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Tampering and Reverse Engineering on Android Android's openness makes it a favorable environment for reverse engineers. In the following chapter, we'll look at some peculiarities of Android reversing and OS-specific tools as processes. Android offers reverse engineers big advantages that are not available with 'the other' mobile OS. 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. Even on standard retail devices it is possible to do things like activating developer mode and sideloading apps without jumping through many hoops. 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. However, there are also a few Android-specific challenges.
For example, you'll need to deal with both Java bytecode and native code. 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). 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. You'll need at least a working knowledge of both the Java-based Android environment and the Linux OS and Kernel, on which Android is based. You'll also need the right toolset to deal with both native code and bytecode running on the Java virtual machine. Note that we'll use the as examples for demonstrating various reverse engineering techniques in the following sections, so expect partial and full spoilers. We encourage you to have a crack at the challenges yourself before reading on!
What You Need Make sure that the following is installed on your system:. The newest SDK Tools and SDK Platform-Tools packages. These packages include the Android Debugging Bridge (ADB) client and other tools that interface with the Android platform. The Android NDK. This is the Native Development Kit that contains prebuilt toolchains for cross-compiling native code for different architectures.
In addition to the SDK and NDK, you'll also need something to make Java bytecode more human-readable. Fortunately, Java decompilers generally handle Android bytecode well. Popular free decompilers include,. For convenience, we have packed some of these decompilers into our. This script completely automates the process of extracting Java code from release APK files and makes it easy to experiment with different backends (we'll also use it in some of the following examples). Other tools are really a matter of preference and budget.
A ton of free and commercial disassemblers, decompilers, and frameworks with different strengths and weaknesses exist; we'll cover some of them. Setting up the Android SDK Local Android SDK installations are managed through Android Studio.
Create an empty project in Android Studio and select 'Tools-Android-SDK Manager' to open the SDK Manager GUI. The 'SDK Platforms' tab lets you install SDKs for multiple API levels. Recent API levels are:. API 23: Android 6.0.
API 24: Android 7.0. API 25: Android 7.1. API 26: Android 8.0. API 27: Android 8.1. API 28: Android 9 Installed SDKs are found at the following locations: Windows: C: Users AppData Local Android sdk MacOS: /Users//Library/Android/sdk Note: On Linux, you'll need to pick your own SDK location. /opt, /srv, and /usr/local are common locations.
Setting up the Android NDK The Android NDK contains prebuilt versions of the native compiler and toolchain. Both the GCC and Clang compilers have traditionally been supported, but active support for GCC ended with NDK revision 14.
The device architecture and host OS determine the appropriate version. The prebuilt toolchains are in the toolchains directory of the NDK, which contains one subdirectory for each architecture. Architecture Toolchain name ARM-based arm-linux-androideabi- x86-based x86- MIPS-based mipsel-linux-android- ARM64-based aarch64-linux-android- X86-64-based x8664- MIPS64-based mips64el-linux-android- Besides picking the right architecture, you need to specify the correct sysroot for the native API level you want to target. The sysroot is a directory that contains the system headers and libraries for your target. Native APIs vary by Android API level. Possible sysroots for each Android API level are in $NDK/platforms/.
Each API level directory contains subdirectories for the various CPUs and architectures. One possibility for setting up the build system is exporting the compiler path and necessary flags as environment variables. To make things easier, however, the NDK allows you to create a so-called standalone toolchain—a 'temporary' toolchain that incorporates the required settings. To set up a standalone toolchain, download the. Extract the ZIP file, change into the NDK root directory, and run the following command.
$ export TOOLCHAIN=/tmp/android-7-toolchain Enabling Developer Mode You must enable USB debugging on the device in order to use the ADB debugging interface. Since Android 4.2, the 'Developer options' sub menu in the Settings app is hidden by default. To activate it, tap the 'Build number' section of the 'About phone' view seven times. Note that the build number field's location varies slightly by device—for example, on LG Phones, it is under 'About phone - Software information.'
Once you have done this, 'Developer options' will be shown at bottom of the Settings menu. Once developer options are activated, you can enable debugging with the 'USB debugging' switch.
Once USB debugging is enabled, connected devices can be viewed with the following command. $ adb devices List of devices attached BAZ5ORFARKOZYDFA device Building a Reverse Engineering Environment for Free With a little effort, you can build a reasonable GUI-based reverse engineering environment for free. For navigating the decompiled sources, we recommend, a relatively lightweight IDE that works great for browsing code and allows basic on-device debugging of the decompiled apps. However, if you prefer something that's clunky, slow, and complicated to use, is the right IDE for you (based on the author's personal bias). If you don't mind looking at Smali instead of Java, you can use the for debugging.
