Frida cheat sheet

6 minute read

Frida is particularly useful for dynamic analysis on Android/iOS/Windows applications. It allows us to set up hooks on the target functions so that we can inspect/modify the parameters and return value. We can also alter the entire logic of the hooked function. This article shows the most useful code snippets for copy&paste to save time reading the lengthy documentation page.

Frida python binding

Python binding to attach to an app:

import frida, sys
 
ss = """
Java.perform(function () {
    // declare classes that are going to be used
    const System = Java.use('java.lang.System');
    const Log = Java.use("android.util.Log");
    const Exception = Java.use("java.lang.Exception");
    
    System.exit.implementation = function() {
        // console.log(Log.getStackTraceString(Exception.$new()));
    };
});
"""
device = frida.get_device_manager().enumerate_devices()[-1]
session = device.attach("com.example.test")
script = session.create_script(ss)
script.load()
sys.stdin.read()

Python binding to spawn an app:

import frida, sys
 
ss = """
Java.perform(function () {
    // declare classes that are going to be used
    const System = Java.use('java.lang.System');
    const Log = Java.use("android.util.Log");
    const Exception = Java.use("java.lang.Exception");
    
    System.exit.implementation = function() {
        // console.log(Log.getStackTraceString(Exception.$new()));
    };
});
"""
device = frida.get_usb_device()
pid = device.spawn(["com.example.test"])
session = device.attach(pid)
script = session.create_script(ss)
script.load()
device.resume(pid)
sys.stdin.read()

Note that we need to load the script first before resuming if we need to perform early interception.

frida-trace

Attach to Chrome app on an Android phone and trace two native functions open and strcmp

$ frida-trace -U -i open -i strcmp -f com.android.chrome

Launch SnapChat app on an iPhone and trace CommonCrypto API calls

$ frida-trace -U -I "libcommonCrypto*" -f com.toyopagroup.picaboo

Trace an Obj-C method of Safari app

$ frida-trace -U -m "-[NSView drawRect:]" Safari

Trace a all Java methods of class BitmapFactory that contain native in method name

$ frida-trace -U -j '*BitmapFactory*!*native*' com.android.chrome

Frida over network

On the target device (192.168.1.3)

$ frida-server -l 0.0.0.0

On the attacking machine

frida-trace -H 192.168.1.3 -i "open*"

Enable Javascript v8

--runtime=duk|v8

Common scripts

Convert IDA address to memory address and vice versa

function memAddress(memBase, idaBase, idaAddr) {
    var offset = ptr(idaAddr).sub(idaBase);
    var result = ptr(memBase).add(offset);
    return result;
}

function idaAddress(memBase, idaBase, memAddr) {
    var offset = ptr(memAddr).sub(memBase);
    var result = ptr(idaBase).add(offset);
    return result;
}

C: Hook a C function and print out the params

Interceptor.attach(Module.findExportByName("liba.so", "HMAC"), {
    onEnter: function (args) {
        var keySize = args[2].toInt32();
        var keyDump = Memory.readByteArray(args[1], keySize);
        console.log('HMAC Key found at ' + args[1]);
        console.log('HMAC Key size = ' + keySize);
        console.log(hexdump(keyDump, { offset: 0, length: keySize, header: false, ansi: false }));  
    }
});

C: Hook a static function by resolving its address

const membase = Module.findBaseAddress('libtest.so');
const fstatat = memAddress(membase, '0x0', '0x69E238');
Interceptor.attach(fstatat, {
    onEnter: function (args) {
        console.log('[+] fstatat: ' + Memory.readUtf8String(args[1]));
        Memory.writeUtf8String(args[1], "/empty");
    }
});

C:

const dlopen = new NativeFunction(Module.findExportByName(null, 'dlopen'), 'pointer', ['pointer', 'int']);
const dlerror = new NativeFunction(Module.findExportByName(null, 'dlerror'), 'pointer', []);
const path = Memory.allocUtf8String("/data/local/tmp/libdummy.so");
var ret = dlopen(path, 2);
console.log("ret = " + ret);
var error = dlerror();
console.log("error = " + Memory.readUtf8String(error));

