For instance, you'd like to log everything that's being sent over the network without having to perform a MITM attack. Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. For example, \e search.quiet=true; will print only the results and hide search progress: For now, we'll continue with the defaults and concentrate on string search. This app triggers anti-debugging and anti-tampering controls within the onCreate method. APKLab The ultimate Android RE experience right inside your VS Code . The final solution script is presented below: As discussed previously in the section "Dynamic Binary Instrumentation", the symbolic execution engine constructs a binary tree of the operations for the program input given and generates a mathematical equation for each possible path that might be taken. Method hooking with an Xposed module is one way to do this (see "Android Basic Security Testing" for more details on Xposed installation and basics). When you add the -s flag, all strings are extracted from the dumped raw memory files and added to the file strings.txt, which is also stored in the dump directory. 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. This app is not exactly spectacular, all it does is show a label with the text "Hello from C++". It contains prebuilt versions of gdbserver for various architectures. See the section "Reviewing Decompiled Java Code" below to learn on how to proceed when inspecting the decompiled Java code. You can scroll and zoom the timeline view with the mouse or trackpad. If you want to look directly into Java source code on a GUI, simply open your APK using jadx or Bytecode Viewer. Now, open the Uncrackable-Level1/src directory in a file browser and drag the sg directory into the now empty Java folder in the IntelliJ project view (hold the "alt" key to copy the folder instead of moving it). Note: When using these tools, you might get several memory access violation errors which can be normally ignored. In mobile application security however, the term loosely refers to all kinds of runtime manipulation, including overriding methods to change behavior. Binary analysis frameworks typically use a technique called symbolic execution, which allow to determine the conditions necessary to reach a specific target. Most of the apps you might encounter connect to remote endpoints. Therefore, having an insight on which files are being opened or which strings are being compared may be very valuable. if not signed use the keytool in the java bin directory to sign apk. Specify the kernel offset, ramdisk offset, tags offset, and command line (use the values listed in your extracted bootimg.cfg) in addition to the newly built kernel and the original ramdisk. This should highlight the Java_sg_vantagepoint_helloworld_ MainActivity_stringFromJNI function. It is not easy to reverse engineer a release version of a flutter app because the tooling is not available and the flutter engine itself changes rapidly. command for all @@ hits matching the glob hit0_*: Additionally, you can search for occurrences of the wide version of the string (\/w) and, again, check their memory regions: They are in the same rw- region as one of the previous strings (0x0000007d30a00000). Next, the endness parameter in the script specifies that the data is stored in "little-endian" fashion, which is the case for almost all of the Android devices. Note: If you experience JRE compatibility issues with apksigner, you can use jarsigner instead. Click the stop button to stop recording, after which the Traceview tool will open and show the recorded trace. You'll usually have the application package in Android's APK format, which can be installed on an Android device or reverse engineered as explained in the section "Disassembling and Decompiling". KProbes inserts a breakpoint instruction at the specified address. To get started, open the ELF executable in a disassembler such as Cutter. Remember that you can use the flag -A to run the aaa command right after loading the binary in order to analyze all referenced code. In this situation, you can inject the Gadget library by using the same methods you're going to learn in this section. Following the example from "Disassembling Native Code" we will use different disassemblers to review the disassembled native code. To do that you have to perform one additional step: patch the APK to include the Frida gadget library. Non-rooted devices have the benefit of replicating an environment that the application is intended to run on. In this section we can look into the decompilation of the function. Execute gdbserver to attach to the suspended app. Connect and share knowledge within a single location that is structured and easy to search. Running this script should return the following output: Now you can run the validate binary in your Android device to verify the solution as indicated here. Finally, search for the input string in the dump directory: The "owasp-mstg" string can be found in one of the dump files as well as in the processed strings file. traffic capture and analysis), you can obtain some initial inputs or entry points by enumerating the domains to which the application is supposed to communicate to. To bypass the pinning check, add the return-void opcode to the first line of each method. The SHA256 digest for the sample is 86a1ba436be2d1ee480e1d40418df3aa5f47571d154a8c41e74c439fbfb04b27. It seems plausible that a.a verifies user input, so we'll refactor the code to reflect this. If the system property ro.debuggable is set to "0", Android disallows both JDWP and native debugging of release builds. Also, it may appear as if the script is simply reading the solution string from the memory of the script. IDA Pro. Once you've unpacked the app (e.g. If you reverse Android apps on a regular basis, building your own reverse engineering sandbox is a no-brainer. Finally, you need to re-sign the application before using it. Q&A for work. Following the example from "Decompiling Java Code", we assume that you've successfully decompiled and opened the UnCrackable App for Android Level 1 in IntelliJ. The ABI defines how the application's machine code is supposed to interact with the system at runtime. Assuming that everything worked, /bin/cat shouldn't be able to see the file. Remember to always type ? We assume that you've successfully opened lib/armeabi-v7a/libnative-lib.so in IDA pro. This is known as DLL injection on Windows (broadly used to modify and bypass anti-cheat mechanisms in games), LD_PRELOAD on Linux and DYLD_INSERT_LIBRARIES on macOS. The disassembly of this function is shown below. \ii libandroid_runtime.so~.. (if not, for this binary, you'd get almost 2500 lines printed to your terminal). Explore . The corresponding C code for the disassembled function is shown in the Decompiler window. 