Obviously, this method displays the dialog box. In the Developer options, pick Uncrackable1 as the debugging application and activate the "Wait for Debugger" switch. Therefore, a good stating point for NFC API analysis would be to consult the Android Developer Documentation to get some ideas and start searching for critical functions such as processCommandApdu from the android.nfc.cardemulation.HostApduService class. Some static analyzers rely on the availability of the source code; others take the compiled APK as input. There are many RE tools that support retrieving Java cross references. Remember to always type ? It can decode resources to nearly original form and rebuild them after making some modifications; it makes possible to debug smali code step by step. This, however, may provide a lot of output. To quickly verify that the correct kernel is running, navigate to Settings -> About phone and check the kernel version field. QEMU emulates a guest CPU by translating the guest instructions on-the-fly into instructions the host processor can understand. You signed in with another tab or window. If you look at the class sg.vantagepoint.uncrackable1.a, you can see the encrypted string with which your input gets compared: Look at the string.equals comparison at the end of the a method and the creation of the string arrby in the try block above. Once you're done, click the same button to stop the recording. We will be using it to keep a tab on method inputs in the NRC APK and also make custom calls to different methods. In the chapter "Reverse Engineering and Tampering", we talked about JDWP, the protocol used for communication between the debugger and the Java Virtual Machine. Download ApkTool, its needed to reverse engineer the apk files, link is here: To get started, open the ELF executable in a disassembler such as Cutter. Patching APKs to load frida-gadget.so on start. This extracts the source code into the HelloWord-JNI/src directory. In the above output, the most relevant fields for us are: This can be extremely useful to spot unusual files when monitoring applications using obfuscation or other anti-reverse engineering techniques, without having to reverse the code. Developers sometimes use the native layer to "hide" data and functionality, and they may structure their apps such that execution frequently jumps between the two layers. Just try to be fair with authors of an app, that you use and probably like. apktool.jar keytool.exe View code README.md Reverse-Engineering-Android-APK-s This is a reference to reverse engineer personal applications built in order to do modifications, as well as to check the source of android applications built by others to determine if theirs any malicious content stored in the apps. There are many of these: open, openat, access, accessat, facessat, stat, fstat, etc. When System.loadLibrary is called, the loader selects the correct version based on the device that the app is running on. The following approach can be used in order to patch the JavaScript file: In the previous section we learned about patching application code to assist in our analysis, but this approach has several limitations. To learn more, please refer to the r2frida wiki. Instrumenting the live environment at strategic points gives you useful tracing functionality and the ability to manipulate the environment, which will help you bypass any anti-tampering defenses the app might implement. Once you've opened your file in radare2 you should first get the address of the function you're looking for. The Mobile Application Security Testing Guide (MASTG) is a comprehensive manual for mobile app security testing and reverse engineering. This defeats the first root detection control of the app. This gives you an advantage because most integrity checks and anti-tampering features ultimately rely on services performed by the kernel. Tools Apktool A tool for reverse engineering APKs. This decompiled C code makes it much easier to understand the function call being made. Then, single-step until you reach the call to String.equals. For white-box source code testing, you'll need a setup similar to the developer's setup, including a test environment that includes the Android SDK and an IDE. One way to achieve this is by using automated tools such as APKEnum or MobSF. In order to extract strings from native code used in an Android application, you can use GUI tools such as Ghidra or Cutter or rely on CLI-based tools such as the strings Unix utility (strings ) or radare2's rabin2 (rabin2 -zz ). Java Native Interface (JNI) is sometimes deliberately used to confuse reverse engineers (to be fair, there are legitimate reasons for using JNI, such as improving performance or supporting legacy code). Which are empty in this case. Before moving ahead, pay attention to the first parameter passed to the current JNI function. * Java Decompiling with Six different decompilers (DJ-GUI/Core, Procyon, CFR, Fernflower, Krakatau . Thus, setting LD_PRELOAD on Zygote is not possible. However, there's a downside: as strace depends on the ptrace system call to attach to the target process, once anti-debugging measures become active it will stop working. Source both your user id and a user session; run yarn to install the deps; run yarn start to start the client; Payloads/Events Many command line tools are not shipped with the Android firmware to reduce the size, but can be easily installed on a rooted device using BusyBox. An interface pointer is a pointer to a pointer. Identifying calls to these functions can help with understanding library functionality, such as what strings are created or Java methods are called. Are you sure you want to create this branch? It is a common notion that analyzing compiled native code is