Port / TCP
1723PPTP
PPTP's control channel for a legacy Microsoft VPN protocol that is no longer considered safe.
If it is internet-facing: Finding 1723 open to the internet is a finding in its own right — it warrants an explanation, not just a rule.
Exposure
Trusted network only
Transport
Encrypted by default
Risk if public
High
Exposure verdict
Port 1723 belongs on a trusted internal network. If a perimeter scan finds it answering, treat that as a firewall defect rather than a configuration preference.
The session is protected from the first byte, so there is no plaintext phase for an on-path attacker to strip or read.
Registered with IANA on request, but bindable by any unprivileged user. Registration records intent — it does not reserve the port, so collisions between products are common.
Grouped with the other network services ports so a rule written for one can be checked against its neighbours.
What actually goes wrong
2 notes- MS-CHAPv2 authentication can be reduced to a single DES key recovery, so PPTP encryption is effectively broken.
- Migrate to IKEv2/IPsec, WireGuard, or OpenVPN — there is no configuration that makes PPTP safe.
Recommended firewall rule
Allow only from 10.0.0.0/8, drop the rest
Swap in the CIDR your administrators or application tier actually come from — the point is that the default answer is no.
ufw
sudo ufw allow from 10.0.0.0/8 to any port 1723 proto tcpiptables
sudo iptables -A INPUT -p tcp -s 10.0.0.0/8 --dport 1723 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 1723 -j DROPIf you do need it open to everyone
sudo ufw allow 1723/tcpWrite down why. An unexplained allow rule outlives the person who added it, and the next reviewer has no way to tell a deliberate exception from an accident.
Check what is really there
Listening locally
sudo ss -tlpn 'sport = :1723'The bind address is the answer that matters: 127.0.0.1 is local-only, 0.0.0.0 and :: mean every interface.
Listening (macOS)
sudo lsof -nP -iTCP:1723 -sTCP:LISTENmacOS ships lsof rather than ss; this names the owning process and user.
From outside
nmap -sV -Pn -p 1723 <host>A version probe confirms whether PPTP is actually what answers, rather than trusting the number. Only scan hosts you are authorised to test.
Reachability
nc -vz <host> 1723The quick yes/no when you only need to know whether a path exists through the firewall.
The safer path
port 1194Move to port 1194 — OpenVPN
OpenVPN does the same job with the session protected from the first byte, so there is no plaintext phase for an on-path attacker to read or strip. Migrating is usually a client configuration change plus a firewall swap.