What are the most commonly attacked network ports?

The most targeted ports are: SSH (22), RDP (3389), SMB (445), Telnet (23), FTP (21), HTTP (80), MySQL (3306), and MSSQL (1433). These are frequently targeted by automated scanners and botnets.

How do I check which ports are open on my server?

Use 'nmap -sV localhost' to scan local ports, or 'sudo ss -tlnp' to see listening ports. For external scanning, use 'nmap -sV your-public-ip' from another machine or use online tools like shodan.io.

Should I change the default SSH port?

Changing SSH from port 22 to a non-standard port reduces automated brute-force attempts significantly (security through obscurity). It's not a replacement for key-based auth and fail2ban, but it reduces log noise by 99%.

Network Port Security Guide | Common Ports Cheat Sheet

By FixTheVuln Team Peer-reviewed security content Sources: CISA, NVD, OWASP

Why Port Security Matters: Open ports are entry points into your systems. Each unnecessary open port is a potential attack vector. Proper port management is fundamental to network security.

Critical Ports to Secure

Port	Service	Risk Level	Common Attacks
21	FTP	High	Unencrypted transmission, brute force, bounce attacks
22	SSH	Medium	Brute force, weak credentials, exploits
23	Telnet	Critical	Unencrypted, credential sniffing, MitM
25	SMTP	Medium	Spam relay, email spoofing
53	DNS	Medium	DNS amplification, cache poisoning, zone transfer
80	HTTP	Medium	MitM, session hijacking, XSS, injection
443	HTTPS	Low	SSL/TLS vulnerabilities, weak ciphers
445	SMB	High	EternalBlue, ransomware, lateral movement
3306	MySQL	High	Unauthorized access, SQL injection, data theft
3389	RDP	High	Brute force, BlueKeep, credential theft
5432	PostgreSQL	High	Unauthorized access, SQL injection
6379	Redis	High	Unauthorized access, data exfiltration
27017	MongoDB	High	Unauthorized access, ransomware

Port-Specific Security Hardening

Port 22 - SSH

Service: Secure Shell

Risk: Brute force attacks, weak credentials

Hardening Steps:

                    # Edit /etc/ssh/sshd_config

# 1. Change default port
Port 2222

# 2. Disable root login
PermitRootLogin no

# 3. Use key-based authentication only
PasswordAuthentication no
PubkeyAuthentication yes

# 4. Limit user access
AllowUsers username1 username2

# 5. Disable empty passwords
PermitEmptyPasswords no

# 6. Set login grace time
LoginGraceTime 30

# 7. Max authentication attempts
MaxAuthTries 3

# 8. Use Protocol 2 only
Protocol 2

# Restart SSH service
sudo systemctl restart sshd
                

Additional Security:

• Install fail2ban to block repeated failed attempts
• Use key-based authentication with strong passphrases
• Implement IP whitelisting if possible
• Enable two-factor authentication (2FA)

See detailed SSH hardening guide →

Key Takeaways

Close all unnecessary ports — only open what your services actively require
Common attack targets: SSH (22), RDP (3389), SMB (445), Telnet (23)
Use firewall rules to restrict port access to specific IP ranges
Regularly scan your systems with nmap to verify only expected ports are open
Move SSH to a non-standard port and disable Telnet completely

Port 3389 - RDP (Remote Desktop Protocol)

Service: Windows Remote Desktop

Risk: Brute force, BlueKeep vulnerability, credential theft

Hardening Steps:

                    # PowerShell commands

# 1. Enable Network Level Authentication (NLA)
Set-ItemProperty -Path 'HKLM:\System\CurrentControlSet\Control\Terminal Server\WinStations\RDP-Tcp' -Name "UserAuthentication" -Value 1

# 2. Change default port (optional, not security through obscurity alone)
Set-ItemProperty -Path 'HKLM:\System\CurrentControlSet\Control\Terminal Server\WinStations\RDP-Tcp' -Name "PortNumber" -Value 3390

# 3. Enable firewall rules for specific IPs only
New-NetFirewallRule -DisplayName "RDP-Restricted" -Direction Inbound -LocalPort 3389 -Protocol TCP -RemoteAddress 192.168.1.100 -Action Allow

# 4. Disable RDP if not needed
Set-ItemProperty -Path 'HKLM:\System\CurrentControlSet\Control\Terminal Server' -Name "fDenyTSConnections" -Value 1
                

Additional Security:

• Use VPN instead of exposing RDP to internet
• Implement account lockout policies
• Use strong, unique passwords
• Enable Windows Defender Credential Guard
• Keep Windows patched (especially for BlueKeep)

See detailed RDP hardening guide →

Port 445 - SMB (Server Message Block)

Service: File sharing (Windows)

Risk: EternalBlue, ransomware, WannaCry, lateral movement

Hardening Steps:

                    # PowerShell commands

# 1. Disable SMBv1 (vulnerable to EternalBlue)
Disable-WindowsOptionalFeature -Online -FeatureName SMB1Protocol

# 2. Verify SMBv1 is disabled
Get-SmbServerConfiguration | Select EnableSMB1Protocol

# 3. Enable SMB signing
Set-SmbServerConfiguration -RequireSecuritySignature $true -EnableSecuritySignature $true

# 4. Enable SMB encryption
Set-SmbServerConfiguration -EncryptData $true

# 5. Block port 445 at firewall for external traffic
New-NetFirewallRule -DisplayName "Block SMB" -Direction Inbound -LocalPort 445 -Protocol TCP -Action Block
                

Additional Security:

• Never expose SMB to the internet
• Use VPN for remote file access
• Implement strong authentication
• Regular Windows updates
• Monitor for unusual SMB traffic

See detailed SMB hardening guide →

Port 3306 - MySQL

Service: MySQL Database

Risk: Unauthorized access, SQL injection, data theft

Hardening Steps:

                    # MySQL configuration (/etc/mysql/my.cnf)

# 1. Bind to localhost only (if remote access not needed)
bind-address = 127.0.0.1

# 2. Disable remote root login
# In MySQL console:
DELETE FROM mysql.user WHERE User='root' AND Host NOT IN ('localhost', '127.0.0.1', '::1');
FLUSH PRIVILEGES;

# 3. Remove anonymous users
DELETE FROM mysql.user WHERE User='';
FLUSH PRIVILEGES;

# 4. Remove test database
DROP DATABASE IF EXISTS test;
DELETE FROM mysql.db WHERE Db='test' OR Db='test\\_%';
FLUSH PRIVILEGES;

# 5. Use strong passwords
ALTER USER 'root'@'localhost' IDENTIFIED BY 'StrongP@ssw0rd!';

# 6. Create specific user accounts
CREATE USER 'appuser'@'localhost' IDENTIFIED BY 'StrongP@ss';
GRANT SELECT, INSERT, UPDATE, DELETE ON mydb.* TO 'appuser'@'localhost';
FLUSH PRIVILEGES;
                

Firewall Rules:

                    # Only allow specific IPs if remote access needed
sudo ufw allow from 192.168.1.100 to any port 3306

# Or block external access entirely
sudo ufw deny 3306
                

See detailed MySQL hardening guide →

Port 27017 - MongoDB

Service: MongoDB Database

Risk: Unauthorized access, ransomware attacks, data exposure

Hardening Steps:

                    # MongoDB configuration (/etc/mongod.conf)

# 1. Bind to localhost
net:
  bindIp: 127.0.0.1
  port: 27017

# 2. Enable authentication
security:
  authorization: enabled

# 3. Create admin user (in mongo shell)
use admin
db.createUser({
  user: "admin",
  pwd: "StrongP@ssw0rd!",
  roles: [ { role: "userAdminAnyDatabase", db: "admin" } ]
})

# 4. Create application user
use mydb
db.createUser({
  user: "appuser",
  pwd: "AppP@ssw0rd!",
  roles: [ { role: "readWrite", db: "mydb" } ]
})

# 5. Enable TLS/SSL
net:
  ssl:
    mode: requireSSL
    PEMKeyFile: /path/to/mongodb.pem
                

See detailed MongoDB hardening guide →

General Port Security Best Practices

1. Port Scanning & Discovery

                    # Scan your own system for open ports
sudo nmap -sS -sV localhost

# Check listening ports
sudo netstat -tulpn

# Or using ss (modern alternative)
sudo ss -tulpn

# Check open ports (Linux)
sudo lsof -i -P -n | grep LISTEN
                

2. Firewall Configuration

                    # UFW (Ubuntu/Debian)
sudo ufw default deny incoming
sudo ufw default allow outgoing
sudo ufw allow 22/tcp
sudo ufw allow 80/tcp
sudo ufw allow 443/tcp
sudo ufw enable

# iptables (Advanced)
iptables -P INPUT DROP
iptables -P FORWARD DROP
iptables -P OUTPUT ACCEPT
iptables -A INPUT -i lo -j ACCEPT
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A INPUT -p tcp --dport 22 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j ACCEPT

# Windows Firewall
New-NetFirewallRule -DisplayName "Allow HTTP" -Direction Inbound -LocalPort 80 -Protocol TCP -Action Allow
New-NetFirewallRule -DisplayName "Allow HTTPS" -Direction Inbound -LocalPort 443 -Protocol TCP -Action Allow
                

3. Close Unnecessary Ports

                    # Stop and disable unused services (Linux)
sudo systemctl stop service_name
sudo systemctl disable service_name

# Windows
Stop-Service -Name "ServiceName"
Set-Service -Name "ServiceName" -StartupType Disabled
                

Security Checklist

☐ Scan for open ports regularly
☐ Close all unnecessary ports
☐ Use firewall to restrict access
☐ Change default ports where appropriate
☐ Implement strong authentication
☐ Enable encryption (TLS/SSL)
☐ Use VPN for remote access to sensitive services
☐ Monitor logs for unauthorized access attempts
☐ Keep all services updated
☐ Implement intrusion detection (fail2ban, etc.)

Important: Security through obscurity (changing default ports) is NOT sufficient alone. Always combine with strong authentication, encryption, and firewall rules.

Related Resources

Linux Hardening Windows Hardening OSI Layer Attacks

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