tougher than analyzing an equivalent compiled Java code, and hence, critical business logic is often written in native. The TCG block is compiled into a block of host instructions, stored in a code cache, and executed. Next, single-step through the onCreate method by clicking "Force Step Into" in Debugger view. in the Debugging section), so don't delete the project yet! CFR was released under an MIT license, so you can use it freely even though its source code is not available. It can decode resources to nearly original form and rebuild them after making some modifications; it makes possible to debug smali code step by step. Next, the current value of PC is added to R1, resulting in the absolute address of the static string "Hello from C++" (PC + offset). You signed in with another tab or window. In mobile application security however, the term loosely refers to all kinds of runtime manipulation, including overriding methods to change behavior. From the powerful tools shipping with the SDK to the wide range of available reverse engineering tools, there's a lot of niceties to make your life easier. There are three user defined functions: FUN_001004d0, FUN_0010051c, and Java_sg_vantagepoint_helloworldjni_MainActivity_stringFromJNI. To build the AOSP kernel, you need a toolchain (a set of programs for cross-compiling the sources) and the appropriate version of the kernel sources. Runtime reverse engineering can be seen as the on-the-fly version of reverse engineering where you don't have the binary data to your host computer. This is has been documented for reference. Moving forward, we will start analyzing the binary from the entry function at offset 0x00001874, and keep a note of all the information easily available to us. If you discover a security vulnerability within Apktool, please send an e-mail to Connor Tumbleson at connor.tumbleson(at)gmail.com. To learn more about all options for advanced usage, check the documentation on the jnitrace GitHub page. APK Decompiler for Windows, Mac or Linux 4. armeabi: ABI is for ARM-based CPUs that support at least the ARMv5TE instruction set. Please refer to r2frida's official installation instructions. The keyword "native" tells the Java compiler that this method is implemented in a native language. As you can see, there is no obfuscation at all on the Goatlin. Inside the current function there is a call to another function, whose address is obtained by accessing an offset in the JNIEnv pointer (found as plParm1). To modify initrd on any Android device, back up the original boot image with TWRP or dump it with the following command: To extract the contents of the boot image, use the abootimg tool as described in Krzysztof Adamski's how-to : Note the boot parameters written to bootimg.cfg; you'll need them when booting your new kernel and ramdisk. Since this function is small and extremely simple, the decompilation output is very accurate, this can change drastically when dealing with complex functions. frida-trace is a very versatile tool and there are multiple configuration options available such as: Many binaries are stripped and don't have function name symbols available with them. My task is finished, but I'm getting c. Tracing functions by address when no function name symbols are available (stripped binaries), e.g. You can calculate everything else with offsets taken from the kernel headers (hopefully, you didn't delete them yet). This instruction loads the function pointer from offset 0x29C into the JNI function pointer table pointed to by R2. First, resume execution of the Java VM by attaching jdb. To conclude, learning symbolic execution might look a bit intimidating at first, as it requires deep understanding and extensive practice. denuvosoftwaresolutions.github.io. Create a file called kmem_util.c with the following code: Beginning with Android 5.0 (API level 21), all executables must be compiled with PIE support. The native library for the current application is relatively very small. Open Source APK Decompilers 2. It is decrypted with 128bit AES, then compared with the user input. This prevents writing to kernel memory regions marked as read-only, so any attempt to patch kernel code or the system call table result in a segmentation fault and reboot. setCancelable(true) was called, so this can't be the call we're looking for. Free and Open Source Reverse Engineering Platform powered by rizin. Moving forward, at offset 0x000017dc, the XOR decoded value obtained from above is being compared against the return value from a sub-function call at 0x000017e8. Therefore, having an insight on which files are being opened or which strings are being compared may be very valuable. Next, we'll have a look at dynamic analysis methods that help you do just that. This makes the problem significantly easier to solve because you avoid symbolically executing the Base32 implementation. If problems in the converting into .zip please refers link. The main activity is found in the file HelloWord-JNI/src/sg/vantagepoint/helloworldjni/MainActivity.java. If you followed the instructions at the beginning of this chapter, you should already have the Android NDK. t can decode resources to the nearly original form and rebuild them after making some modifications. XOR is a very commonly used technique to encrypt information where obfuscation is the goal rather than security. Next, we are able to find the "Hello from C++" strings with radare2: Alternatively you can use Fridump. Bionic adds support for important Android-specific services such as system properties and logging, and it is not fully POSIX-compatible. When you do this, zipalign must be called after signing. If you recall from the "Platform Overview" section, every application in Android is forked from Zygote, which is started very early during the Android boot-up. The decompilation process of Android source code is usually a two-step process. 