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"

…