没有发现任何的解密中央，猜测既然是xposed插件，那一定会有findAndHookMethod的中央，以及beforeHook和afterHook，直接去查找，找到如下代码

[AppleScript] 纯文本查看 复制代码

protected void afterHookedMethod(MethodHookParam methodHookParam) {
    super.afterHookedMethod(methodHookParam);
    Context context = (Context) methodHookParam.thisObject;
    String curProcessName = this.ʼ.getCurProcessName(context);
    XposedBridge.log("processName = " + curProcessName);
    if (context.getPackageName().equalsIgnoreCase(curProcessName)) {
        File dir = context.getDir("forward_so", 0);
        File dir2 = context.getDir("forward_dex", 0);
        String absolutePath = new File(dir, "libJpush.so").getAbsolutePath();
        Context createPackageContext = context.createPackageContext(XposedEntry.pkgName, 2);
        this.ʼ.copyFileFromAssets(createPackageContext.getAssets().open("libJpush.so"), absolutePath);
        this.ʼ.copyFileFromAssets(createPackageContext.getAssets().open("appcompat_v4.dex"), new File(dir2, "appcompat_v4.dex").getAbsolutePath());
        this.ʼ.copyFileFromAssets(createPackageContext.getAssets().open("gson.dex"), new File(dir2, "gson.dex").getAbsolutePath());
        System.load(absolutePath);
        Class cls = (Class) JniUtil.getXClass(context, dir.getAbsolutePath(), dir2.getAbsolutePath());
        cls.getMethod(JniUtil.getXMethodName(), new Class[]{LoadPackageParam.class, Context.class}).invoke(cls.newInstance(), new Object[]{this.ʻ, context});
    }
}

程序读取assets中的文件，并加载了assets下的so文件，调用了一个名为JniUtil的getXClass函数，传入了三个参数，分别是context和两个途径，此处没有看到DexClassLoader，猜测这里的context是用作后面classloader运用的，IDA载入libJpush.so(竟然用极光推送的称号)

[AppleScript] 纯文本查看 复制代码

if ( ((int (__fastcall *)(JavaVM *, JNIEnv **, signed int))(*v2)->GetEnv)(v2, &env, 0x10004)
    || (v3 = env, (v4 = (*env)->FindClass(env, "wechat/simpleforwarder/util/JniUtil")) == 0)
    || ((int (__fastcall *)(JNIEnv *, jclass, JNINativeMethod *, signed int))(*v3)->RegisterNatives)(
         v3,
         v4,
         gMethods,
         2) < 0 )
  {
    result = 0xFFFFFFFF;
  }

在jni_Onload处看到动态动态了函数，直接双击gMethods跳过去

[AppleScript] 纯文本查看 复制代码

int __fastcall getXClass(JNIEnv *env, jclass jls, jobject context, jstring soDir, jstring dexDir)
{
  jobject v5; // ST08_4@1
  jclass v6; // r7@1
  JNIEnv *v7; // r4@1
  jstring v8; // r6@1
  unsigned __int8 *v9; // r5@3
  unsigned __int8 *v10; // r0@3
  int v11; // r5@3
  unsigned __int8 *v12; // r6@3
  unsigned __int8 *v13; // r0@3
  jstring dexPath; // [sp+0h] [bp-28h]@1
  jstring gsonDexPath; // [sp+4h] [bp-24h]@1
  jstring soDira; // [sp+Ch] [bp-1Ch]@1

  v5 = context;
  v6 = jls;
  soDira = soDir;
  v7 = env;
  dexPath = appendCharStr(env, dexDir, string27);
  v8 = appendCharStr(v7, dexDir, string28);
  gsonDexPath = appendCharStr(v7, dexDir, string29);
  if ( getSignatureHashCode(v7, v6, v5) != 0x962F5B7 )
    killSelf(v7);
  v9 = (unsigned __int8 *)((int (__fastcall *)(JNIEnv *, jstring, _DWORD))(*v7)->GetStringUTFChars)(v7, v8, 0);
  v10 = (unsigned __int8 *)((int (__fastcall *)(JNIEnv *, jstring, _DWORD))(*v7)->GetStringUTFChars)(v7, dexPath, 0);
  decryptFun(v9, v10);
  v11 = DexClassLoader(v7, dexPath, gsonDexPath, soDira);
  v12 = (unsigned __int8 *)((int (__fastcall *)(JNIEnv *, jstring, _DWORD))(*v7)->GetStringUTFChars)(v7, v8, 0);
  v13 = (unsigned __int8 *)((int (__fastcall *)(JNIEnv *, jstring, _DWORD))(*v7)->GetStringUTFChars)(v7, dexPath, 0);
  copyFun(v12, v13);
  return v11;
}

这里就来到了getXClass处，这里函数并没有做混杂，所以很好剖析，看到一个DecryptFun函数，跟进去

[AppleScript] 纯文本查看 复制代码

void __fastcall decryptFun(unsigned __int8 *path, unsigned __int8 *dePath)
{
  unsigned __int8 *v2; // r4@1
  FILE *v3; // r0@1
  FILE *v4; // r5@1
  unsigned int v5; // r6@2
  FILE *v6; // r7@2
  unsigned int i; // r6@2
  int v8; // r4@3

  v2 = dePath;
  v3 = j_j_fopen((const char *)path, "r");
  v4 = v3;
  if ( v3 )
  {
    v5 = j_j_fgetc(v3) << 0x18;
    v6 = j_j_fopen((const char *)v2, "w");
    for ( i = v5 >> 0x18; ; j_j_fputc(v8 ^ i, v6) )
    {
      v8 = (unsigned __int8)j_j_fgetc(v4);
      if ( j_j_feof(v4) )
        break;
    }
    j_j_fclose(v4);
    j_j_fclose(v6);
  }
}

这里就能够看到两个传入的途径，分别是文件所在途径和保管的解密文件的途径，而且包括下面一个DexClassLoader函数都没有做remove文件操作，所以也能够在程序执行后运用re把真实的dex拖出来

上面的DecryptFun函数，能够看到翻开了加密dex文件，并停止异或后保管在v6中，fgetc会每次读取一个字符，这里v5即是该文件的第一个字符，0x2D-->45 ，所以能够推导出该文件的每个字符会和45停止异或，

[AppleScript] 纯文本查看 复制代码

 public static void main(String[] args) throws Exception {
     FileInputStream fis = new FileInputStream(new File("appcompat_v4.dex"));
     byte[] b = new byte[fis.available()];
     fis.read(b);
     int len = b.length;
     for(int i=0;i<len;i++){
         b  = (byte) (b^45);

     }
     FileOutputStream fos = new FileOutputStream("appcompat_v43333.dex");
     fos.write(b);

 } 

异或完成后运用010翻开

去除掉第一个00字符，然后重新运用jadx翻开





联系我时，请说是在 挂海论坛 上看到的，谢谢！