Smalidea supports single-stepping through the bytecode and identifier renaming, and it watches for non-named registers, which makes it much more powerful than a JD + IntelliJ setup. Is a popular free tool that can extract and disassemble resources directly from the APK archive and disassemble Java bytecode to Smali format (Smali/Baksmali is an assembler/disassembler for the Dex format. It's also Icelandic for 'Assembler/Disassembler'). APKTool allows you to reassemble the package, which is useful for patching and applying changes to the Manifest. You can accomplish more elaborate tasks (such as program analysis and automated de-obfuscation) with open source reverse engineering frameworks such as. You'll find usage examples for many of these free tools and frameworks throughout the guide. Commercial Tools Although working with a completely free setup is possible, you should consider investing in commercial tools.
The main advantage of these tools is convenience: they come with a nice GUI, lots of automation, and end user support. If you earn your daily bread as a reverse engineer, they will save you a lot of time. JEB, a commercial decompiler, packs all the functionality necessary for static and dynamic analysis of Android apps into an all-in-one package. It is reasonably reliable and includes prompt support. It has a built-in debugger, which allows for an efficient workflow—setting breakpoints directly in the decompiled (and annotated) sources is invaluable, especially with ProGuard-obfuscated bytecode. Of course, convenience like this doesn't come cheap, and now that JEB is provided fvia a subscription-based license, you'll have to pay a monthly fee to use it.
IDA Pro is compatible with ARM, MIPS, Java bytecode, and, of course, Intel ELF binaries. It also comes with debuggers for both Java applications and native processes. With its powerful scripting, disassembling, and extension capabilities, IDA Pro works great for static analysis of native programs and libraries.
However, the static analysis facilities it offers for Java code are rather basic—you get the Smali disassembly but not much more. You can't navigate the package and class structure, and some actions (such as renaming classes) can't performed, which can make working with more complex Java apps tedious. Reverse Engineering Reverse engineering is the process of taking an app apart to find out how it works.
You can do this by examining the compiled app (static analysis), observing the app during run time (dynamic analysis), or a combination of both. Statically Analyzing Java Code Java bytecode can be converted back into source code without many problems unless some nasty, tool-breaking anti-decompilation tricks have been applied.
We'll be using UnCrackable App for Android Level 1 in the following examples, so download it if you haven't already. First, let's install the app on a device or emulator and run it to see what the crackme is about.
$ apkx UnCrackable-Level1.apk Extracting UnCrackable-Level1.apk to UnCrackable-Level1 Converting: classes.dex - classes.jar (dex2jar) dex2jar UnCrackable-Level1/classes.dex - UnCrackable-Level1/classes.jar Decompiling to UnCrackable-Level1/src (cfr) You should now find the decompiled sources in the directory Uncrackable-Level1/src. To view the sources, a simple text editor (preferably with syntax highlighting) is fine, but loading the code into a Java IDE makes navigation easier. Let's import the code into IntelliJ, which also provides on-device debugging functionality.
Open IntelliJ and select 'Android' as the project type in the left tab of the 'New Project' dialog. Enter 'Uncrackable1' as the application name and 'vantagepoint.sg' as the company name. This results in the package name 'sg.vantagepoint.uncrackable1,' which matches the original package name. Using a matching package name is important if you want to attach the debugger to the running app later on because Intellij uses the package name to identify the correct process. In the next dialog, pick any API number; you don't actually want to compile the project, so the number doesn't matter.
Click 'next' and choose 'Add no Activity,' then click 'finish.' Once you have created the project, expand the '1: Project' view on the left and navigate to the folder app/src/main/java.
Right-click and delete the default package 'sg.vantagepoint.uncrackable1' created by IntelliJ. 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). You'll end up with a structure that resembles the original Android Studio project from which the app was built. As soon as IntelliJ has indexed the code, you can browse it just like you'd browse any other Java project. Note that many of the decompiled packages, classes, and methods have weird one-letter names; this is because the bytecode has been 'minified' with ProGuard at build time. This is a basic type of obfuscation that makes the bytecode a little more difficult to read, but with a fairly simple app like this one it won't cause you much of a headache.
When you're analyzing a more complex app, however, it can get quite annoying. When analyzing obfuscated code, annotating class names, method names, and other identifiers as you go along is a good practice.
Open the MainActivity class in the package sg.vantagepoint.uncrackable1. The method verify is called when you tap the 'verify' button. This method passes user input to a static method called a.a, which returns a boolean value. It seems plausible that a.a verifies user input, so we'll refactor the code to reflect this.
Right-click the class name—the first a in a.a—and select Refactor-Rename from the drop-down menu (or press Shift-F6). Change the class name to something that makes more sense given what you know about the class so far. For example, you could call it 'Validator' (you can always revise the name later).