7 comments Copy this post's permalink to the clipboard redditads Promoted Additional post actions. In this example, this requirement is satisfied by the following C or C++ function: So where is the native implementation of this function? After doing some tutorials, I can reverse engineer basic APK file (with no proguard/dexguard). DDMS is somewhat confusing, however; it can be launched several ways, and different trace viewers will be launched depending on how a method was traced. Are you sure you want to create this branch? On the device, choose the app as debug app on the "Developer options" (UnCrackable App for Android Level 1 in this tutorial), and make sure you've switched on the "Wait For Debugger" feature. Once execution reaches a location where the su binary would be detected, modify one of the variables holding the file name or directory name by pressing F2 or right-clicking and choosing "Set Value". This skipping makes following the code flow crucial to debugging decompiled applications. replace a function that should give random numbers). The main function is located at offset 0x00001874 in the disassembly. Save the above script as uncrackable1.js and load it: After you see the "MainActivity.a modified" message and the app will not exit anymore. ", sg.vantagepoint.helloworldjni.MainActivity., com.acme.bob.mobile.android.core.BobMobileApplication.. Setting ro.debuggable to "1" makes all running apps debuggable (i.e., the debugger thread will run in every process), regardless of the value of the android:debuggable attribute in the Android Manifest. The loop is more clearly visible in the graph view below. Because Android is open-source, you can study its source code at the Android Open Source Project (AOSP) and modify the OS and its standard tools any way you want. Apps that run additional integrity checks beyond default Android code-signing are an exception. In the "static analysis" section, you saw that the string is decrypted with AES, then compared with the string input to the message box. Neither the string nor the pointer to the string actually exist. If you want to look directly into Java source code on a GUI, simply open your APK using jadx or Bytecode Viewer. It describes the technical processes for verifying the controls listed in the OWASP Mobile Application Security Verification Standard (MASVS). APK adalah format aplikasi Android, berupa file ZIP yang di dalamnya ada classes.dex yang berisi kode Java, resource dalam bentuk XML (yang diencode khusus), file-file gambar, video, suara, dsb. Native libraries are code that the developer wrote and then compiled for a specific computer architecture. So we want the return value of sg.vantagepoint.a.a.a. Tracing all JNI functions in an Android application using. string.equals comparison compares your input with arrby. Specifically, stock Lollipop and Marshmallow kernels are built with the CONFIG_STRICT_MEMORY_RWX option enabled. Another important restriction is that line breakpoints won't work because the release bytecode doesn't contain line information. Instrumenting and intercepting calls into the kernel is an effective method for getting a rough idea of what a user process is doing, and often the most efficient way to deactivate low-level tampering defenses. https://rmusser.net/git/admin-2/Infosec_Reference, https://code.google.com/archive/p/ipv6-hosts/. In the remaining sections, we'll introduce a few advanced subjects, including process exploration, kernel modules and dynamic execution. This is a page about tools for analyzing APK files, which are application packages for Android, and how to use them. Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. arrby is the return value of the function sg.vantagepoint.a.a.a. A tag already exists with the provided branch name. if not signed use the keytool in the java bin directory to sign apk. . In order to start the reversing process the APK file of the target application is needed. Thus, it is recommended to start with keyword-based searching and go through the list only when keyword search does not help. Deploying a kernel that abuses this trust and unabashedly lies about itself and the environment, goes a long way in defeating most reversing defenses that malware authors (or normal developers) can throw at you. Show References to in Ghidra or Find Usage in jadx. You can use the same techniques on Android apps that ship with obfuscated native libraries (in fact, obfuscated code is often put into native libraries specifically to make de-obfuscation more difficult). You can either pull it from the device (I am going the tell how in a minute) or download it from web sites like apkmirror.com For example, vulnerabilities regarding data storage might be sometimes hard to catch during static analysis, but in dynamic analysis you can easily spot what information is stored persistently and if the information is protected properly. The native code is present in the form of ELF shared libraries. Instead of reversing the decryption routines to reconstruct the secret key, you can simply ignore all the decryption logic in the app and hook the sg.vantagepoint.a.a.a function to catch its return value. Changing this file can make the Android environment easier to reverse engineer. Additional post actions People that are used to IDA or Hopper just load the binary, go out to make a coffee and then when the analysis is done, they start doing the manual analysis to understand what the program is doing. Once the breakpoint is reached, activate method tracing with the trace go methods command and resume execution. However, instead of relying on widely used C libraries (such as glibc), Android binaries are built against a custom libc named Bionic. When analyzing obfuscated code, annotating class names, method names, and other identifiers as you go along is a good practice. This makes starting times pretty heavy, compared to just loading the headers and strings information like its done by default. The only problem is that you're already too late to debug the JNI function StringFromJNI; it only runs once, at startup. 