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Port 21 Details

known port assignments and vulnerabilities

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Port(s)	Protocol	Service	Details	Source
21	tcp	FTP	File Transfer Protocol [RFC 959] - some network devices may be listening on this port, such as NAT routers for remote access/private cloud storage and network attached multi-function printers (scan to ftp feature). Asus RT routers may open an internet accessible FTP server for USB-attached storage, configurable in administration panel under "USB Application > Servers Center > FTP Share" Trojan horses/backdoors that also use this port: 7tp trojan, MBT, Back Construction, Blade Runner, Cattivik FTP Server, CC Invader, Dark FTP, Doly Trojan, Fore, Invisible FTP, Juggernaut 42, Larva, MotIv FTP, Nerte 7.8.1, Net Administrator, Ramen, Senna Spy FTP server, The Flu, Traitor 21, WebEx, WinCrash, W32.Mytob.AE@mm [Symantec-2005-040915-5504-99], W32.Sober.N@mm [Symantec-2005-041910-4132-99], W32.Bobax.AF@mm [Symantec-2005-081611-4121-99] - a mass-mailing worm that opens a backdoor and lowers security settings on the compromised computer. It exploits the MS Plug and Play Buffer Overflow vulnerability (MS Security Bulletin [MS05-039]) on port 21/tcp., and by sending copies of itself to gathered email addresses. Also opens a backdoor on a random tcp port and/or port 80/udp. W32.Loxbot.C [Symantec-2006-010515-3159-99] (2006-01-05) FTP proxy server for Novell BorderManager 3.6 SP 1a allows remote attackers to cause a denial of service (network connectivity loss) via a connection to port 21 with a large amount of random data. References: [CVE-2002-0779] TURCK BL20 / BL67 could allow a remote attacker to bypass security restrictions, caused by the use of hardcoded credentials for the FTP service. An attacker could exploit this vulnerability using TCP port 21 to gain administrative access to the device. References: [CVE-2012-4697], [XFDB-84351] The FTP service in QNAP iArtist Lite before 1.4.54, as distributed with QNAP Signage Station before 2.0.1, has hardcoded credentials, which makes it easier for remote attackers to obtain access via a session on TCP port 21. References: [CVE-2015-7261] The FTP service on Janitza UMG 508, 509, 511, 604, and 605 devices has a default password, which makes it easier for remote attackers to read or write to files via a session on TCP port 21. References: [CVE-2015-3968] A vulnerability was discovered in Siemens OZW672 (all versions) and OZW772 (all versions) that could allow an attacker with access to port 21/tcp to access or alter historical measurement data stored on the device. References: [CVE-2017-6872], [BID-99473] A vulnerability has been identified in SiNVR 3 Central Control Server (CCS) (all versions), SiNVR 3 Video Server (all versions). The two FTP services (default ports 21/tcp and 5411/tcp) of the SiNVR 3 Video Server contain a path traversal vulnerability that could allow an authenticated remote attacker to access and download arbitrary files from the server, if the FTP services are enabled. References: [CVE-2019-19296] Backdoor.Win32.Delf.zho / Authentication Bypass RCE - the malware listens on TCP port 21 and TCP ports 14920 to 14923. Third-party attackers who can reach the system can logon using any username/password combination. Attackers may then upload executables using ftp PASV, STOR commands, this can result in remote code execution. References: [MVID-2021-0205] ReverseTrojan by satan_addict listens on TCP ports, 12000 and 21. The malware accepts empty credentials for authentication as the default settings are set to blank. Third-party attackers who can reach an infected host can potentially gain access to the machine before or if no password is set. References: [MVID-2021-0256] Backdoor.Win32.Wollf.16 / Authentication Bypass - the malware listens on TCP port 1015 and has an FTPD feature that when enabled listens on TCP port 21. Third-party attackers who can reach an infected system can logon using any username/password combination. References: [MVID-2022-0462] Backdoor.Win32.Hellza.120 / Unauthorized Remote Command Execution - the malware listens on TCP ports 12122, 21. Third-party adversarys who can reach infected systems can issue commands made available by the backdoor. References: [MVID-2022-0641]	SG