A.a now becomes Validator.a. Follow the same procedure to rename the static method a to checkinput. Congratulations—you just learned the fundamentals of static analysis! 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. Next, Ctrl+click (or Command+click on Mac) on the checkinput method.
This takes you to the method definition. The decompiled method looks like this. JNIEXPORT jstring JNICALL JavasgvantagepointhelloworldMainActivitystringFromJNI(JNIEnv.env, jobject) So where is the native implementation of this function? If you look into the lib directory of the APK archive, you'll see eight subdirectories named after different processor architectures.
Each of these directories contains a version of the native library libnative-lib.so that has been compiled for the processor architecture in question. When System.loadLibrary is called, the loader selects the correct version based on the device that the app is running on. Following the naming convention mentioned above, you can expect the library to export a symbol called JavasgvantagepointhelloworldMainActivitystringFromJNI.
On Linux systems, you can retrieve the list of symbols with readelf (included in GNU binutils) or nm. Do this on Mac OS with the greadelf tool, which you can install via Macports or Homebrew. The following example uses greadelf. $ greadelf -W -s libnative-lib.so grep Java 3: 00004e49 112 FUNC GLOBAL DEFAULT 11 JavasgvantagepointhelloworldMainActivitystringFromJNI This is the native function that eventually gets executed when the stringFromJNI native method is called. To disassemble the code, you can load libnative-lib.so into any disassembler that understands ELF binaries (i.e., any disassembler). 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.
Each version is compiled from the same source and implements the same functionality. However, if you're planning to debug the library on a live device later, it's usually wise to pick an ARM build. To support both older and newer ARM processors, Android apps ship with multiple ARM builds compiled for different Application Binary Interface (ABI) versions.
The ABI defines how the application's machine code is supposed to interact with the system at run time. The following ABIs are supported:. armeabi: ABI is for ARM-based CPUs that support at least the ARMv5TE instruction set. armeabi-v7a: This ABI extends armeabi to include several CPU instruction set extensions. arm64-v8a: ABI for ARMv8-based CPUs that support AArch64, the new 64-bit ARM architecture. Most disassemblers can handle any of those architectures. Below, we'll be viewing the armeabi-v7a version in IDA Pro.
It is in lib/armeabi-v7a/libnative-lib.so. If you don't own an IDA Pro license, you can do the same thing with the demo or evaluation version available on the Hex-Rays website. Open the file in IDA Pro. In the 'Load new file' dialog, choose 'ELF for ARM (Shared Object)' as the file type (IDA should detect this automatically), and 'ARM Little-Endian' as the processor type. Once the file is open, click into the 'Functions' window on the left and press Alt+t to open the search dialog. Enter 'java' and hit enter.
This should highlight the JavasgvantagepointhelloworldMainActivitystringFromJNI function. Double-click the function to jump to its address in the disassembly Window.
'Ida View-A' should now show the disassembly of the function. Not a lot of code there, but you should analyze it. The first thing you need to know is that the first argument passed to every JNI is a JNI interface pointer. An interface pointer is a pointer to a pointer.
This pointer points to a function table—an array of even more pointers, each of which points to a JNI interface function (is your head spinning yet?). The function table is initialized by the Java VM and allows the native function to interact with the Java environment.
With that in mind, let's have a look at each line of assembly code. LDR R2, R0 Remember: the first argument (in R0) is a pointer to the JNI function table pointer.
The LDR instruction loads this function table pointer into R2. LDR R1, =aHelloFromC This instruction loads into R1 the pc-relative offset of the string 'Hello from C.' Note that this string comes directly after the end of the function block at offset 0xe84. Addressing relative to the program counter allows the code to run independently of its position in memory. LDR.W R2, R2, #0x29C This instruction loads the function pointer from offset 0x29C into the JNI function pointer table pointed to by R2.
This is the NewStringUTF function. You can look at the list of function pointers in jni.h, which is included in the Android NDK. The function prototype looks like this. Jstring (.NewStringUTF)(JNIEnv., const char.); The function takes two arguments: the JNIEnv pointer (already in R0) and a String pointer. Next, the current value of PC is added to R1, resulting in the absolute address of the static string 'Hello from C' (PC + offset). ADD R1, PC Finally, the program executes a branch instruction to the NewStringUTF function pointer loaded into R2: BX R2 When this function returns, R0 contains a pointer to the newly constructed UTF string. This is the final return value, so R0 is left unchanged and the function returns.
