# Hermes Dec Bytecode Reverse Engineering > Disassemble and decompile React Native Hermes bytecode (HBC) files for reverse engineering - Skill: `aradotso/hermes-dec-bytecode-reverse-engineering` (Agent Skill) - Install (CLI): `npx skillmds@latest add aradotso/hermes-dec-bytecode-reverse-engineering` - Raw SKILL.md: https://api.skillmd.com/api/skills/aradotso/hermes-dec-bytecode-reverse-engineering/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Web & Frontend - Author: aradotso (https://skillmd.com/u/aradotso) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/aradotso/hermes-dec-bytecode-reverse-engineering --- # Hermes-Dec Bytecode Reverse Engineering > Skill by [ara.so](https://ara.so) — Hermes Skills collection. ## Overview `hermes-dec` is a reverse-engineering tool for disassembling and decompiling React Native applications compiled to Hermes VM bytecode (HBC) format. It provides three main utilities: - **hbc-file-parser**: Parse and display HBC file headers and metadata - **hbc-disassembler**: Convert bytecode to assembly-like representation (.hasm) - **hbc-decompiler**: Decompile bytecode to pseudo-JavaScript code Hermes is the default JavaScript engine for React Native on Android (since RN 0.70) and is commonly found in mobile apps at `assets/index.android.bundle`. ## Installation ```bash # Using pipx (recommended for CLI tools) pipx install hermes-dec # Using uv uv tool install hermes-dec # Using pip pip install hermes-dec # From source git clone https://github.com/P1sec/hermes-dec.git cd hermes-dec pip install -e . ``` ### System-specific installations ```bash # Ubuntu (Snap) sudo snap install hermes-dec # Commands become: hermes-dec.hbc-disassembler, hermes-dec.hbc-file-parser, hermes-dec.hbc-decompiler # Arch Linux (AUR) yay -S hermes-dec ``` ## Extracting Hermes Bytecode from APK React Native Android apps package Hermes bytecode in APK files. Extract it first: ```bash # APK files are ZIP archives unzip my_application.apk -d my_application/ cd my_application/ # Verify it's Hermes bytecode file assets/index.android.bundle # Output: assets/index.android.bundle: Hermes JavaScript bytecode, version 84 # Or use 7z 7z x my_application.apk ``` Common locations in APK: - `assets/index.android.bundle` (main bundle) - `assets/*.hbc` (additional bundles) - `assets/index.android.bundle.hbc` ## Core Commands ### 1. Parse File Headers Inspect HBC file structure, bytecode version, and metadata: ```bash # Display file information to stdout hbc-file-parser assets/index.android.bundle # Save output to file hbc-file-parser assets/index.android.bundle > analysis.txt ``` **Example output:** ``` Hermes Bytecode File v84 File size: 2,453,678 bytes SHA1 hash: a1b2c3d4... Number of functions: 1,234 String table size: 45,678 entries Debug info: present ``` ### 2. Disassemble to Assembly Convert bytecode to human-readable assembly (.hasm format): ```bash # Output to stdout hbc-disassembler assets/index.android.bundle # Save to file hbc-disassembler assets/index.android.bundle output.hasm # Typical workflow hbc-disassembler index.android.bundle disassembled.hasm ``` **Example .hasm output:** ```assembly Function(3 params, 15 registers): Offset in debug table: source 0x0000 LoadParam r2, 1 LoadParam r3, 2 LoadConstString r1, "username" GetByVal r0, r2, r1 LoadConstString r1, "password" GetByVal r4, r2, r1 Call2 r0, r3, r0, r4 Ret r0 ``` ### 3. Decompile to Pseudo-JavaScript Generate pseudo-JavaScript code (not yet valid JS, missing control flow): ```bash # Output to stdout hbc-decompiler assets/index.android.bundle # Save to file hbc-decompiler assets/index.android.bundle decompiled.js # Typical workflow hbc-decompiler index.android.bundle output.js ``` **Example decompiled output:** ```javascript function loginUser(param0, param1, param2) { var r0, r1, r2, r3, r4; r2 = param1; r3 = param2; r1 = "username"; r0 = r2[r1]; r1 = "password"; r4 = r2[r1]; r0 = r3(r0, r4); return r0; } ``` ## Practical Workflows ### Full Analysis Pipeline ```bash #!