C: Print the backtraces of a list of functions

const membase = Module.findBaseAddress('libtest.so');
const funcs = [ '0x21B248', '0x21D0C8', '0x234730', '0x23F718', '0x259E68' ];
for (var i in funcs) {
    var funcPtr = memAddress(membase, '0x0', funcs[i]);
    var handler = (function() {
        var name = funcs[i];
        return function(args) {
            console.log(name + ': ');
            var trace = Thread.backtrace(this.context, Backtracer.ACCURATE).map(DebugSymbol.fromAddress);
            for (var j in trace)
                console.log(trace[j]);
        };
    })();
    Interceptor.attach(funcPtr, {onEnter: handler});
}

C: Print the execution traces of a list of functions with Stalker

const funcs = [ '0x870FF0', '0x871BA0' ];
const STALKED = 12345;
const STARTING_ADDRESS = "0x102FE0";
const ENDING_ADDRESS = "0x89BE04";
const base = Module.findBaseAddress('libtest.so');
var threads = [];
for (var i in funcs) {
    console.log('Hooking funcs[' + i + '] ' + funcs[i]);
    Interceptor.attach(memAddress(base, '0x0', funcs[i]), {
        onEnter: function (args) {
            var tid = Process.getCurrentThreadId();
            if (threads[tid] == STALKED)
                return;
            Stalker.follow(tid, {
                events: {
                    call: true, // CALL instructions: yes please
                    ret: false, // RET instructions: no thanks
                    exec: false // all instructions: no thanks
                },
                onCallSummary: function (summary) {
                    var log = []
                    for (i in summary) {
                        var addr = idaAddress(base, '0x0', i);
                        if (addr.compare(ptr(STARTING_ADDRESS)) >= 0 && addr.compare(ptr(ENDING_ADDRESS)) <= 0)
                            log.push(addr);
                    }
                    console.log(JSON.stringify(log));
                }
            });
            threads[tid] = STALKED;
        },
        onLeave: function (retval) {
            var tid = Process.getCurrentThreadId();
            if (threads[tid] == STALKED)
                return;
            Stalker.unfollow(tid);
            Stalker.garbageCollect();
        }
    });
}

C: Invoke a libc function

var openPtr = Module.findExportByName("libc.so", "open");
var open = new NativeFunction(openPtr, 'int', ['pointer', 'int']);
var fd = open(Memory.allocUtf8String('/tmp/test.txt'), 0);

C: Read and write std string

function readStdString(str) {
    const isTiny = (str.readU8() & 1) === 0;
    if (isTiny)
        return str.add(1).readUtf8String();
    return str.add(2 * Process.pointerSize).readPointer().readUtf8String();
}

function writeStdString(str, content) {
    const isTiny = (str.readU8() & 1) === 0;
    if (isTiny)
        str.add(1).writeUtf8String(content);
    else
        str.add(2 * Process.pointerSize).readPointer().writeUtf8String(content);
}

Android: Hook C remove() function to save a files that is going to be deleted

const File = Java.use("java.io.File");
const FileInputStream = Java.use("java.io.FileInputStream");
const FileOutputStream = Java.use("java.io.FileOutputStream");
const ActivityThread = Java.use("android.app.ActivityThread");
var name = 0;
Interceptor.attach(Module.findExportByName(null, "remove"), {
    onEnter: function (args) {
        path = Memory.readUtf8String(args[0]);
        Java.perform(function () {
            // create the input channel
            var f = File.$new(path);
            var fis = FileInputStream.$new(f);
            var inChannel = fis.getChannel();
            // create the output channet
            var application = ActivityThread.currentApplication();
            if (application == null)
                return;
            var context = application.getApplicationContext();
            var fos = context.openFileOutput('deleted_' + name, 0);
            name = name + 1;
            var outChannel = fos.getChannel();
            // transfer the file from the input channel to the output channel
            inChannel.transferTo(0, inChannel.size(), outChannel);
            fis.close();
            fos.close();
        });
    }
});

iOS: Hook an Obj-C method

const sendMessage = ObjC.classes.SecureStorage["- readFile:"];
Interceptor.attach(sendMessage.implementation, {
    onLeave: function (retval) {
        var message = ObjC.Object(retval);
        console.log("- [SecureStorage readFile:] -->\n\"" + message.toString() + "\"");
    }
});

