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John the Ripper

John the Ripper is a free and open-source password security auditing and password recovery tool available for many operating systems. It is designed to detect weak passwords in Unix/Linux and Windows systems.

Basic Usage

# Basic usage with automatic format detection
john hash.txt

# Specify a format
john --format=raw-md5 hash.txt

# Use wordlist
john --wordlist=/usr/share/wordlists/rockyou.txt hash.txt

# Show cracked passwords
john --show hash.txt

Extracting Hashes

Shadow File (Linux)

# First, combine /etc/passwd and /etc/shadow
unshadow /etc/passwd /etc/shadow > hashes.txt

# Then crack with John
john hashes.txt

Windows NTLM Hashes

# Using pwdump or similar tools to extract
john --format=NT hash.txt

Zip Files

# Extract hash from password-protected zip
zip2john file.zip > zip.hash

# Crack the hash
john zip.hash

RAR Files

# Extract hash from password-protected rar
rar2john file.rar > rar.hash

# Crack the hash
john rar.hash

PDF Files

# Extract hash from password-protected PDF
pdf2john file.pdf > pdf.hash

# Crack the hash
john pdf.hash

SSH Keys

# Extract hash from encrypted SSH private key
ssh2john id_rsa > ssh.hash

# Crack the hash
john ssh.hash

Attack Modes

Dictionary Attack

# Basic dictionary attack
john --wordlist=/usr/share/wordlists/rockyou.txt hash.txt

# With rules
john --wordlist=/usr/share/wordlists/rockyou.txt --rules hash.txt

Incremental Mode (Brute Force)

# Brute force attack (slower but tries all possibilities)
john --incremental hash.txt

# Limit to certain character sets
john --incremental=Digits hash.txt  # Only digits
john --incremental=Alpha hash.txt   # Only letters

Rules-Based Attack

# Apply rules to transform words from wordlist
john --wordlist=/usr/share/wordlists/rockyou.txt --rules hash.txt

# Use specific rule set
john --wordlist=/usr/share/wordlists/rockyou.txt --rules=Jumbo hash.txt

Common Hash Formats

# MD5
john --format=raw-md5 hash.txt

# SHA1
john --format=raw-sha1 hash.txt

# SHA256
john --format=raw-sha256 hash.txt

# SHA512
john --format=raw-sha512 hash.txt

# Windows NTLM
john --format=NT hash.txt

# Linux /etc/shadow (SHA512CRYPT)
john --format=sha512crypt hash.txt

# MySQL
john --format=mysql-sha1 hash.txt

# PostgreSQL
john --format=postgres hash.txt

Advanced Options

# Set maximum execution time
john --max-run-time=3600 hash.txt  # Run for 1 hour

# Use multiple cores
john --fork=4 hash.txt  # Use 4 cores

# Session management
john --session=mysession hash.txt  # Start a named session
john --restore=mysession           # Restore a session

# Show statistics during cracking
john --status=mysession

# Automatically detect hash type
john --format=auto hash.txt

Custom Rules

John's custom rules allow you to create complex password transformations. Add these to the john.conf file:

# Example rule to append years
$[append_year] $[l] [0-9][0-9]

# Example rule to replace characters
$[replace_chars] s $s0 $i1 $e3 $a4 $t7

Integration in Pentesting Workflow

  1. Extract hashes from the target system
  2. Identify hash types (hashid or hash-identifier can help)
  3. Select appropriate attack method based on hash type and context
  4. Begin with quick wordlist attacks using common passwords
  5. Move to rule-based attacks if simple wordlists fail
  6. Use incremental (brute force) mode as a last resort for short passwords

Tips for Effective Usage

  • Always start with the most likely password patterns for your target
  • Use the --pot option to save cracked passwords for future reference
  • Leverage session management for long-running cracks
  • Use custom rules based on the target organization (company name, founding year, etc.)
  • For Linux shadow files, target lower-privileged users first as they often have weaker passwords
  • Use --show to display already cracked passwords without rerunning the attack

Real-World Example: Cracking Linux Passwords

# Step 1: Extract shadow and passwd files from target
# Assuming you have both files

# Step 2: Combine them
unshadow passwd shadow > linux_hashes.txt

# Step 3: Crack with wordlist first
john --wordlist=/usr/share/wordlists/rockyou.txt linux_hashes.txt

# Step 4: If that fails, try with rules
john --wordlist=/usr/share/wordlists/rockyou.txt --rules linux_hashes.txt

# Step 5: Show cracked passwords
john --show linux_hashes.txt

Additional Resources