Port 1433 (Microsoft SQL Server)

Default instance of Microsoft SQL Server (and Azure SQL).

Registered (user) port, 1024-49151 TCP Databases Do not expose to the internet

What port 1433 is used for

Default instance of Microsoft SQL Server (and Azure SQL).

Security: should port 1433 be open?

Do not expose to the internet. Do not expose. Internet-facing SQL Servers are brute-forced for the 'sa' account and used to run xp_cmdshell for malware. Disable sa, use Windows or Entra auth, and allow only app servers.

IANA assignment

Service nameProtoDescriptionAssigneeReference

ms-sql-s TCP Microsoft-SQL-Server - -

ms-sql-s UDP Microsoft-SQL-Server - -

Check what is listening on port 1433

Linux: is anything listening on TCP 1433?

sudo ss -tlnp 'sport = :1433'

Linux / macOS: which process owns the port

sudo lsof -nP -i :1433

Older Linux / BSD (net-tools)

sudo netstat -tlnp | grep ':1433 '

Windows (Command Prompt)

netstat -ano | findstr :1433

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 1433 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine

nc -vz -w 5 HOST 1433

From Windows

Test-NetConnection HOST -Port 1433

Block port 1433 in the firewall

ufw

sudo ufw deny 1433/tcp

iptables

sudo iptables -A INPUT -p tcp --dport 1433 -j DROP

nftables

sudo nft add rule inet filter input tcp dport 1433 drop

Windows Firewall

New-NetFirewallRule -DisplayName 'Block TCP 1433' -Direction Inbound -Protocol TCP -LocalPort 1433 -Action Block

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Assignment data: IANA port number registry (public domain), retrieved 2026-11-10.