Port / TCP/UDP
88Kerberos
The Kerberos KDC issues ticket-granting tickets and service tickets that underpin Active Directory authentication.
If it is internet-facing: Port 88 is legitimate in the right place, but exposed carelessly it widens the attack surface noticeably.
Exposure
Trusted network only
Transport
Encrypted by default
Risk if public
Elevated
Exposure verdict
Port 88 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.
Assigned by IANA through a review process. On Unix systems a process needs root or the CAP_NET_BIND_SERVICE capability to bind here, which is why containers so often remap these to a higher port.
Grouped with the other directory & auth ports so a rule written for one can be checked against its neighbours.
What actually goes wrong
2 notes- Reachability lets an attacker request tickets for offline cracking (AS-REP roasting and Kerberoasting).
- It belongs on internal networks; remote users should arrive over a VPN first.
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 88 proto tcp
sudo ufw allow from 10.0.0.0/8 to any port 88 proto udpiptables
sudo iptables -A INPUT -p tcp -s 10.0.0.0/8 --dport 88 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 88 -j DROP
sudo iptables -A INPUT -p udp -s 10.0.0.0/8 --dport 88 -j ACCEPT
sudo iptables -A INPUT -p udp --dport 88 -j DROPIf you do need it open to everyone
sudo ufw allow 88/tcp
sudo ufw allow 88/udpWrite 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 -tulpn 'sport = :88'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:88 -sTCP:LISTENmacOS ships lsof rather than ss; this names the owning process and user.
From outside
nmap -sSUV -Pn -p 88 <host>A version probe confirms whether Kerberos is actually what answers, rather than trusting the number. Only scan hosts you are authorised to test.
Reachability
nc -vz <host> 88The quick yes/no when you only need to know whether a path exists through the firewall.