When you execute chmod -R 777 /, you're recursively granting read, write, and execute permissions to all users (owner, group, others) on every file and directory starting from the root (/). This includes:

• System binaries in /bin, /sbin
• Configuration files in /etc
• Temporary directories like /tmp
• Even device files in /dev

Linux relies on file permissions for:

# Example of critical system files that MUST remain restricted:
-rw------- 1 root root /etc/shadow   # Contains password hashes
-rwsr-xr-x 1 root root /bin/su       # SUID binary
crw-rw---- 1 root root /dev/sda      # Direct disk access

When these lose their protections:

• SUID/SGID binaries become security holes (any user can modify them)
• Service accounts (like www-data) gain root-equivalent access
• Package managers fail because they verify file permissions

Imagine a web server where:

# Before chmod:
drwxr-x--- 5 root     www-data /var/www
# After chmod:
drwxrwxrwx 5 root     www-data /var/www

Now any process can:

1. Modify PHP files to inject malware
2. Tamper with session files
3. Overwrite log files to hide attacks

Modern Linux systems use permission-aware daemons:

// Example from systemd source (simplified):
if (stat("/etc/systemd/system", &st) == -1) {
    log_error("Permissions too open");
    exit(EXIT_FAILURE);
}

Common failure modes include:

Service	Failure Reason
SSH	Refuses to start if ~/.ssh has world-writable permissions
MySQL	Aborts if data directory permissions are insecure

While a full reinstall is recommended, for critical systems you might:

# 1. Reset permissions for core directories
find / -type d -exec chmod 755 {} \;
find / -type f -exec chmod 644 {} \;

# 2. Special cases (run as root)
chmod 700 /root /etc/shadow
chmod 4755 /bin/su /usr/bin/sudo

Then verify with:

# Check for world-writable files
find / -perm -2 ! -type l -ls

---

When you execute chmod -R 777 /, you're recursively granting read, write, and execute permissions to every user and process on the system for all files and directories starting from the root (/). This includes:

- Critical system binaries (/bin, /sbin)
- Configuration files (/etc)
- Temporary directories (/tmp)
- Home directories (/home)
- Device files (/dev)

Linux relies on file permissions for:

• Security isolation: Many services run as specific users with limited permissions
• System integrity: Critical files should only be modified by root or specific system users
• Process behavior: Some programs check permissions before operating

Example of a broken scenario:

$ sudo chmod -R 777 /
$ sudo su -
# ls -l /etc/shadow
-rwxrwxrwx 1 root root 1234 Jan 1 00:00 /etc/shadow  # Now world-writable!

SSH Daemon often fails because it checks that key files have strict permissions:

$ tail /var/log/auth.log
sshd[1234]: Bad permissions for /etc/ssh/ssh_host_rsa_key
sshd[1234]: Authentication refused: bad ownership or modes

SUID binaries like passwd become security risks when world-writable:

$ ls -l /usr/bin/passwd
-rwsrwxrwx 1 root root 59976 Nov 24  2022 /usr/bin/passwd

There's no perfect way to restore original permissions because:

• System packages don't track original permission states
• Many files are created dynamically during operation
• Permission inheritance gets broken

Partial recovery attempt (not recommended for production):

# For Debian/Ubuntu systems
find / -type d -exec chmod 755 {} \;
find / -type f -exec chmod 644 {} \;
# Then reinstall critical packages
apt-get --reinstall install `dpkg -S /bin /sbin /usr/bin /usr/sbin | cut -d: -f1`

Always:

1. Double-check paths before recursive operations
2. Use --preserve-root flag (default in modern chmod)
3. Consider using setfacl for complex permission needs

Example safer alternative:

# Instead of 777:
chmod -R u=rwX,g=rX,o=rX /path/to/dir
find /path/to/dir -type d -exec chmod 755 {} \;