Debugging and Tracing So far, you've been using static analysis techniques without running the target apps. In the real world—especially when reversing malware or more complex apps—pure static analysis is very difficult. Observing and manipulating an app during run time makes it much, much easier to decipher its behavior. Next, we'll have a look at dynamic analysis methods that help you do just that. Android apps support two different types of debugging: Debugging on the level of the Java runtime with the Java Debug Wire Protocol (JDWP), and Linux/Unix-style ptrace-based debugging on the native layer, both of which are valuable to reverse engineers.
Debugging Release Apps Dalvik and ART support the JDWP, a protocol for communication between the debugger and the Java virtual machine (VM) that it debugs. JDWP is a standard debugging protocol that's supported by all command line tools and Java IDEs, including JDB, JEB, IntelliJ, and Eclipse. Android's implementation of JDWP also includes hooks for supporting extra features implemented by the Dalvik Debug Monitor Server (DDMS. A JDWP debugger allows you to step through Java code, set breakpoints on Java methods, and inspect and modify local and instance variables.
You'll use a JDWP debugger most of the time you debug 'normal' Android apps (i.e., apps that don't make many calls to native libraries). In the following section, we'll show how to solve the UnCrackable App for Android Level 1 with JDB alone. Note that this is not an efficient way to solve this crackme—you can do it much faster with Frida and other methods, which we'll introduce later in the guide. This, however, serves as an introduction to the capabilities of the Java debugger. Repackaging Every debugger-enabled process runs an extra thread for handling JDWP protocol packets. This thread is started only for apps that have the android:debuggable='true' tag set in their manifest file's element. This is the typical configuration of Android devices shipped to end users.
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When reverse engineering apps, you'll often have access to the target app's release build only. Release builds aren't meant to be debugged—after all, that's the purpose of debug builds. If the system property ro.debuggable is set to '0,' Android disallows both JDWP and native debugging of release builds. Although this is easy to bypass, you're still likely to encounter limitations, such as a lack of line breakpoints. Nevertheless, even an imperfect debugger is still an invaluable tool— being able to inspect the run time state of a program makes understanding the program a lot easier. To 'convert' a release build into a debuggable build, you need to modify a flag in the app's manifest file.
This modification breaks the code signature, so you'll also have to re-sign the altered APK archive. To re-sign, you first need a code-signing certificate. If you have built a project in Android Studio before, the IDE has already created a debug keystore and certificate in $HOME/.android/debug.keystore.
The default password for this keystore is 'android,' and the key is called 'androiddebugkey.' The standard Java distribution includes keytool for managing keystores and certificates. You can create your own signing certificate and key, then add it to the debug keystore.
$ adb install UnCrackable-Repackaged.apk The “Wait For Debugger” Feature The UnCrackable App is not stupid: it notices that it has been run in debuggable mode and reacts by shutting down. A modal dialog is shown immediately, and the crackme terminates once you tap 'OK.' Fortunately, Android's 'Developer options' contain the useful 'Wait for Debugger' feature, which allows you to automatically suspend an app doing startup until a JDWP debugger connects. With this feature, you can connect the debugger before the detection mechanism runs, and trace, debug, and deactivate that mechanism. It's really an unfair advantage, but, on the other hand, reverse engineers never play fair! In the Developer options, pick Uncrackable1 as the debugging application and activate the 'Wait for Debugger' switch.
Note: Even with ro.debuggable set to 1 in default.prop, an app won't show up in the 'debug app' list unless the android:debuggable flag is set to true in the Manifest. The Android Debug Bridge The adb command line tool, which ships with the Android SDK, bridges the gap between your local development environment and a connected Android device. You'll usually debug apps on the emulator or a device connected via USB. Use the adb devices command to list the connected devices. $ adb forward tcp:1234 tcp:1234 You'll now use the prebuilt version of gdb included in the NDK toolchain (if you haven't already, follow the instructions above to install it).
$ $TOOLCHAIN/bin/gdb libnative-lib.so GNU gdb (GDB) 7.11 (.) Reading symbols from libnative-lib.so.(no debugging symbols found).done. (gdb) target remote:1234 Remote debugging using:1234 0xb6e0f124 in?? You have successfully attached to the process! The only problem is that you're already too late to debug the JNI function StringFromJNI; it only runs once, at startup. You can solve this problem by activating the 'Wait for Debugger' option. Go to 'Developer Options' - 'Select debug app' and pick HelloWorldJNI, then activate the 'Wait for debugger' switch.
Then terminate and re-launch the app. It should be suspended automatically. Our objective is to set a breakpoint at the first instruction of the native function JavasgvantagepointhelloworldjniMainActivitystringFromJNI before resuming the app. Unfortunately, this isn't possible at this point in the execution because libnative-lib.so isn't yet mapped into process memory—it is loaded dynamically during run time. To get this working, you'll first use JDB to gently change the process into the desired state.
