seikaikyo/performing-firmware-extraction-with-binwalk

Performing Firmware Extraction with Binwalk

When to Use

• Analyzing IoT device firmware downloaded from vendor sites or extracted from flash chips
• Reverse engineering router, camera, or embedded device firmware for vulnerability research
• Identifying embedded filesystems (SquashFS, CramFS, JFFS2, UBIFS) within firmware blobs
• Detecting encrypted or compressed regions using entropy analysis
• Extracting hardcoded credentials, API keys, certificates, or configuration files from firmware
• Performing security assessments of embedded devices in authorized penetration tests

Do not use for analyzing standard desktop application binaries or malware samples that are not firmware images; use dedicated malware analysis tools instead.

Prerequisites

• binwalk v3.x installed (pip install binwalk3 or from system package manager)
• Python 3.8+ with standard libraries (struct, math, hashlib, subprocess)
• SquashFS tools (unsquashfs) for mounting extracted SquashFS filesystems
• Jefferson for JFFS2 filesystem extraction (pip install jefferson)
• Sasquatch for non-standard SquashFS variants used by vendors like TP-Link and D-Link
• strings utility (GNU binutils) for string extraction
• Optional: firmware-mod-kit for repacking modified firmware images

Workflow

Step 1: Initial Firmware Reconnaissance

Perform a signature scan to identify embedded file types and their offsets:

# Basic signature scan - identify all recognized file types
binwalk firmware.bin

# Scan with verbose output showing confidence levels
binwalk -v firmware.bin

# Scan for specific file types only
binwalk -y "squashfs" firmware.bin
binwalk -y "gzip\|lzma\|xz" firmware.bin

# Opcode scan to identify CPU architecture
binwalk -A firmware.bin

# Scan for raw strings to find version info, URLs, credentials
binwalk -R "password" firmware.bin
binwalk -R "http://" firmware.bin

Step 2: Entropy Analysis

Analyze entropy to identify encrypted, compressed, and plaintext regions:

# Generate entropy plot
binwalk -E firmware.bin

# Entropy with specific block size for higher resolution
binwalk -E -K 256 firmware.bin

# Combined entropy and signature scan
binwalk -BE firmware.bin

Interpreting entropy values:

• 0.0 - 1.0: Empty or padding regions (null bytes, 0xFF fill)
• 1.0 - 5.0: Plaintext data, code, ASCII strings, configuration
• 5.0 - 7.0: Compressed data (gzip, LZMA, zlib)
• 7.0 - 7.99: Strongly compressed or encrypted data
• ~8.0: Maximum entropy, likely encrypted or random data

Step 3: Extract Embedded Files

Extract all identified components from the firmware image:

# Automatic extraction of known file types
binwalk -e firmware.bin

# Recursive extraction (matryoshka mode) for nested archives
binwalk -Me firmware.bin

# Recursive extraction with depth limit
binwalk -Me -d 5 firmware.bin

# Extract specific file type with custom handler
binwalk -D "squashfs filesystem:squashfs:unsquashfs %e" firmware.bin

# Manual extraction of data at a known offset
dd if=firmware.bin of=extracted.squashfs bs=1 skip=327680 count=4194304

Step 4: Mount and Inspect Extracted Filesystems

Mount extracted filesystems for deep inspection:

# Mount SquashFS filesystem
mkdir /tmp/squashfs_root
unsquashfs -d /tmp/squashfs_root extracted.squashfs

# Mount CramFS filesystem
mkdir /tmp/cramfs_root
mount -t cramfs -o loop extracted.cramfs /tmp/cramfs_root

# Extract JFFS2 filesystem
jefferson extracted.jffs2 -d /tmp/jffs2_root

# Inspect the extracted filesystem
ls -la /tmp/squashfs_root/
find /tmp/squashfs_root -name "*.conf" -o -name "*.cfg" -o -name "*.key"
find /tmp/squashfs_root -name "passwd" -o -name "shadow"

Step 5: String Analysis and Credential Discovery

Search extracted filesystem and raw firmware for sensitive data:

# Extract all printable strings
strings -a firmware.bin > all_strings.txt
strings -n 12 firmware.bin | sort -u > long_strings.txt

# Search for credentials and secrets
grep -rni "password\|passwd\|secret\|api_key\|token" /tmp/squashfs_root/etc/
grep -rni "BEGIN.*PRIVATE KEY" /tmp/squashfs_root/

# Find hardcoded URLs and endpoints
grep -rnoE "https?://[a-zA-Z0-9./?=_-]+" /tmp/squashfs_root/

# Search for certificate files
find /tmp/squashfs_root -name "*.pem" -o -name "*.crt" -o -name "*.key" -o -name "*.p12"

# Identify busybox and service versions
strings /tmp/squashfs_root/bin/busybox | grep "BusyBox v"
cat /tmp/squashfs_root/etc/banner 2>/dev/null

