Reverse engineering ARM binaries involves taking apart and analyzing ARM executable files to understand how they work. This allows researchers and developers to dig into closed-source ARM software, find vulnerabilities, modify behavior, or recreate functionality. The key steps in reverse engineering ARM binaries include:
1. Obtaining the ARM Binary
The first step is to acquire the ARM binary executable you want to reverse engineer. This may be a closed-source application, firmware image, kernel driver, or other ARM program. You’ll need the actual binary file, not just the installed application. ARM binaries typically have file extensions like .exe, .ko, .fw, or custom extensions.
2. Determining the ARM Architecture
There are many ARM architectures, like ARMv7, ARMv8, Cortex-A53 etc. You’ll need to identify which specific architecture the binary targets. This determines the instruction set and calling conventions used. Tools like ArmBinCheck can automate this process. You may also find architecture info in firmware header files.
3. Selecting a Disassembler
A disassembler converts binary machine code into human-readable assembly code. Good disassemblers for ARM include IDA Pro, Ghidra, Radare2 and Hopper. They can parse ARM executable formats like ELF and PE. IDA Pro has the most ARM support but is expensive. Ghidra is an open source alternative from the NSA.
4. Disassembling the ARM Code
Feed your ARM binary into the disassembler tool. It will parse the executable format and extract the machine code instructions and data into an intermediate database. You can then explore and navigate through the disassembled ARM assembly code.
5. Analyzing the Disassembly
Carefully study the disassembled code to understand the program logic and flow. Look for key functions, APIs, strings and data references that may offer clues. Important areas to focus on are entrypoints, wrappers, dispatch tables and security checks.
6. Identifying Functional Code Blocks
Divide code into functional blocks like functions, loops, if/else blocks etc. Name or annotate these blocks based on what they appear to do. This creates a hierarchical understanding of the code structure and flow. Use cross-references and string references to identify functions.
7. Linking Code to Behaviors
Connect the code logic to actual program behaviors by debugging or dynamic analysis. Set breakpoints at key code locations, then observe results as you step through the executable. Modify input data and code to see impact on outputs.
8. Documenting Findings
As you analyze, document your findings on code purpose, data structures, algorithms, vulnerabilities, etc. Keep detailed notes on a code wiki page. Your insights will guide how you interact with or modify the binary in later stages.
9. Modifying and Augmenting
With detailed understanding of the disassembled code, you can now start modifying it or adding new functionality. Be very careful making code changes to avoid crashes or bugs. Use insights from earlier phases to surgically modify logic and data.
10. Reassembling and Testing
After making changes, reassemble the code using the disassembler or an ARM assembler. This generates a modified ARM executable to test and validate your code changes. You may need to fix up addresses and offsets impacted by modifications.
Reverse engineering ARM binaries takes time and advanced skills. But the insight it provides into closed-source ARM software makes it a valuable endeavor for researchers, hackers, and developers. With the right tools and techniques, determined engineers can dissect nearly any ARM binary executable.
Challenges You May Face
Here are some common challenges faced when reverse engineering ARM binaries and how to tackle them:
Vendors often obfuscate code to deliberately make reverse engineering harder. Look for patterns like opaque predicate branches, overlapped instructions, intermixed data and code, and flattened control flows. Use emulation or symbolic execution to simplify the code logic.
Key symbols like function and variable names are stripped out to hide purpose. Give your own descriptive names to code blocks during analysis. Look for clues in string references. In some cases, you may be able to recover symbols from debug files.
Code will actively detect and block debugging or emulation using tactics like debugger checks, timing checks, instruction traps. Anti-debugging defenses can be spotted and neutralized by patching checks or via dynamic instrumentation.
The code may rely on specific ARM chip features for timing, random numbers, or instructions. Emulate dependent hardware via QEMU models orHelpers when running on different ARM platforms. Analyze code carefully to find hardware touch points.
Lack of Documentation
Unlike open source projects, closed-source ARM code lacks internal documentation. Lean heavily on static analysis of the binaries themselves. Also seek clues from related codebases or external product docs and guides.
With patience and advanced disassembly skills, you can work around these challenges to crack open nearly any ARM binary. The reward is worth the effort.
Reverse engineering raises potential legal concerns:
- Respect copyright and license terms of commercial ARM software you analyze.
- Do not redistribute disassembled code without permission if it contains copyrighted code or data.
- Be careful not to violate anti-circumvention laws by bypassing technical protections.
- Only use and share binaries you have legal rights to, avoid pirated code.
- Do not violate company policies by reverse engineering work software.
Stay ethical and legal during your research. Only share findings in educational contexts and do not use insights for malicious aims.
Tools of the Trade
Here are some essential tools for effectively reverse engineering ARM binaries:
- Disassemblers: IDA Pro, Ghidra, Radare2, Hopper, Capstone
- Debuggers: IDA Pro, GDB, Medusa, Immunity Debugger
- Emulators: QEMU, Unicorn Engine
- Decompilers: Hex-Rays, Ghidra, RetDec
- Utilities: ObjectDump, ArmBinCheck, Binwalk, Jadx
Using a combination of static and dynamic analysis tools gives you maximum insight into ARM binary code and data.
For further learning on reverse engineering ARM binaries, check out these resources:
- Open Security Training’s Intro to ARM Reverse Engineering video course
- Beginner’s Guide to Reversing ARM Binaries on Azeria Labs
- Reverse Engineering ARM Course on Binary Security
- ARM Assembly Basics on Azeria Labs
- Exploit Database for ARM binary samples
Remember to always dig into binaries legally and ethically. Reverse engineering unlocks unique insights that can benefit determined developers, researchers, and hackers.