First, resume execution of the Java VM by attaching JDB. You don't want the process to resume immediately though, so pipe the suspend command into JDB. $ adb forward tcp:1234 tcp:1234 $ $TOOLCHAIN/arm-linux-androideabi-gdb libnative-lib.so GNU gdb (GDB) 7.7 Copyright (C) 2014 Free Software Foundation, Inc. (.) (gdb) target remote:1234 Remote debugging using:1234 0xb6de83b8 in?? Execute the resume command in JDB to resume execution of the Java runtime (you're done with JDB, so you can detach it too).
You can start exploring the process with GDB. The info sharedlibrary command displays the loaded libraries, which should include libnative-lib.so. The info functions command retrieves a list of all known functions.
The JNI function javasgvantagepointhelloworldjniMainActivitystringFromJNI should be listed as a non-debugging symbol. Set a breakpoint at the address of that function and resume the process.
(gdb) info sharedlibrary (.) 0xa3522e3c 0xa3523c90 Yes (.) libnative-lib.so (gdb) info functions All defined functions: Non-debugging symbols: 0x00000e78 JavasgvantagepointhelloworldjniMainActivitystringFromJNI (.) 0xa3522e78 JavasgvantagepointhelloworldjniMainActivitystringFromJNI (.) (gdb) b.0xa3522e78 Breakpoint 1 at 0xa3522e78 (gdb) cont Your breakpoint should be reached when the first instruction of the JNI function is executed. You can now display a disassembled version of the function with the disassemble command. Breakpoint 1, 0xa3522e78 in JavasgvantagepointhelloworldjniMainActivitystringFromJNI from libnative-lib.so (gdb) disass $pc Dump of assembler code for function JavasgvantagepointhelloworldjniMainActivitystringFromJNI: = 0xa3522e78: ldr r2, r0, #0 0xa3522e7a: ldr r1, pc, #8; (0xa3522e84 ) 0xa3522e7c: ldr.w r2, r2, #668; 0x29c 0xa3522e80: add r1, pc 0xa3522e82: bx r2 0xa3522e84: lsrs r4, r7, #28 0xa3522e86: movs r0, r0 End of assembler dump. From here on, you can single-step through the program, print the contents of registers and memory, or tamper with them to explore the JNI function (which, in this case, simply returns a string).
Use the help command to get more information on debugging, running, and examining data. Execution Tracing Besides being useful for debugging, the JDB command line tool offers basic execution tracing functionality. To trace an app right from the start, you can pause the app with the Android 'Wait for Debugger' feature or a kill –STOP command and attach JDB to set a deferred method breakpoint on any initialization method. Once the breakpoint is reached, activate method tracing with the trace go methods command and resume execution. JDB will dump all method entries and exits from that point onwards. $ while true; do pid= $(pgrep 'targetprocess ' head -1 ); if -n ' $pid ' ; then strace -s 2000 - e '!read ' -ff -p ' $pid '; break; fi; done Ftrace Ftrace is a tracing utility built directly into the Linux kernel.
On a rooted device, ftrace can trace kernel system calls more transparently than strace can (strace relies on the ptrace system call to attach to the target process). Conveniently, the stock Android kernel on both Lollipop and Marshmallow include ftrace functionality. The feature can be enabled with the following command. $ echo 1 /proc/sys/kernel/ftraceenabled The /sys/kernel/debug/tracing directory holds all control and output files related to ftrace.
The following files are found in this directory:. availabletracers: This file lists the available tracers compiled into the kernel. currenttracer: This file sets or displays the current tracer. tracingon: Echo 1 into this file to allow/start update of the ring buffer. Echoing 0 will prevent further writes into the ring buffer. KProbes The KProbes interface provides an even more powerful way to instrument the kernel: it allows you to insert probes into (almost) arbitrary code addresses within kernel memory.
KProbes inserts a breakpoint instruction at the specified address. Once the breakpoint is reached, control passes to the KProbes system, which then executes the user-defined handler function(s) and the original instruction.
Besides being great for function tracing, KProbes can implement rootkit-like functionality, such as file hiding. Jprobes and Kretprobes are other KProbes-based probe types that allow hooking of function entries and exits. The stock Android kernel comes without loadable module support, which is a problem because Kprobes are usually deployed as kernel modules. The strict memory protection the Android kernel is compiled with is another issue becauseit prevents the patching of some parts of Kernel memory.
Elfmaster's system call hooking method causes a Kernel panic on stock Lollipop and Marshmallow because the syscalltable is non-writable. You can, however, use KProbes in a sandbox by compiling your own, more lenient Kernel (more on this later). Emulation-based Analysis The Android emulator is based on QEMU, a generic and open source machine emulator.
