1. 背景
此题比较简单，网上也有各种各样的wp。wp可以分为两类，一类分析算法，另一类是爆破。此题采用的是异或算法，所以分析算法求flag是最快的。而爆破的话，可以重写成等价的代码，如C++、python，或者直接采用主动调用的方式进行爆破，如Frida的主动调用。采用主动调用的好处是不用重写，而我采用Frida RPC进行主动调用的目的是想利用python丰富的库，方便爆破，缺点相对于直接用JS代码进行爆破是太慢了，慢了200多倍。
- 用到的工具：
- pixel 3 android 9.0（不能用模拟器，因为被hook的libmyjni.so只有arm架构的）
- jadx
- IDA
- Frida12.8
- Pycharm
- VSCode
2. 分析过程
输入注册码，显示如下：
在代码中搜索"您的注册码已保存"，相关代码如下：
public class RegActivity extends Activity {
private Button btn_reg;
private EditText edit_sn;
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_reg);
this.btn_reg = (Button) findViewById(R.id.button1);
this.edit_sn = (EditText) findViewById(R.id.editText1);
this.btn_reg.setOnClickListener(new View.OnClickListener() {
/* class com.gdufs.xman.RegActivity.AnonymousClass1 */
public void onClick(View v) {
String sn = RegActivity.this.edit_sn.getText().toString().trim();
if (sn == null || sn.length() == 0) {
Toast.makeText(RegActivity.this, "您的输入为空", 0).show();
return;
}
((MyApp) RegActivity.this.getApplication()).saveSN(sn);
new AlertDialog.Builder(RegActivity.this).setTitle("回复").setMessage("您的注册码已保存").setPositiveButton("好吧", new DialogInterface.OnClickListener() {
/* class com.gdufs.xman.RegActivity.AnonymousClass1.AnonymousClass1 */
public void onClick(DialogInterface dialog, int which) {
Process.killProcess(Process.myPid());
}
}).show();
}
});
}
}
然后跳转到saveSN函数所在的类MyApp，代码如下：
public class MyApp extends Application {
public static int m = 0;
public native void initSN();
public native void saveSN(String str);
public native void work();
static {
System.loadLibrary("myjni");
}
public void onCreate() {
initSN();
Log.d("com.gdufs.xman m=", String.valueOf(m));
super.onCreate();
}
}
接着分析libmyjni.so文件，在JNI_OnLoad函数中注册了initSN、saveSN和work函数，代码如下：
2.1 分析initSN函数
首先分析initSN函数，其中setValue函数的作用是设置com/gdufs/xman/MyApp类的静态字段m的值，
initSN函数只做了一件事，即如果/sdcard/reg.dat文件的内容是"EoPAoY62@ElRD"，com/gdufs/xman/MyApp类的静态字段m的值则设置为1，否则设置为0
通过对com/gdufs/xman/MyApp类的静态字段m交叉引用发现，如果m的值为1，则显示已注册，所以我们怀疑输入的注册码通过一系列的计算后得到的值会保存到/sdcard/reg.dat文件中，如果得到的值为"EoPAoY62@ElRD"，则输入的注册码即为flag
2.2 分析 saveSN函数
saveSN也只干了一件事，把输入的注册码经过异或运算之后存到了/sdcard/reg.dat文件中
我们可以试试，直接把/sdcard/reg.dat文件的内容替换成"EoPAoY62@ElRD"，会是什么效果，如下：
则我们的flag格式为xman{注册码}
3. 编写脚本
通过上面的分析过程可知，我们可以通过Frida主动调用的方式爆破出flag，首先给出直接用JS代码进行爆破的脚本，再给出RPC爆破的脚本。
3.1 JS代码爆破
var fputs_str = null;
function Hook() {
Java.perform(function () {
const imports = Module.enumerateImportsSync("libmyjni.so");
const imports_len = imports.length;
var fputs_addr = null;
for (var i = 0; i < imports_len; i++) {
if (imports[i].name == "fputs") {
fputs_addr = imports[i].address;
break;
}
}
if (fputs_addr != null) {
Interceptor.attach(fputs_addr, {
onEnter: function (args) {
fputs_str = args[0].readCString();
},
onLeave: function (retval) {
}
})
}
})
}
function Invoke(try_str) {
Java.perform(function () {
Java.choose("com.gdufs.xman.MyApp", {
