Internet TCP port 21 is primarily used by File Transfer Protocol (FTP) for data transfer over the internet. FTP is a standard network protocol used for the transfer of computer files between a client and server on a computer network. Examples of software that use this port include FileZilla, WinSCP, and CuteFTP. These applications use port 21 to establish a connection for transferring files.
TCP port 21 is commonly used for FTP (File Transfer Protocol) services. In the past, hackers have exploited this port to perform brute force attacks, attempting to guess the username and password to gain unauthorized access to the system. They have also exploited vulnerabilities in the FTP service itself to execute arbitrary code or cause a denial of service. Additionally, hackers have used port 21 to distribute malware or carry out FTP bounce attacks, using the port to mask their identity while attacking another host. Furthermore, if the data transferred over this port is not encrypted, hackers can potentially intercept and manipulate it.