/bin/bash # analyze_hbc.sh - Complete Hermes bytecode analysis APK_FILE="$1" OUTPUT_DIR="analysis_$(date +%Y%m%d_%H%M%S)" mkdir -p "$OUTPUT_DIR" # Extract APK echo "[+] Extracting APK..." unzip -q "$APK_FILE" -d "$OUTPUT_DIR/extracted" # Find HBC files echo "[+] Locating HBC files..." find "$OUTPUT_DIR/extracted" -name "*.bundle" -o -name "*.hbc" > "$OUTPUT_DIR/hbc_files.txt" # Process each HBC file while IFS= read -r hbc_file; do basename=$(basename "$hbc_file") echo "[+] Processing $basename..." # Parse headers hbc-file-parser "$hbc_file" > "$OUTPUT_DIR/${basename}.info.txt" # Disassemble hbc-disassembler "$hbc_file" "$OUTPUT_DIR/${basename}.hasm" # Decompile hbc-decompiler "$hbc_file" "$OUTPUT_DIR/${basename}.js" done < "$OUTPUT_DIR/hbc_files.txt" echo "[+] Analysis complete in $OUTPUT_DIR" ``` ### Searching for Specific Functions ```bash # Disassemble and search for API endpoints hbc-disassembler index.android.bundle output.hasm grep -i "https://" output.hasm # Find authentication-related code grep -i -E "(token|auth|password|login)" output.hasm # Decompile and search for specific strings hbc-decompiler index.android.bundle output.js grep -i "api_key" output.js ``` ### Comparing Versions ```bash # Compare two versions of the same app hbc-disassembler old_version/index.android.bundle old.hasm hbc-disassembler new_version/index.android.bundle new.hasm # Diff the assembly diff -u old.hasm new.hasm > changes.diff # Or use a better diff tool code --diff old.hasm new.hasm ``` ## Python API Usage While hermes-dec is primarily a CLI tool, you can import its modules in Python: ```python #!/usr/bin/env python3 """ Example: Programmatic access to hermes-dec functionality """ import sys from pathlib import Path # Import hermes-dec modules (structure may vary by version) # Note: Internal API may change; CLI is the stable interface def analyze_hbc_file(hbc_path: Path): """ Analyze an HBC file programmatically """ if not hbc_path.exists(): print(f"Error: {hbc_path} not found") return # Read file header with open(hbc_path, 'rb') as f: magic = f.read(8) if magic[:4] != b'\xC6\x1F\xBC\x03': print("Not a valid Hermes bytecode file") return # Version is at offset 4 version = int.from_bytes(magic[4:8], 'little') print(f"Hermes bytecode version: {version}") # For detailed parsing, use the CLI tools import subprocess # Get file info result = subprocess.run( ['hbc-file-parser', str(hbc_path)], capture_output=True, text=True ) if result.returncode == 0: print("\n=== File Info ===") print(result.stdout) if __name__ == "__main__": if len(sys.argv) < 2: print("Usage: python analyze.py ") sys.exit(1) analyze_hbc_file(Path(sys.argv[1])) ``` ### Batch Processing Script ```python #!/usr/bin/env python3 """ Batch process multiple APK files for Hermes bytecode analysis """ import subprocess import zipfile from pathlib import Path from typing import List def extract_hbc_from_apk(apk_path: Path, output_dir: Path) -> List[Path]: """ Extract HBC files from an APK Returns list of extracted HBC file paths """ hbc_files = [] output_dir.mkdir(parents=True, exist_ok=True) with zipfile.ZipFile(apk_path, 'r') as apk: for name in apk.namelist(): if name.endswith('.bundle') or name.endswith('.hbc'): # Extract file target = output_dir / Path(name).name with apk.open(name) as source, open(target, 'wb') as dest: dest.write(source.read()) # Verify it's HBC if is_hermes_bytecode(target): hbc_files.append(target) print(f"[+] Extracted {name} -> {target}") return hbc_files def is_hermes_bytecode(file_path: Path) -> bool: """Check if file is Hermes bytecode""" try: with open(file_path, 'rb') as f: magic = f.read(4) return magic == b'\xC6\x1F\xBC\x03' except: return False def decompile_hbc(hbc_path: Path, output_path: Path): """Decompile HBC file to pseudo-JavaScript""" subprocess.run( ['hbc-decompiler', str(hbc_path), str(output_path)], check=True ) print(f"[+] Decompiled {hbc_path.name} -> {output_path}") def main(): apk_dir = Path("apks") output_base = Path("decompiled") for apk in apk_dir.glob("*.apk"): print(f"\n[*] Processing {apk.name}") # Create output directory for this APK apk_output = output_base / apk.stem apk_output.mkdir(parents=True, exist_ok=True) # Extract HBC files hbc_files = extract_hbc_from_apk(apk, apk_output / "extracted") # Decompile each for hbc in hbc_files: js_output = apk_output / f"{hbc.stem}.js" try: decompile_hbc(hbc, js_output) except subprocess.CalledProcessError as e: print(f"[!] Failed to decompile {hbc.name}: {e}") if __name__ == "__main__": main() ``` ## Understanding Output Formats ### Assembly (.hasm) Format The disassembler outputs Hermes VM assembly instructions: ```assembly