QEMU emulates a guest CPU by translating the guest instructions on-the-fly into instructions the host processor can understand. Each basic block of guest instructions is disassembled and translated into an intermediate representation called Tiny Code Generator (TCG). The TCG block is compiled into a block of host instructions, stored in a code cache, and executed. After execution of the basic block, QEMU repeats the process for the next block of guest instructions (or loads the already translated block from the cache). The whole process is called dynamic binary translation. Because the Android emulator is a fork of QEMU, it comes with all QEMU features, including monitoring, debugging, and tracing facilities. QEMU-specific parameters can be passed to the emulator with the -qemu command line flag.
Youcan use QEMU's built-in tracing facilities to log executed instructions and virtual register values. Starting qemu with the '-d' command line flag will cause it to dump the blocks of guest code, micro operations, or host instructions being executed. With the –dasm option, QEMU logs all basic blocks of guest code as they enter QEMU's translation function. The following command logs all translated blocks to a file. $ emulator -show-kernel -avd Nexus4API19 -snapshot default-boot -no-snapshot-save -qemu -d inasm,cpu 2/tmp/qemu.log 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. For example, if a block is repeatedly executed in a loop, only the first iteration will be printed to the log. There's no way to disable TB caching in QEMU (besides hacking the source code).
Nevertheless, the functionality is sufficient for basic tasks, such as reconstructing the disassembly of a natively executed cryptographic algorithm. Dynamic analysis frameworks, such as PANDA and DroidScope, build on QEMU's tracing functionality. PANDA/PANDROID is the best choice if you're going for a CPU-trace based analysis because it allows you to easily record and replay a full trace and is relatively easy to set up if you follow the build instructions for Ubuntu.
DroidScope DroidScope—an extension to the —is a malware analysis engine based on QEMU. It instrumentats the emulated environment on several context levels, making it possible to fully reconstruct the semantics on the hardware, Linux and Java levels. DroidScope exports instrumentation APIs that mirror the different context levels (hardware, OS, and Java) of a real Android device. Analysis tools can use these APIs to query or set information and register callbacks for various events. For example, a plugin can register callbacks for native instruction start and end, memory reads and writes, register reads and writes, system calls, and Java method calls. All of this makes it possible to build tracers that are practically transparent to the target application (as long as we can hide the fact that it is running in an emulator). One limitation is that DroidScope is compatible with the Dalvik VM only.
PANDA is another QEMU-based dynamic analysis platform. Similar to DroidScope, PANDA can be extended by registering callbacks that are triggered by certain QEMU events. The twist PANDA adds is its record/replay feature. This allows an iterative workflow: the reverse engineer records an execution trace of the target app (or some part of it), then replays it repeatedly, refining the analysis plugins with each iteration. PANDA comes with pre-made plugins, including a stringsearch tool and a syscall tracer. Most importantly, it supports Android guests, and some of the DroidScope code has even been ported. Building and running PANDA for Android ('PANDROID') is relatively straightforward.
To test it, clone Moiyx's git repository and build PANDA. $ cd qemu $./configure -target-list=arm-softmmu -enable-android $ makee As of this writing, Android versions up to 4.4.1 run fine in PANDROID, but anything newer than that won't boot. Also, the Java level introspection code only works on the Android 2.3 Dalvik runtime.
Older versions of Android seem to run much faster in the emulator, so sticking with Gingerbread is probably best if you plan to use PANDA. For more information, check out the extensive documentation in the PANDA git repo. VxStripper Another very useful tool built on QEMU is. VXStripper is specifically designed for de-obfuscating binaries. By instrumenting QEMU's dynamic binary translation mechanisms, it dynamically extracts an intermediate representation of a binary.
It then applies simplifications to the extracted intermediate representation and recompiles the simplified binary with LLVM. This is a very powerful way of normalizing obfuscated programs. See for more information.
Tampering and Runtime Instrumentation First, we'll look at some simple ways to modify and instrument mobile apps. Tampering means making patches or run-time changes to the app to affect its behavior. For example, you may want to deactivate SSL pinning or binary protections that hinder the testing process. Runtime Instrumentation encompasses adding hooks and runtime patches to observe the app's behavior.
In mobile app-sec however, the term loosely refers to all kinds of run-time manipulation, including overriding methods to change behavior. Patching and Re-Packaging Making small changes to the app Manifest or bytecode is often the quickest way to fix small annoyances that prevent you from testing or reverse engineering an app. On Android, two issues in particular happen regularly:. You can't attach a debugger to the app because the android:debuggable flag is not set to true in the Manifest.
