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Port 1723 Details

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Port(s) Protocol Service Details Source
1723 tcp,udp PPTP PPTP VPN (Point-to-Point Tunneling Protocol Virtual Private Networking).

PPTP has a number of known vulnerabilities. It is no longer considered secure, as cracking the initial MS-CHAPv2 authentication can be reduced to the difficulty of cracking a single DES 56-bit key, which can be brute-forced in a short period of time. It is prone to MITM (man in the middle) attacks, where an attacker can capture the handshake and do an offline attack to derive the RC4 key and decrypt the traffic. PPTP is also vulnerable to bit-flipping attacks, i.e. an attacker can modify PPTP packets without possibility of detection. OpenVPN with AES encryption is a much more secure choice.

See also:
port 500/udp (IPSec IKE)
port 1701/tcp (L2TP)
port 1194/udp (OpenVPN)

QNAP NAS uses port 1723/TCP for PPTP VPN. It can also use 1194/UDP (OpenVPN), and a number of other ports, as follows: 80,8081/TCP (web server), 443,8080/TCP (web admin), 20,21,22/TCP (FTP/SSH), 13131/TCP (telnet), 873,8899/TCP (remote replication), 20001/UDP (CloudLink - optional, only required for access without manual port forwarding)

Mac OS X Server VPN service also uses port 1723 (TCP).

The Siemens Gigaset SE361 WLAN router allows remote attackers to cause a denial of service (device reboot) via a flood of crafted TCP packets to port 1723.
References: [CVE-2009-3322] [BID-36366]

SMC Networks Barricade Wireless Cable/DSL Broadband Router SMC7004VWBR allows remote attackers to cause a denial of service via certain packets to PPTP port 1723 on the internal interface.
References: [CVE-2003-0419]

The PPTP implementation in Cisco IOS 12.2 and 15.0 through 15.3, when NAT is used, allows remote attackers to cause a denial of service (device reload) via crafted TCP port-1723 packets, aka Bug ID CSCtq14817.
References: [CVE-2013-5481]
1723 tcp,udp Microsoft Point-to-Point Tunneling Protocol (PPTP) (official) Wikipedia
1723 tcp pptp Point-to-point tunnelling protocol SANS
1723 udp pptp pptp SANS
1723 tcp applications Point-to-Point Tunneling Protocol Portforward
1723 tcp pptp Point-to-point tunnelling protocol Nmap
1723 tcp,udp pptp pptp IANA
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Related ports: 500  1194  1701  4500  

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External Resources
SANS Internet Storm Center: port 1723

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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