21	udp	FSP	FSP/FTP [RFC959]	SG
21	tcp		FTP - control (command) (official)	Wikipedia
21	tcp	trojan	ADM worm, Back Construction, Blade Runner, BlueFire, Bmail, Cattivik FTP Server, CC Invader, Dark FTP, Doly Trojan, FreddyK, Invisible FTP, KWM, MscanWorm, NerTe, NokNok, Pinochet, Ramen, Reverse Trojan, RTB 666, The Flu, WinCrash, Voyager Alpha Force	Trojans
21	tcp,udp	ftp	File Transfer [Control]	SANS
21	tcp	applications	GeoVision DMIP	Portforward
21, 80, 3389, 4550, 5550, 6550, 9650	tcp	applications	GeoVision TwinDVR with Webcam	Portforward
21,80,3389,4550,5550,6550	tcp	applications	GeoVision Webcam	Portforward
21,50000-50004	tcp	applications	Serv-U	Portforward
21,1983	tcp	applications	Tales of Pirates	Portforward
20,21	tcp	applications	vsftp	Portforward
21	tcp,udp	ftp	File Transfer [Control]	Nmap
21	tcp	BackConstruction	[trojan] Back Construction	Neophasis
21	tcp	BladeRunner	[trojan] BladeRunner	Neophasis
21	tcp	CattivikFTPServer	[trojan] Cattivik FTP Server	Neophasis
21	tcp	CCInvader	[trojan] CC Invader	Neophasis
21	tcp	DarkFTP	[trojan] Dark FTP	Neophasis
21	tcp	DolyTrojan	[trojan] Doly Trojan	Neophasis
21	tcp	Fore	[trojan] Fore	Neophasis
21	tcp	FreddyK	[trojan] FreddyK	Neophasis
21	tcp	InvisibleFTP	[trojan] Invisible FTP	Neophasis
21	tcp	Juggernaut42	[trojan] Juggernaut 42	Neophasis
21	tcp	Larva	[trojan] Larva	Neophasis
21	tcp	MotIvFTP	[trojan] MotIv FTP	Neophasis
21	tcp	NetAdministrator	[trojan] Net Administrator	Neophasis
21	tcp	Ramen	[trojan] Ramen	Neophasis
21	tcp	RTB666	[trojan] RTB 666	Neophasis
21	tcp	SennaSpyFTPserver	[trojan] Senna Spy FTP server	Neophasis
21	tcp	Traitor21	[trojan] Traitor 21	Neophasis
21	tcp	[trojan]TheFlu	[trojan] The Flu	Neophasis
21	tcp	WebEx	[trojan] WebEx	Neophasis
21	tcp	WinCrash	[trojan] WinCrash	Neophasis
21	tcp	AudioGalaxy	AudioGalaxy file sharing app	Neophasis
21	tcp	threat	Back Construction	Bekkoame
21	tcp	threat	Blade Runner	Bekkoame
21	tcp	threat	Cattivik FTP Server	Bekkoame
21	tcp	threat	CC Invader	Bekkoame
21	tcp	threat	Dark FTP	Bekkoame
21	tcp	threat	Doly Trojan	Bekkoame
21	tcp	threat	Fore	Bekkoame
21	tcp	threat	Invisible FTP	Bekkoame
21	tcp	threat	Juggernaut 42	Bekkoame
21	tcp	threat	Larva	Bekkoame
21	tcp	threat	MotIv FTP	Bekkoame
21	tcp	threat	Net Administrator	Bekkoame
21	tcp	threat	Ramen	Bekkoame
21	tcp	threat	Senna Spy FTP server	Bekkoame
21	tcp	threat	The Flu	Bekkoame
21	tcp	threat	Traitor 21	Bekkoame
21	tcp	threat	W32.Bobax	Bekkoame
21	tcp	threat	W32.Loxbot	Bekkoame
21	tcp	threat	W32.Mytob	Bekkoame
21	tcp	threat	WebEx	Bekkoame
21	tcp	threat	WinCrash	Bekkoame
21	tcp,udp	ftp	File Transfer Protocol [Control] [RFC959]	IANA
21	sctp	ftp	FTP [RFC4960]	IANA

56 records found

SG security scan: port 21

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Related ports: 20 443 1234 1235 1239 5410 12000 12122 14920 14923

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External Resources
SANS ISC: port 21

Notes:
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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User Reviews/Comments:

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by clairmont32 - 2014-02-18 11:29

Used by the Qakbot worm, which is known to do data exfiltration through FTP. Look for network logs showing seclog*.kcb files to determine if the exfiltration is attempting to take place.