3. Typically these domains will be present as strings within the binary of the application. An ELF executable includes a list of shared libraries (dependencies) that are linked to the executable for it to function optimally. The main activity is found in the file HelloWord-JNI/src/sg/vantagepoint/helloworldjni/MainActivity.java. You should get an output similar to the following: The hooked function outputted the decrypted string. Android apps have several ways to interact with the OS. 5) Remove the pinning code. From the reversing perspective, we need to be aware of two key data structures: JavaVM and JNIEnv. First, unzip the APK file ( unzip UnCrackable-Level1.apk -d UnCrackable-Level1) and look at the content. Once you "Force Step Into", the debugger will stop at the beginning of the next method, which is the a method of the class sg.vantagepoint.a.c. ", "sg.vantagepoint.uncrackable1.MainActivity", sg.vantagepoint.helloworldjni.MainActivity.stringFromJNI, sg.vantagepoint.helloworldjni.MainActivity. For hacking, I recommend an AOSP-supported device. Each DEX file contains a list of string identifiers (strings_ids), which contains all the string identifiers used in the binary whenever a string is referred, including internal naming (e.g, type descriptors) or constant objects referred by the code (e.g hardcoded strings). If you are looking for the Apktool website. Therefore, when the injected Frida agent tries to read a region that's not readable, it'll return the corresponding memory access violation errors. It should be suspended automatically. Note that the file can easily be found using other syscalls, and you need to do a lot more to properly hide a file, including hooking stat, access, and other system calls. As on other Linux-based operating systems, native code is packaged (compiled) into ELF dynamic libraries (*.so), which the Android app loads at runtime via the System.load method. After saving the script to a file called java_class_listing.js, you can tell Frida CLI to load it by using the flag -l and inject it to the process ID specified by -p. Given the verbosity of the output, the system classes can be filtered out programmatically to make output more readable and relevant to the use case. With Ghidra, strings can be obtained by simply loading the DEX file and selecting Window -> Defined strings in the menu. You can seek to that address with s 0x7d1c49954c, analyze that function af and print 10 lines of its disassembly pd 10: Note that the line tagged with ; hit0_4 corresponds to the string that we've previously found: 0x7d1c499560 hit0_4 Hello from C++. First, we'll look at some simple ways to modify and instrument mobile apps. Further information on how to analyze Android's cryptographic APIs is discussed in the section "Android Cryptographic APIs". The standard Java distribution includes keytool for managing KeyStores and certificates. In the "static analysis" section, you saw that the string is decrypted with AES, then compared with the string input to the message box. For example, getting the disassembly of a specific function is a trivial task that can be performed in one command. Goal in the Debugging section), so don't delete the project yet! 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). Although regular Android apps are hopelessly restricted and sandboxed, you, the reverser, can customize and alter the behavior of the operating system and kernel any way you wish. To use these tools on a rooted phone, simply have frida-server installed and running. On Android, which is a Linux based OS, you can load an additional library by setting the LD_PRELOAD environment variable. On ARM Linux, system calls are invoked via the SVC instruction, which triggers a software interrupt. To create a toolchain for Android 7.0 (API level 24), run make-standalone-toolchain.sh from the Android NDK package: Set the CROSS_COMPILE environment variable to point to your NDK directory and run "make" to build These tools inject a Frida agent and try to dump all the mapped memory of the app regardless of the access permissions (read/write/execute). Immediately after that, the input string is passed to a Base32 decoding function at offset 0x00001340. A tag already exists with the provided branch name. You can now try to input a "secret string". Click here. The CLI version of JStillery or the local server should be preferred instead of using the online version as otherwise source code is sent and disclosed to a 3rd party. \is libnative-lib.so. After the breakpoint has been reached, execute the step up command, which will resume the process until loadLibraryreturns. Going through the complete list, specially for big binaries, can be very cumbersome. Identify where the code should be patched in the temporary file and implement the changes. This is one of those cases where we can speed up our workflow because you're focusing on some specific part of an app. To conclude, learning symbolic execution might look a bit intimidating at first, as it requires deep understanding and extensive practice. The method verify is called when you tap the "verify" button. In this case, the code block that prints the "Incorrect serial" message at 0x00001854 is not interesting. A FREE comprehensive reverse engineering tutorial covering x86, x64, 32-bit ARM & 64-bit ARM architectures. Using "Force