Android: Hook constructor method of SecretKeySpec to print out the key byte array

Java.perform(function () {
    var SecretKeySpec = Java.use('javax.crypto.spec.SecretKeySpec');
    SecretKeySpec.$init.overload('[B', 'java.lang.String').implementation = function(p0, p1) {
        console.log('SecretKeySpec.$init("' + bytes2hex(p0) + '", "' + p1 + '")');
        return this.$init(p0, p1);
    };
});
function bytes2hex(array) {
    var result = '';
    console.log('len = ' + array.length);
    for(var i = 0; i < array.length; ++i)
        result += ('0' + (array[i] & 0xFF).toString(16)).slice(-2);
    return result;
}

Android: Hook the library loading

Java.perform(function() {
    const System = Java.use('java.lang.System');
    const Runtime = Java.use('java.lang.Runtime');
    const VMStack = Java.use('dalvik.system.VMStack');

    System.loadLibrary.implementation = function(library) {
        try {
            console.log('System.loadLibrary("' + library + '")');
            Runtime.getRuntime().loadLibrary0(VMStack.getCallingClassLoader(), library);
        } catch(ex) {
            console.log(ex);
        }
    };
    
    System.load.implementation = function(library) {
        try {
            console.log('System.load("' + library + '")');
            Runtime.getRuntime().nativeLoad(library, VMStack.getCallingClassLoader());
        } catch(ex) {
            console.log(ex);
        }
    };
});
var dlopen = new NativeFunction(Module.findExportByName(null, 'dlopen'), 'pointer', ['pointer', 'int']);
Interceptor.replace(dlopen, new NativeCallback(function(path, mode) {
    console.log("dlopen(" + "path=\"" + Memory.readUtf8String(path) + "\"" + ", mode=" + mode + ")");
    var name = Memory.readUtf8String(path);
    if (name !== null) {
        console.log("[*] dlopen " + name);
    }
    return dlopen(path, mode);
}, 'pointer', ['pointer', 'int']));

Android: create Java byte array

var buffer = Java.array('byte', [ 13, 37, 42 ]);
var buffer = Java.array('byte', new Array(1024).fill(0));

Android: Java bytearray dumping

function bytes2hex(array) {
    var result = '';
    for(var i = 0; i < array.length; ++i)
        result += ('0' + (array[i] & 0xFF).toString(16)).slice(-2);
    result += ' (' + array.length + ' bytes)'
    return result;
}

function jhexdump(array) {
    var ptr = Memory.alloc(array.length);
    for(var i = 0; i < array.length; ++i)
        Memory.writeS8(ptr.add(i), array[i]);
    console.log(hexdump(ptr, { offset: 0, length: array.length, header: false, ansi: false }));
}

Android: Java inspect a Java class of an Java object

const Class = Java.use("java.lang.Class");
function inspectObject(obj) {
    const obj_class = Java.cast(obj.getClass(), Class);
    const fields = obj_class.getDeclaredFields();
    const methods = obj_class.getMethods();
    console.log("Inspecting " + obj.getClass().toString());
    console.log("\tFields:");
    for (var i in fields)
        console.log("\t\t" + fields[i].toString());
    console.log("\tMethods:");
    for (var i in methods)
        console.log("\t\t" + methods[i].toString());
}

TODO: add references https://awesomeopensource.com/project/iddoeldor/frida-snippets

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