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Port 123 Details

known port assignments and vulnerabilities
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Port(s) Protocol Service Details Source
123 udp NTP Network Time Protocol (NTP) - used for time synchronization [RFC 5905]

Security Concerns:
It provides both information and possible avenue of attack for intruders. Info gathered can include system uptime, time since reset, time server pkt, I/O & memory statistics and ntp peer list. If a host is susceptible to time altering via ntp an attacker can possibly:
1) Run replay attacks using captured OTP and Kerberos tickets before they expire.
2) Stop security-related cron jobs from running or cause them to run at incorrect times.
3) Make system and audit logs unreliable since time is alterable.

Vodafone Sure Signal also uses this port
123 tcp trojan Net Controller trojan

Network Time Protocol (NTP), as specified in RFC 5905, uses port 123 even for modes where a fixed port number is not required, which makes it easier for remote attackers to conduct off-path attacks.
References: [CVE-2019-11331], [BID-108010], [XFDB-159889]
123 udp Network Time Protocol (NTP) - used for time synchronization (official) Wikipedia
123 tcp trojan [trojan] Net Controller Trojans
123 tcp NetController [trojan] Net Controller SANS
123 tcp,udp applications NTP Portforward
123 tcp threat Madfind Bekkoame
123 tcp,udp ntp Network Time Protocol [RFC5905] IANA
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External Resources
SANS Internet Storm Center: port 123

Port numbers in computer networking represent communication endpoints. Ports are unsigned 16-bit integers (0-65535) that identify a specific process, or network service. IANA is responsible for internet protocol resources, including the registration of commonly used port numbers for well-known internet services.
Well Known Ports: 0 through 1023.
Registered Ports: 1024 through 49151.
Dynamic/Private : 49152 through 65535.

TCP ports use the Transmission Control Protocol, the most commonly used protocol on the Internet and any TCP/IP network. TCP enables two hosts to establish a connection and exchange streams of data. TCP guarantees delivery of data and that packets will be delivered in the same order in which they were sent. Guaranteed communication/delivery is the key difference between TCP and UDP.

UDP ports use the Datagram Protocol. Like TCP, UDP is used in combination with IP (the Internet Protocol) and facilitates the transmission of datagrams from one computer to applications on another computer, but unlike TCP, UDP is connectionless and does not guarantee reliable communication; it's up to the application that received the message to process any errors and verify correct delivery. UDP is often used with time-sensitive applications, such as audio/video streaming and realtime gaming, where dropping some packets is preferable to waiting for delayed data.

When troubleshooting unknown open ports, it is useful to find exactly what services/processes are listening to them. This can be accomplished in both Windows command prompt and Linux variants using the "netstat -aon" command. We also recommend runnig multiple anti-virus/anti-malware scans to rule out the possibility of active malicious software. For more detailed and personalized help please use our forums.

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