You can't intercept HTTPS traffic with a proxy because the app employs SSL pinning. In most cases, both issues can be fixed by making minor changes to the app and then re-signing and re-packaging it. Apps that run additional integrity checks beyond default Android code-signing are an exception—in these cases, you have to patch the additional checks as well. Example: Disabling Certificate Pinning Certificate pinning is an issue for security testers who want to intercept HTTPS communication for legitimate reasons. Patching bytecode to deactivate SSL pinning can help with this. To demonstrate bypassing certificate pinning, we'll walk through an implementation in an example application.
The first step is disassembling the APK with apktool. $ apktool d targetapk.apk You then locate the certificate pinning checks in the Smali source code. Searching the code for keywords such as 'X509TrustManager' should point you in the right direction.
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In our example, a search for 'X509TrustManager' returns one class that implements a custom Trustmanager. The derived class implements the methods checkClientTrusted, checkServerTrusted, and getAcceptedIssuers. To bypass the pinning check, add the return-void opcode to the first line of each method. This opcode causes the checks to return immediately. With this modification, no certificate checks are performed, and the application accepts all certificates. Michael@sixtyseven: /Development/frida$ frida -U -l uncrackable1.js sg.vantagepoint.uncrackable1 / Frida 9.1.16 - A world-class dynamic instrumentation framework ( Commands: // help - Displays the help system.
Display information about 'object '. Exit/quit - Exit. More info at.
Starting script USB::Android Emulator 5554::sg.vantagepoint.uncrackable1-. onClick handler modified. sg.vantagepoint.a.a.a modified. onClick called. Decrypted: I want to believe The hooked function outputted the decrypted string.
You extracted the secret string without having to dive too deep into the application code and its decryption routines. You've now covered the basics of static/dynamic analysis on Android. Of course, the only way to really learn it is hands-on experience: build your own projects in Android Studio, observe how your code gets translated into bytecode and native code, and try to crack our challenges. In the remaining sections, we'll introduce a few advanced subjects, including kernel modules and dynamic execution. Binary Analysis Frameworks Binary analysis frameworks give you powerful ways to automate tasks that would be almost impossible to do manually. In this section, we'll look at Angr, a Python framework for analyzing binaries. It is useful for both static and dynamic symbolic ('concolic') analysis.
Angr operates on the VEX intermediate language and comes with a loader for ELF/ARM binaries, so it is perfect for dealing with native Android binaries. Our target program is a simple license key validation program. Granted, you won't usually find license key validators like this, but the example should demonstrate the basics of static/symbolic analysis of native code. 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). Installing Angr Since version 8 Angr is based on Python 3, and it's available from PyPI.
With pip, it's easy to install on.nix operating systems and Mac OS. $ pip install angr Creating a dedicated virtual environment with Virtualenv is recommended because some of its dependencies contain forked versions Z3 and PyVEX, which overwrite the original versions. You can skip this step if you don't use these libraries for anything else.
Comprehensive documentation, including an installation guide, tutorials, and usage examples, is available on Gitbooks. A complete API reference is also available. Using the Disassembler Backends - Symbolic Execution Symbolic execution allows you to determine the conditions necessary to reach a specific target. It translates the program's semantics into a logical formula in which some variables are represented by symbols with specific constraints. By resolving the constraints, you can find the conditions necessary for the execution of some branch of the program.
Symbolic execution is useful when you need to find the right input for reaching a certain block of code. In the following example, you'll use Angr to solve a simple Android crackme in an automated fashion. The crackme takes the form of a native ELF binary that you can download here: Running the executable on any Android device should give you the following output. (angr) $ python solve.py WARNING 2017-01-09 17:17:03,664 cle.loader The main binary is a position-independent executable. It is being loaded with a base address of 0x400000. JQAE6ACMABNAAIIA Customizing Android for Reverse Engineering Working on real devices has advantages, especially for interactive, debugger-supported static/dynamic analysis.
For example, working on a real device is simply faster. Also, Running the target app on a real device is less likely to trigger defenses. 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. Customizing the RAMDisk Initramfs is a small CPIO archive stored inside the boot image. It contains a few files that are required at boot, before the actual root file system is mounted.
On Android, initramfs stays mounted indefinitely. It contains an important configuration file, default.prop, that defines some basic system properties. Changing this file can make the Android environment easier to reverse engineer. For our purposes, the most important settings in default.prop are ro.debuggable and ro.secure. $ cd initrd $ find.
cpio -create -format= 'newc ' gzip./myinitd.img Customizing the Android Kernel The Android kernel is a powerful ally to the reverse engineer. 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. This gives you an advantage because most integrity checks and anti-tampering features ultimately rely on services performed by the kernel. 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. Android apps have several ways to interact with the OS. Interacting through the Android Application Framework's APIs is standard.