65. DDMS also offers a convenient heap dump button that will dump the Java heap of a process to a .hprof file. Making small changes to the Android Manifest or bytecode is often the quickest way to fix small annoyances that prevent you from testing or reverse engineering an app. Capstone disassembly/disassembler framework: Core (Arm, Arm64, BPF, EVM, M68K, M680X, MOS65xx, Mips, PPC, RISCV, Sparc, SystemZ, TMS320C64x, Web Assembly, X86, X86_64, XCore) + bindings. On ARM Linux, system calls are invoked via the SVC instruction, which triggers a software interrupt. You can create your own signing certificate and key, then add it to the debug KeyStore: After the certificate is available, you can re-sign the APK with it. Android decompilers go one step further and attempt to convert Android bytecode back into Java source code, making it more human-readable. You've now covered the basics of static/dynamic analysis on Android. Thus, it's more convenient to use Frida scripts to e.g. You signed in with another tab or window. Now you're getting somewhere: it's simply standard AES-ECB. The zipalign and apksigner tools are in this directory. Then again, reversing code is never easy, and efficiently navigating and debugging plain old Java code is a pretty convenient way of doing it. Once you tap the app icon from the launcher, it will be suspended in "Wait For Debugger" mode. An open-source x64/x32 debugger for windows. It is the same JNIEnv data structure which was discussed earlier in this section. The UnCrackable App for Android Level 1 is not stupid: it notices that it has been run in debuggable mode and reacts by shutting down. If you look into the "lib" directory of the unzipped APK archive, you'll see several subdirectories (one per supported processor architecture), each of them containing a version of the native library, in this case libnative-lib.so. Reverse Engineering - Kotlin SCP. If the app ships with binaries for different architectures, you can theoretically pick the architecture you're most familiar with, as long as it is compatible with the disassembler. In the following chapter, we'll look at some peculiarities of Android reversing and OS-specific tools as processes. Although this is easy to bypass, you're still likely to encounter limitations, such as a lack of line breakpoints. For file-hiding, you'll need to hook one of the system calls used to open (or check for the existence of) files. If you are looking for the Apktool website. You need to perform several steps to initialize Angr's symbolic execution engine: Load the binary into a Project, which is the starting point for any kind of analysis in Angr. With this modification, no certificate checks are performed, and the application accepts all certificates. Ctrl-click it to find out. Seems like we're expected to find some kind of secret code! The process of analyzing a subject system to identify the system's components and their interrelationships and to create representations of the system in another form or at a higher level of abstraction. prints the complete list of commands. Open the command prompt from directory and use the following command: Or from any directory open the command prompt and use this command with path to keytool: Password for key signature can be cracked with any password cracker. To convert a release build into a debuggable build, you need to modify a flag in the Android Manifest file (AndroidManifest.xml). What exactly does this particular a do? For the analysis of android applications we need the apk file. The instructions in the function Java_sg_vantagepoint_helloworldjni_MainActivity_stringFromJNI are already discussed in detail in previous sections. The app is now going to exit. Let's assume you're testing an app that's stubbornly quitting on your rooted device. We'll use Frida to solve the UnCrackable App for Android Level 1 and demonstrate how we can easily bypass root detection and extract secret data from the app. The file "nowyouseeme" is now somewhat hidden from all user mode processes. the regions provoking memory access violations. You'll end up with a structure that resembles the original Android Studio project from which the app was built. The "Hello World" text view is populated in the onCreate method: Note the declaration of public native String stringFromJNI at the bottom. JD-GUI Java Decompiler is a tool to decompile and. First, unzip the APK file (unzip UnCrackable-Level1.apk -d UnCrackable-Level1) and look at the content. However, the effort is justified considering the valuable time it can save in contrast to analyzing complex disassembled instructions manually. Q&A for work. If you want to inspect the app's smali code (instead of Java), you can open your APK in Android Studio by clicking Profile or debug APK from the "Welcome screen" (even if you don't intend to debug it you can take a look at the smali code). Another thing to notice in the output above is that it's colorized. With Frida, you can prevent the app from exiting by hooking the MainActivity.a method or the callback inside it. Using it requires only a few lines of Python code as shown below: In the above example, libinject.so library is injected as a dependency of a native library (libnative.so), which the application already loads by default. But this takes a lot of CPU