Step Into" might be a good idea once you reach the decompiled sources and "Step Out" of the core Java and Android classes. Above we looked into techniques which require some kind of modification of the application's code. * Quickly decompile classes using JD-Core. The adb command line tool was introduced in the "Android Basic Security Testing" chapter. GitHub Instantly share code, notes, and snippets. A moderated community dedicated to all things reverse engineering. to learn about the search command and get a list of options. They can still be reverse engineered, but the process is not automated and requires knowledge of low-level details. It is decrypted with 128bit AES, then compared with the user input. You can look up the address in the /proc/kallsyms file: This is the only memory address you need for writing your kernel module. You can use its adb jdwp command to list the process IDs of all debuggable processes running on the connected device (i.e., processes hosting a JDWP transport). use JD-GUI to extract from the jar file. 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. The decompiled method looks like this: So, you have a Base64-encoded String that's passed to the function a in the package You can easily repackage an app by doing the following: Note that the Android Studio build tools directory must be in the path. This takes you to the method definition. Once the breakpoint is reached, control passes to the KProbes system, which then executes the user-defined handler function(s) and the original instruction. 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. When using the CLI-based ones you can take advantage of other tools such as grep (e.g. XOR should not be used for any serious encryption, as it can be cracked using frequency analysis. First, resume execution of the Java VM by attaching jdb. Clicking any of the methods in the profile panel highlights the selected method in the timeline panel. This method passes the user input to a static method called a.a, which returns a boolean value. Apktool A tool for reverse engineering 3rd party, closed, binary Android apps. Use the java decompiler JD-GUI to extract the source code from the jar file. The LDR instruction loads this function table pointer into R2. 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. Other useful keywords specific to the context of the app can be obtained while you are using the app itself. You can find the function prototypes for all system calls in the kernel header file arch/arm/include/asm/unistd.h. *.java source code. For example, in radare2 you'd run i to display the binary information, but in r2frida you'd use \i. A tag already exists with the provided branch name. Otherwise, identifying the next line to be executed would become complicated. It is all about theorizing, annotating, and gradually revising theories about the analyzed program until you understand it completely or, at least, well enough for whatever you want to achieve. You signed in with another tab or window. Although this is easy to bypass, you're still likely to encounter limitations, such as a lack of line breakpoints. This is the syscall that the /bin/cat program uses when accessing a file, so the call should be suitable for a demonstration. For me, I have a directory called "APKAnalysisTools" and this is where I am doing my APK testing. You also have access to the /dev/kmem interface, which you can use to patch kernel memory on-the-fly. Many Android applications use native code in addition to Java code for various performance and security reasons. a.a now becomes Validator.a. In those cases, you have to patch the additional checks as well. Actually you can do it much faster with Frida and other methods, which we'll introduce later in the guide. 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. A library can also be injected into a process using functionalities offered by the loader of the operating system. Bionic adds support for important Android-specific services such as system properties and logging, and it is not fully POSIX-compatible. Advanced Reverse Engineering on Android (2016) -. reverse engineered java code from signed APK. Internally, Angr explores all the paths between the two points specified by us, and passes the corresponding mathematical equations to the solver to return meaningful concrete results. It may not look like much, but its Java method tracer is one of the most awesome tools you can have in your arsenal, and it is indispensable for analyzing obfuscated bytecode. Thus it is recommended to extract the DEX file by unzipping the APK file and then loading it into Ghidra. When you start the crackme app on an emulator or a rooted device, you'll find that the it presents a dialog box and exits as soon as you press "OK" because it detected root: The main method (decompiled with CFR) looks like this: Notice the "Root detected" message in the onCreate method and the various methods called in the preceding if-statement (which perform the actual root checks). Configure the static analyzer properly to reduce the likelihood of false positives and maybe only select several vulnerability categories in the scan. Discussing all the instructions in the function is beyond the scope of this chapter, instead we will discuss only the important points needed for the analysis. Each basic block of guest instructions is disassembled and translated into an intermediate representation called Tiny Code Generator (TCG). Frida gadget can be injected into an application using this approach as explained in detail in LIEF's documentation. Start jadx by opening the terminal in the VM and running the jadx-gui command in the terminal. class and method). See r2frida's help on the search command (\/?) This time, we will input a string and see if we can find it in the memory dump. It is a Bash script that makes reverse engineering Android apps easier, automating some repetitive tasks like decoding, rebuilding and patching. 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.
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