At the lowest level, however, many important functions (such as allocating memory and accessing files) are translated into old-school Linux system calls. On ARM Linux, system calls are invoked via the SVC instruction, which triggers a software interrupt. This interrupt calls the vectorswi kernel function, which then uses the system call number as an offset into a table (known as syscalltable on Android) of function pointers. The most straightforward way to intercept system calls is to inject your own code into kernel memory, then overwrite the original function in the system call table to redirect execution. Unfortunately, current stock Android kernels enforce memory restrictions that prevent this. Specifically, stock Lollipop and Marshmallow kernels are built with the CONFIGSTRICTMEMORYRWX option enabled. 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.
To get around this, build your own kernel. You can then deactivate this protection and make many other useful customizations that simplify reverse engineering. If you reverse Android apps on a regular basis, building your own reverse engineering sandbox is a no-brainer. For hacking, I recommend an AOSP-supported device. Google's Nexus smartphones and tablets are the most logical candidates because kernels and system components built from the AOSP run on them without issues.
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Sony's Xperia series is also known for its openness. 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. Follow Google's instructions to identify the correct git repo and branch for a given device and Android version. For example, to get kernel sources for Lollipop that are compatible with the Nexus 5, you need to clone the msm repo and check out one of the android-msm-hammerhead branches (hammerhead is the codename of the Nexus 5, and finding the right branch is confusing). Once you have downloaded the sources, create the default kernel config with the command make hammerheaddefconfig (replacing 'hammerhead' with your target device).
$ git clone $ cd msm $ git checkout origin/android-msm-hammerhead-3.4-lollipop-mr1 $ export ARCH=arm $ export SUBARCH=arm $ make hammerheaddefconfig $ vim.config I recommend using the following settings to add loadable module support, enable the most important tracing facilities, and open kernel memory for patching. CONFIGMODULES=Y CONFIGSTRICTMEMORYRWX=N CONFIGDEVMEM=Y CONFIGDEVKMEM=Y CONFIGKALLSYMS=Y CONFIGKALLSYMSALL=Y CONFIGHAVEKPROBES=Y CONFIGHAVEKRETPROBES=Y CONFIGHAVEFUNCTIONTRACER=Y CONFIGHAVEFUNCTIONGRAPHTRACER=Y CONFIGTRACING=Y CONFIGFTRACE=Y CONFIG KDB=Y Once you're finished editing save the.config file, build the kernel. $ fastboot boot zImage-dtb initrd.img -base 0 -kernel-offset 0x8000 -ramdisk-offset 0x2900000 -tags-offset 0x2700000 -c 'console=ttyHSL0,115200,n8 androidboot.hardware=hammerhead userdebug=31 maxcpus=2 msmwatchdogv2.enable=1 ' The system should now boot normally.
To quickly verify that the correct kernel is running, navigate to Settings-About phone and check the 'kernel version' field. System Call Hooking with Kernel Modules System call hooking allows you to attack any anti-reversing defenses that depend on kernel-provided functionality. With your custom kernel in place, you can now use an LKM to load additional code into the kernel. You also have access to the /dev/kmem interface, which you can use to patch kernel memory on-the-fly. This is a classic Linux rootkit technique that has been described for Android by Dong-Hoon You 1.
You first need the address of syscalltable. Fortunately, it is exported as a symbol in the Android kernel (iOS reversers aren't so lucky).
You can look up the address in the /proc/kallsyms file. $ adb shell 'su -c echo ABCD /data/local/tmp/nowyouseeme ' $ adb shell cat /data/local/tmp/nowyouseeme ABCD It's time to write the kernel module. For file-hiding, you'll need to hook one of the system calls used to open (or check for the existence of) files. There are many of these—open, openat, access, accessat, facessat, stat, fstat, etc. For now, you'll only hook the openat system call.
This is the syscall the /bin/cat program uses when accessing a file, so the call should be suitable for a demonstration. You can find the function prototypes for all system calls in the kernel header file arch/arm/include/asm/unistd.h. Create a file called kernelhook.c with the following code. $ make (.) $ adb push kernelhook.ko /data/local/tmp/ 100% /data/local/tmp/kernelhook.ko $ adb shell su -c insmod /data/local/tmp/kernelhook.ko $ adb shell lsmod kernelhook 1160 0 permanent, Live 0xbf000000 (PO) Now you'll access /dev/kmem to overwrite the original function pointer in syscalltable with the address of your newly injected function (this could have been done directly in the kernel module, but /dev/kmem provides an easy way to toggle your hooks on and off). I have adapted the code from for this purpose.
However, I used the file interface instead of mmap because I found that the latter caused kernel panics. Create a file called kmemutil.c with the following code.