JNIHook

Hook Java methods natively from C/C++

License

This project is licensed under the GNU AGPL-3.0. No later version is allowed.

Read the file LICENSE for more information.

Example

Hooking the function static int myFunction(int mynumber, String name) from a class Dummy:

jint hkMyFunction(JNIEnv *jni, jclass clazz, jint number, jstring name)
{
	// Print parameters
	std::cout << "[*] mynumber value: " << mynumber << std::endl;
	std::cout << "[*] name object: " << name << std::endl;

	// Get unhooked class for accessing original methods
	jclass originalClass = JNIHook_GetOriginalClass("dummy/Dummy");

	// Call the original method with 'mynumber' set to '1337'
	jni->CallNonvirtualIntMethod(dummyClass, callableMethod, 1337, name);

	return 42; // Modify the return value to '42'
}

void start(JavaVM *jvm, JNIEnv *env)
{
	jnihook_t jnihook;
	jclass dummyClass = env->FindClass("dummy/Dummy");
	jmethodID myFunctionID = env->GetStaticMethodID(dummyClass, "myFunction",
							"(ILjava/lang/String;)I");

	JNIHook_Init(env, &jnihook);
	JNIHook_Attach(&jnihook, myFunctionID, hkMyFunction, NULL);
}

Building

To build this, you can either compile all the files in src into your project, or use CMake to build a static library, which can be compiled into your project.

NOTE: This requires C++17, which is available on newer versions of Visual Studio and GCC.

The steps ahead are for building a static library with CMake.

1. Setup your JAVA_HOME environment variable. It will be used for accessing jni.h, jvmti.h, and linking the jvm library. On Linux, it's usually on /usr/lib/jvm/<java release>, and on Windows at %ProgramFiles%\Java\jdk-<version>.

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2. Create a build directory inside the root directory of the repository and enter it:

NOTE: Use the x64 Native Tools Command Prompt on Windows.

mkdir build
cd build

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3. Run CMake to setup the project

Linux/*nix:

cmake ..

Windows:

cmake .. -G "NMake Makefiles" -DCMAKE_CXX_STANDARD=17

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4. Build using nmake (Windows) or make (*nix):

Linux/*nix:

make

Windows:

nmake

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After running these commands, you will have libjnihook.a or jnihook.lib in your build directory, which you can compile along with your project.

NOTE: Don't forget to include JNIHook's include dir in your project so that you can #include <jnihook.h>.

Acknowledgements

Special thanks to @Lefraudeur for helping me with information about JVM functionality