Android Decompilers and Countermeasures Quiz

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Questions and Answers

Which of the following is NOT a pure Dalvik decompiler?

dex2jar
Androguard's DAD
JEB
Apktool (correct)

What is the primary benefit of using a pure Dalvik decompiler over a traditional Java decompiler for Android apps?

Pure Dalvik decompilers produce better decompiled output by skipping the conversion to Java bytecode (correct)
Pure Dalvik decompilers are faster than Java decompilers
Pure Dalvik decompilers are open source, while Java decompilers are typically commercial
Pure Dalvik decompilers are better at handling obfuscated code

Which of the following is a recommended countermeasure against decompiling, disassembly, and repackaging of Android apps?

Avoid using any obfuscation techniques
Obfuscate the source code using tools like ProGuard or Arxan (correct)
Store sensitive information on the client-side
Rely on client-side authentication and validation

What is the primary purpose of ProGuard in the context of Android app development?

It is an open-source optimizer and obfuscator for Java bytecode (B) Signup and view all the answers

Which of the following tools is commonly used for decompiling Android apps to Java source code?

dex2jar and JD-GUI (D) Signup and view all the answers

What is the purpose of using declared registers in Dalvik bytecode?

To specify object types (D) Signup and view all the answers

What is the main drawback of transforming DEX to JAR when decompiling Android apps?

Loss of important metadata (B) Signup and view all the answers

Which tool is commonly used to sign an Android app with a generated key?

jarsigner (A) Signup and view all the answers

What is a potential malicious action that can be performed by inserting Trojan code into an Android app?

Modify variables for client-side authentication bypass (D) Signup and view all the answers

Which type of register in Dalvik bytecode is used for parameters?

p0 (D) Signup and view all the answers

What is the consequence of changing variables to bypass client-side authentication or input validation in an Android app?

Security vulnerabilities may be introduced (D) Signup and view all the answers

What is the primary reason for decompiling Android apps to an intermediate format like Smali rather than directly to Java?

Decompiling to an intermediate format preserves app functionality, while decompiling to Java does not. (C) Signup and view all the answers

Which of the following tools is used to convert the Dalvik Executable (DEX) format used by Android to the Java Archive (JAR) format?

Dex2jar (B) Signup and view all the answers

What is the primary advantage of using a runtime debugger to manipulate an Android app, compared to decompiling and modifying the app's code directly?

Using a runtime debugger allows you to observe and modify the app's behavior in real-time. (A) Signup and view all the answers

Which of the following statements is true regarding decompiling an Android app to Java bytecode?

Decompiling an Android app to Java bytecode is more readable than decompiling to Smali. (D) Signup and view all the answers

Which of the following tools is used to view the decompiled Java bytecode of an Android app after converting it from the DEX format using Dex2jar?

JD-GUI (A) Signup and view all the answers

What is the purpose of the `Baksmali` tool in the context of decompiling Android apps?

Baksmali is used to decompile the Dalvik bytecode of an Android app to Smali assembly code. (B) Signup and view all the answers

Which of the following is a potential countermeasure against command injection vulnerabilities in Android apps?

Performing input validation on all data received from Intents and blocking special characters. (C) Signup and view all the answers

What is the primary disadvantage of using open-source or free application analysis tools for Android apps?

Open-source tools may be buggy or problematic, as they are developed by enthusiasts in their spare time. (C) Signup and view all the answers

Which of the following statements is true regarding the decompilation of iOS apps?

iOS apps are generally more difficult to decompile than Android apps due to the additional security measures implemented by Apple. (A) Signup and view all the answers

What is the primary purpose of the `Apktool` utility in the context of decompiling Android apps?

Apktool is used to decompile the Dalvik bytecode of an Android app to Smali assembly code. (A) Signup and view all the answers

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Study Notes

Mobile Application Analysis and Reverse Engineering

Application analysis provides security information, but manual analysis is time-consuming and requires a developed skill set.
Automated analysis can be done using third-party tools, which are faster but often limited and buggy.

Application Analysis Tools

Snoop-it: a user-friendly runtime iOS application analysis tool that requires a jailbroken iOS device.
iNalyzer: a tool that decrypts an app installed on an iOS 6.1 device and generates a HTML interface to explore classes, plist files, strings, and view controllers.
Xcode Clang Static Analyzer: a direct static analysis tool that integrates with the Xcode build system and helps developers.
Androwarn: an automated static analysis tool for Android applications that reads APK files and reports on potentially malicious behavior.

Android Components and Intents

Android components are declared in the AndroidManifest.xml file, including filters and who/what can invoke the intent.
Parameters for Intent (Extras) are not declared and must be retrieved from decompiled source.
Intents are the primary inter-process communication (IPC) method used by Android apps.
Malicious intents can launch other apps and inject data into them.

Command Injection

Command injection can occur when an app saves files on the SD card with a vulnerable Intent Filter.
Exploitation can expose secrets the app can access.
Countermeasures include not making Intents "exported" unless necessary, performing input validation, and blocking special characters in filenames.

Decompilation and Disassembly

Decompilation and disassembly can be used to analyze binary, compiled apps.
Smali assembly code is used for Android apps.
iOS code is generally difficult to decompile.
Decompiling an Android app to Java is possible, but not recompiling from Java.
Apktool and Dex2jar can be used to decompile Android apps to bytecode.

Decompilation Tools

Baksmali/Smali: an intermediate disassembler/assembler designed for Dalvik executables.
Java vs. Pure Dalvik decompilers: Java decompilers skip the step of converting to bytecode, producing better output.
Open-source decompilers include Androguard's DAD, while commercial options include JEB.

Countermeasures

Every binary can be reverse-engineered given enough time and effort.
Never store secrets on the client-side or rely on client-side authentication or validation.
Obfuscate the source code using tools like ProGuard or Arxan.

Dalvik Bytecode

Dalvik bytecode uses declared registers as placeholders for objects, variables.
Syntax is always destination, then source.
Object types are specified with capital letter identifying type.
Types in Dalvik bytecode use uppercase letter to denote type.

Signing an Android App

Create a key with keytool and sign with jarsigner.
Keytool is used to generate a key, while jarsigner is used to sign the APK file.

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