onMatch: function (instance) {
instance.saveSN(try_str);
},
onComplete: function () {
}
})
})
}
function Main() {
Hook();
Java.perform(function () {
const three_character_array = new Array("EoP", "AoY", "62@", "ElR");
const last_character = "D";
const my_dict = "abcdefghijklmnopqrstuvwxyz1234567890ABCDEFGHIJKLMNOPQRSTUVWXYZ!@#$%^&*()?_"
const myapp_class_obj = Java.use("com.gdufs.xman.MyApp");
const three_character_array_len = three_character_array.length;
const my_dict_len = my_dict.length
const myapp = Java.use("com.gdufs.xman.MyApp").$new();
var flag = "";
for (var i = 0; i < three_character_array_len; i++) {
var found = false;
for (var j = 0; j < my_dict_len; j++) {
if (found == true) {
break;
}
for (var k = 0; k < my_dict_len; k++) {
if (found == true) {
break;
}
for (var m = 0; m < my_dict_len; m++) {
const try_str = my_dict[j] + my_dict[k] + my_dict[m];
console.log(`try_str: ${try_str}`);
myapp.saveSN(try_str);
if (three_character_array[i] == fputs_str) {
flag += try_str;
console.log(`found: ${try_str}`);
found = true;
break;
}
}
}
}
}
for (var i = 0; i < my_dict_len; i++) {
const try_str = my_dict[i];
console.log(`try_str: ${try_str}`);
Invoke(try_str);
if (last_character == fputs_str) {
flag += try_str;
console.log(`found: ${try_str}`);
break;
}
}
console.log(`flag: xman{${flag}}`);
})
}
//不能用setImmediate(Main)，会出现Process crashed: SIGSYS SYS_SECCOMP
//需要延迟一下再爆破
setTimeout(Main, 500);
花了大概两分钟爆破出了flag
3.2 RPC爆破
JavaScript代码
var myapp = null;
function Hook() {
Java.perform(function () {
myapp = Java.use("com.gdufs.xman.MyApp").$new();
const imports = Module.enumerateImportsSync("libmyjni.so");
const imports_len = imports.length;
var fputs_addr = null;
for (var i = 0; i < imports_len; i++) {
if (imports[i].name == "fputs") {
fputs_addr = imports[i].address;
break;
}
}
if (fputs_addr != null) {
Interceptor.attach(fputs_addr, {
onEnter: function (args) {
send(args[0].readCString());
},
onLeave: function (retval) {
}
})
}
})
}
function Invoke(try_str) {
Java.perform(function () {
myapp.saveSN(try_str);
})
}
rpc.exports = {
hook: Hook,
invoke: Invoke
}
Python代码
from itertools import permutations
import sys
import time
import frida
result = ""
received = False
def MessageHandler(message, data):
if message["type"] == "send":
global result
global received
result = message["payload"]
received = True
else:
print(message)
def GeneratePossibilities(dict, count, repetitive=False):
"""
:param dict: Dictionary
:param count: Number of characters per group
:param repetitive: Whether there are duplicate characters in each group
:return: return permutations(dict, count)
"""
if repetitive and (count > 1):
src_dict = dict
for i in range(count - 1):
dict += src_dict
return permutations(dict, count)
device = frida.get_device_manager().add_remote_device("192.168.1.5:8888")
pid = device.spawn("com.gdufs.xman")
