frida-hooking

name	frida-hooking
description	Comprehensive guide and patterns for writing Frida hook scripts for Android (Java & Native).

Frida Hooking Skill

This skill provides a structured approach and reusable patterns for dynamic instrumentation using Frida, based on official documentation and real-world reverse engineering workflows.

Core Concepts

1. Attachment & Environment

• Java.perform(fn): Essential wrapper for any Java-related work. It ensures the thread is attached to the VM.
• Java.use(className): Synchronously gets a JavaScript wrapper for a class. Use this for hooking.
• Java.choose(className, callbacks): Scans the heap for alive instances of a class.

2. Java Hooking Patterns

Method Implementation

var TargetClass = Java.use("com.package.TargetClass");
TargetClass.methodName.implementation = function(arg1, arg2) {
    console.log("Entering methodName");
    var result = this.methodName(arg1, arg2); // Call original
    console.log("Exit methodName, result: " + result);
    return result;
};

Overload Handling

TargetClass.methodName.overload('java.lang.String', 'int').implementation = function(s, i) {
    return this.methodName(s, i);
};

Constructor Hooking

TargetClass.$init.implementation = function(args) {
    return this.$init(args);
};

Field Access

// Accessing a field value
var val = this.field_name.value;
// Modifying a field value
this.field_name.value = newValue;

3. Native (Interceptor) Patterns

Attaching to Exports

Interceptor.attach(Process.findModuleByName("libname.so").findExportByName("symbol_name").base, {
    onEnter: function(args) {
        console.log("Args[0]: " + args[0]);
    },
    onLeave: function(retval) {
        console.log("Return Value: " + retval);
    }
});

Memory Operations

• Reading: ptr.readUtf8String(), ptr.readByteArray(len).
• Writing: ptr.writeUtf8String(str), ptr.writeByteArray(arr).
• Hexdump: console.log(hexdump(ptr, { offset: 0, length: 64, header: true, ansi: true })).

4. Advanced Scenarios

Syscall Tracing (frida-strace, Frida 17.8+)

frida-strace is the syscall-tracing tool officially introduced in Frida 17.8. Built on Android's eBPF backend and iOS's CoreProfile ktrace, it is a powerful tool for analyzing RASP detection and anti-debugging.

# spawn the target app and trace all syscalls
frida-strace -U -f com.target.app

# attach to a running process
frida-strace -U -p <pid>

# spawn multiple processes at once
frida-strace -U -f com.target.app -f com.target.service

# trace all processes for a given user (Android-only)
frida-strace -U -u com.target.app

Typical use cases:

• RASP detection analysis: find syscalls that read process information, such as openat("/proc/self/maps") and openat("/proc/<pid>/status")
• Anti-debugging detection: find ptrace(PTRACE_TRACEME), getppid(), and the like
• File-access monitoring: path arguments are decoded automatically, so you can see exactly which files the app accesses
• The output includes a callstack plus symbolication, letting you jump straight to the calling code

Recommended workflow:

1. First scan all syscalls with frida-strace to quickly locate suspicious detection points
2. Then Interceptor-hook the corresponding userland function to confirm the detection logic
3. Modify the return value to bypass the detection

Hooking JNI RegisterNatives

Essential for discovering dynamically registered native functions in obfuscated binaries. (See template in resources/register_natives_hook.js)

Stalker (Code Tracing)

Use Stalker for deep instruction-level tracing or finding execution paths in heavily obfuscated code.

5. Spawn Mode and the --no-pause Argument

When using -f spawn mode, different Frida versions behave differently with respect to pausing the process:

Frida version	Default behavior	Pause control
≤ 15.x	Pauses after spawn; requires a manual %resume	--no-pause resumes automatically
16.x ~ 17.x	Resumes automatically after spawn	--no-pause has been removed; using it raises an unrecognized arguments error
17.x+	Resumes automatically after spawn	Use --pause to pause (no pause by default)

Important: Never use --no-pause; Frida 17+ exits with an error immediately.

# ❌ Wrong — Frida 16+/17+ errors with: unrecognized arguments: --no-pause
frida -U -f com.target.app --no-pause -l hook.js

# ✅ Correct — Frida 16+/17+ resumes automatically by default
frida -U -f com.target.app -l hook.js

# ✅ To pause (Frida 17+)
frida -U -f com.target.app --pause -l hook.js

If you need to pause the spawned process on 16.x+, use the Python API:

device = frida.get_usb_device()
pid = device.spawn(["com.target.app"])
session = device.attach(pid)
script = session.create_script(open("hook.js").read())
script.load()
# Control when to resume manually
device.resume(pid)

Best Practices

1. Robust Hooking

• Try-Catch Wrapper: Always wrap implementations in try-catch to avoid crashing the target application on script errors.
• Logging: Use console.log for debugging, but be mindful of performance in high-frequency functions.

2. Multi-Architecture Support

• Use Process.pointerSize or Process.arch to handle offsets or data types that vary between arm/arm64.

3. Data Integrity

• When modifying buffers in onEnter, be careful not to trigger memory corruption if the new data is larger than the original allocation.

Resources

• register_natives_hook.js
• generic_interceptor.js
• java_method_tracer.js