Function(params, registers): # Instruction Dest Source1 Source2 LoadParam r0, 1 # Load first parameter LoadConstString r1, "key" # Load string constant GetByVal r2, r0, r1 # Property access: r0[r1] Call r3, r2 # Function call Ret r3 # Return value ``` Common opcodes: - `LoadParam`: Load function parameter - `LoadConst*`: Load constants (string, number, undefined, etc.) - `GetByVal`/`PutByVal`: Property access - `Call*`: Function calls - `NewObject`/`NewArray`: Object/array creation - `Ret`: Return from function - `Jmp*`: Jump instructions (control flow) ### Pseudo-JavaScript Format The decompiler outputs register-based pseudo-code: ```javascript // Not yet valid JavaScript - shows data flow function example(param0, param1) { var r0, r1, r2, r3; // Registers r0 = param0; // Parameter assignment r1 = "property"; // String constant r2 = r0[r1]; // Property access r3 = someFunc(r2); // Function call return r3; } ``` **Limitations:** - No control flow reconstruction (loops, conditionals appear linear) - Register-based (not variable-based) - May have dead code - Requires manual cleanup for valid JS ## Troubleshooting ### Not a Hermes Bytecode File ```bash $ file index.android.bundle index.android.bundle: ASCII text, with very long lines ``` **Solution**: This is minified JavaScript, not Hermes bytecode. The app doesn't use Hermes engine or you have the wrong file. ### Unsupported Bytecode Version ``` Error: Unsupported bytecode version 96 ``` **Solution**: Update hermes-dec to the latest version: ```bash pipx upgrade hermes-dec # or pip install --upgrade hermes-dec ``` If still unsupported, check the [GitHub issues](https://github.com/P1sec/hermes-dec/issues) or open a new one. ### Incomplete Decompilation The decompiler output is incomplete or has missing functions. **Solution**: Try disassembly first to understand structure: ```bash # Disassemble to see raw instructions hbc-disassembler file.bundle output.hasm # Then manually interpret critical sections # The decompiler is best-effort; some complex patterns may not decompile ``` ### Empty Output Files ```bash $ hbc-decompiler index.android.bundle output.js $ wc -l output.js 0 output.js ``` **Solution**: Check file format and permissions: ```bash # Verify it's HBC file index.android.bundle hexdump -C index.android.bundle | head # Check permissions ls -la index.android.bundle # Try with stdout first hbc-decompiler index.android.bundle ``` ### APK Extraction Issues ```bash $ unzip app.apk error: cannot find or open app.apk ``` **Solution**: Ensure APK is not corrupted: ```bash # Verify ZIP structure 7z t app.apk # Try alternative extraction jar xf app.apk # or python -m zipfile -e app.apk output_dir/ ``` ## Advanced Techniques ### Finding Obfuscated Strings ```bash # Decompile and search for base64 hbc-decompiler index.android.bundle output.js grep -E '[A-Za-z0-9+/]{20,}={0,2}' output.js # Look for hex patterns grep -E '0x[0-9a-fA-F]{4,}' output.js ``` ### Extracting API Endpoints ```bash # Search for URLs in disassembly hbc-disassembler index.android.bundle output.hasm grep -oE 'https?://[^"]+' output.hasm | sort -u > endpoints.txt # Find in decompiled code hbc-decompiler index.android.bundle output.js grep -oE '(http|https)://[a-zA-Z0-9./?=_-]*' output.js | sort -u ``` ### Cross-referencing Functions ```bash # Find function definitions and calls hbc-disassembler index.android.bundle output.hasm grep "Function<" output.hasm > functions.txt grep "Call" output.hasm > calls.txt # Analyze function call graph awk '/Function " $0}' output.hasm ``` ## Documentation Resources - [Hermes VM Design Docs](https://hermesengine.dev/docs/vm/) - [Bytecode Format Spec](https://github.com/facebook/hermes/tree/main/doc) - [Opcode Reference](https://p1sec.github.io/hermes-dec/opcodes_table.html) - [React Native Hermes Guide](https://reactnative.dev/docs/hermes) - [P1sec Blog Post](https://www.p1sec.com/blog/releasing-hermes-dec-an-open-source-disassembler-and-decompiler-for-the-react-native-hermes-bytecode) ## Version Compatibility Hermes bytecode versions and React Native releases: - v59: Hermes 0.1.0 (July 2019) - v74: React Native 0.64 (iOS support) - v84: React Native 0.70 (default on Android) - v90+: Recent React Native versions (2023+) Check bytecode version: ```bash hbc-file-parser file.bundle | grep -i version ```