session = device.attach(pid)
with open("index.js") as file_descriptor:
script = session.create_script(file_descriptor.read(), runtime="v8")
script.on("message", MessageHandler)
script.load()
time.sleep(1)
device.resume(pid)
goal_three_character_array = ["EoP", "AoY", "62@", "ElR"]
goal_three_character_array_length = len(goal_three_character_array)
goal_last_character = "D"
my_dict = "abcdefghijklmnopqrstuvwxyz1234567890ABCDEFGHIJKLMNOPQRSTUVWXYZ!@#$%^&*()?_"
flag = ['*'] * 13
possibilities = GeneratePossibilities(my_dict, 3, True)
time.sleep(1)
time_begin = time.time()
script.exports.hook()
first_part_times = 0
try_count = 0
for p in possibilities:
character1, character2, character3 = p
try_str = character1 + character2 + character3
try_count += 1
sys.stdout.write("[{}]Try_str: {}\r".format(try_count, "".join(try_str)))
sys.stdout.flush()
received = False
script.exports.invoke(try_str)
while not received:
pass
for i in range(goal_three_character_array_length):
if result == goal_three_character_array[i]:
if flag[i * 3] == "*":
first_part_times += 1
flag[i * 3: i * 3 + 3] = character1, character2, character3
break
sys.stdout.write(" Flag: xman{{{}}} ".format("".join(flag)))
sys.stdout.flush()
if first_part_times == 4:
break
# 清屏
print("\033c")
try_count = 0
for try_str in my_dict:
sys.stdout.write("[{}]Try_str: {}\r".format(try_count, "".join(try_str)))
script.exports.invoke(try_str)
if result == goal_last_character:
flag[12] = try_str
break
sys.stdout.write(" Flag: xman{{{}}} ".format("".join(flag)))
sys.stdout.flush()
# 清屏
print("\033c")
print(" Flag: xman{{{}}}".format("".join(flag)))
time_end = time.time()
cost_time = time_end - time_begin
print(" cost time: " + str(cost_time // 60) + "min")
采用RPC爆破的方式花了429分钟才爆破出了flag，花的时间是直接使用JS代码进行爆破的214倍
4. 问题答疑
4.1 设备问题
如果出现各种无法解决的问题，尝试换成跟我一样的设备和系统——pixel 3、android 9.0
4.2 出现global reference table overflow
错误是全局引用表溢出了，具体意思是Java对象的全局引用表溢出了，你在循环里new Java对象就可能会溢出，因为frida对Java对象的引用就是用的全局引用，不知道什么时候才释放，反正循环的时候没有释放
4.3 爆破卡住了
多次调用Java.choose程序就会卡住，好像跟frida版本无关，不知道为什么，所以我直接new一个对象进行主动调用
4.4 为什么233字符没有被爆破出来？
排列使用的字典是没有重复字符的，所以3个字符中不可能同时出现两个相同的字符，下面的代码就会出现这种问题
my_dict = "abcdefghijklmnopqrstuvwxyz1234567890ABCDEFGHIJKLMNOPQRSTUVWXYZ!@#$%^&*()?_"
permutations(dict, 3)
解决办法是把字典自加多次，代码如下
def GeneratePossibilities(dict, count, repetitive=False):
"""
:param dict: Dictionary
:param count: Number of characters per group
:param repetitive: Whether there are duplicate characters in each group
:return: return permutations(dict, count)
"""
if repetitive and (count > 1):
src_dict = dict
for i in range(count - 1):
dict += src_dict
return permutations(dict, count)
my_dict = "abcdefghijklmnopqrstuvwxyz1234567890ABCDEFGHIJKLMNOPQRSTUVWXYZ!@#$%^&*()?_"
possibilities = GeneratePossibilities(my_dict, 3, True)
5. 附件
链接：https://pan.baidu.com/s/1NRPGU_j6CK7bgB363FN33w
提取码：09wb
[复制链接]
发表于 2021-4-9 21:04