Step 6: Generate Firmware Analysis Report

Compile comprehensive extraction and analysis findings:

Report should include:
- Firmware metadata (vendor, model, version, build date)
- Identified components with offsets and sizes (bootloader, kernel, filesystem, config)
- Entropy analysis summary with regions of interest
- Extracted filesystem structure and key contents
- Discovered credentials, keys, certificates
- Identified services, daemons, and their versions
- Known CVEs applicable to identified component versions
- Recommendations for hardening or vulnerability remediation

Key Concepts

| Term | Definition | |------|------------| | Firmware | Software embedded in hardware devices providing low-level control; typically contains a bootloader, kernel, root filesystem, and configuration data | | Entropy Analysis | Statistical measurement of randomness in binary data; high entropy indicates encryption or compression, low entropy indicates plaintext or structured data | | SquashFS | Read-only compressed filesystem commonly used in embedded Linux devices; supports LZMA, gzip, LZO, and zstd compression | | Magic Bytes | Known byte sequences at fixed offsets that identify file types; binwalk uses a database of magic signatures to detect embedded files | | Matryoshka Extraction | Recursive extraction mode where binwalk re-scans extracted files for additional embedded content, handling deeply nested archives | | CramFS | Compressed ROM filesystem designed for embedded systems with limited flash storage; supports only zlib compression | | JFFS2 | Journalling Flash File System version 2, designed for NOR and NAND flash memory in embedded devices |

Tools & Systems

• binwalk: Primary firmware analysis tool for signature scanning, entropy analysis, and automated extraction of embedded files
• unsquashfs: SquashFS extraction utility for mounting read-only compressed filesystems found in router and IoT firmware
• jefferson: Python tool for extracting JFFS2 flash filesystem images commonly found in embedded devices
• sasquatch: Patched SquashFS utility supporting non-standard vendor-modified SquashFS variants
• firmware-mod-kit: Toolkit for extracting, modifying, and repacking firmware images for security testing

Common Scenarios

Scenario: Extracting and Auditing Router Firmware for Hardcoded Credentials

Context: A security researcher is performing an authorized assessment of a consumer router. The firmware update file was downloaded from the vendor's support page. The goal is to identify hardcoded credentials, insecure default configurations, and known vulnerable components.

Approach:

1. Run binwalk -e firmware.bin to perform initial extraction
2. Use binwalk -E firmware.bin to check entropy and identify encrypted regions
3. Locate the SquashFS root filesystem in the extracted output
4. Mount with unsquashfs and inspect /etc/passwd, /etc/shadow, and web server configs
5. Search for hardcoded credentials with grep -rni "password" /tmp/root/etc/
6. Identify service versions and cross-reference with CVE databases
7. Check for debug interfaces (telnet, UART, JTAG references) in startup scripts
8. Examine web application code for authentication bypass or command injection

Pitfalls:

• Some vendors use non-standard SquashFS with custom compression; use sasquatch instead of unsquashfs
• Encrypted firmware requires decryption keys often found in bootloader or previous unencrypted versions
• Firmware headers may need to be stripped before binwalk can identify the embedded filesystem
• Obfuscated strings may evade simple grep searches; use entropy analysis to locate data blobs

Output Format

FIRMWARE EXTRACTION REPORT
====================================
Firmware:         TP-Link TL-WR841N v14
File:             wr841nv14_en_3_16_9_up.bin
Size:             3,932,160 bytes (3.75 MB)
SHA-256:          a1b2c3d4e5f6...

SIGNATURE SCAN RESULTS
Offset       Type                          Size
------       ----                          ----
0x00000000   U-Boot bootloader header      64 bytes
0x00020000   LZMA compressed data          1,048,576 bytes
0x00120000   SquashFS filesystem v4.0      2,752,512 bytes
0x003B0000   Configuration partition       131,072 bytes

ENTROPY ANALYSIS
Region 0x000000-0x020000: 4.21 (bootloader - plaintext code)
Region 0x020000-0x120000: 7.89 (kernel - LZMA compressed)
Region 0x120000-0x3B0000: 7.45 (filesystem - SquashFS compressed)
Region 0x3B0000-0x3C0000: 1.12 (config - mostly empty)

EXTRACTED FILESYSTEM
Root filesystem: SquashFS v4.0, LZMA compression
Total files: 847
Total dirs: 112
BusyBox version: 1.19.4

SECURITY FINDINGS
[CRITICAL] Hardcoded root password in /etc/shadow (hash: $1$...)
[HIGH]     Telnet daemon enabled by default in /etc/init.d/rcS
[HIGH]     Private RSA key at /etc/ssl/private/server.key
[MEDIUM]   BusyBox 1.19.4 (CVE-2021-42373, CVE-2021-42374)
[MEDIUM]   Dropbear SSH 2014.63 (CVE-2016-3116)
[LOW]      UPnP service enabled by default