Linux File System Security Guide

Critical File Permissions

System File Permissions

# Secure important system files
sudo chmod 644 /etc/passwd
sudo chmod 600 /etc/shadow
sudo chmod 644 /etc/group
sudo chmod 600 /etc/gshadow

# Verify permissions
ls -l /etc/{passwd,shadow,group,gshadow}

SUID/SGID Control

# Find SUID files
sudo find / -type f -perm -4000 -ls

# Find SGID files
sudo find / -type f -perm -2000 -ls

# Remove unnecessary SUID/SGID bits
sudo chmod u-s /path/to/file

File System Mounting

Secure Mount Options

# Edit fstab
sudo nano /etc/fstab

# Add security options
/dev/sda1 /     ext4  defaults,noexec,nosuid,nodev 0 1
/dev/sda2 /home ext4  defaults,nosuid,nodev 0 2

File Integrity Monitoring

AIDE Setup

# Install AIDE
sudo apt install aide

# Initialize database
sudo aideinit

# Configure daily checks
sudo nano /etc/aide/aide.conf

# Example rule
/etc/passwd Full
/etc/shadow Full