time, and r2 aims to run in many more platforms than just high-end desktop computers. In those cases, you have to patch the additional checks as well. Learn more about Teams From here on you can use this information to derive more insights which might be of use later during your analysis, e.g. When using the CLI-based ones you can take advantage of other tools such as grep (e.g. For this exercise, you will use the app at ~/samples/HonSystemService.apk in the VM. As a workaround for this problem, Android supports the setprop (set property) functionality. Watch on. You can overwrite the onClick method and prevent it from ending the application with the System.exit call. Debugging and fixing elusive runtime bugs on code for which you don't have the original source. strlen is called at offset 0x000018a8, and the returned value is compared to 0x10 at offset 0x000018b0. The Android platform provides many in-built libraries for frequently used functionalities in applications, for example cryptography, Bluetooth, NFC, network or location libraries. A detailed explanation of this process is in the section "Dynamic Analysis on Non-Rooted Devices. The LDR instruction loads this function table pointer into R2. You can also use objection to display the same information. I recommend using the following settings to add loadable module support, enable the most important tracing facilities, and open kernel memory for patching. With such an approach we can directly move to analyzing cryptographic assets, which often are very critical in an application. First start by opening an r2frida session to the target app (e.g. You've now reached a call to setCancelable with the argument false. You may obtain different solutions using the script, as there are multiple valid license keys possible. Echoing "0" will prevent further writes into the ring buffer. A library can also be injected into a process using functionalities offered by the loader of the operating system. The following command logs all translated blocks to a file: Unfortunately, generating a complete guest instruction trace with QEMU is impossible because code blocks are written to the log only at the time they are translated, not when they're taken from the cache. Are you sure you want to create this branch? HelloWorld JNI APK) that should be running on your Android phone (connected per USB). Intro to Android App Reverse Engineering workshop. Most Java decompilers take plain class files or JARs as input, so you need to convert the classes.dex file into a JAR first. A well-known use case of this technique is loading the Frida gadget to an application, specially while working on a non-rooted device (this is what objection patchapk basically does). Write a Frida hook to change how that code works. The ABI defines how the application's machine code is supposed to interact with the system at runtime. Dynamic Analysis tests the mobile app by executing and running the app binary and analyzing its workflows for vulnerabilities. Typically you'd use hybrid techniques, as in the above example, where we performed manual analysis of the disassembled code to provide the correct criteria to the symbolic execution engine. The next thing you might want to look at are the currently loaded Java classes: Note that we've filtered by package name as this is the MainActivity and it includes all methods from Android's Activity class. The other symbols are not user defined and are generated for proper functioning of the shared library. This instruction loads into R1 the PC-relative offset of the string "Hello from C++". Dex2jar Tool to work with android .dex and java .class files. Then terminate and re-launch the app. For this you can target the app binary directly or reverse engineer it and target the disassembled or decompiled code. If you do not have access to C://Windows, you may place the two files anywhere then add that directory to your Environment Variables System PATH variable. Procfs provides a directory-based view of a process running on the system, providing detailed information about the process itself, its threads, and other system-wide diagnostics. This modification will break the APK signature, so you'll also have to re-sign the altered APK archive after repackaging it. It is important to know, the list of strings obtained using the above tools can be very big, as it also includes the various class and package names used in the application. We assume that you've successfully opened lib/armeabi-v7a/libnative-lib.so in IDA pro. The workflow can be further improved by using r2ghidra-dec, a deep integration of Ghidra decompiler for radare2. Certificate pinning is an issue for security testers who want to intercept HTTPS communication for legitimate reasons. Open ThaiCamera.apk in the jadx GUI. It translates the program's semantics into a logical formula in which some variables are represented by symbols with specific constraints. Note that we'll use the OWASP UnCrackable Apps for Android as examples for demonstrating various reverse engineering techniques in the following sections, so expect partial and full spoilers. Addressing relative to the program counter allows the code to run independently of its position in memory. For many of the GUI-based ones, this is usually done by right clicking on the desired function and selecting the corresponding option, e.g. Categories > Software Development > Reverse Engineering. To demonstrate bypassing certificate pinning, we'll walk through an implementation in an example application. Using library injection you can develop reusable libraries and inject them to different applications, effectively making them behave differently without having to modify their original source code.
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