File transfer protocol (FTP) is a service that allows users to transfer files between computers over a network. If enabled within firewall rules, FTP would present the greatest risk, as it can expose sensitive data to unauthorized access, modification, or deletion. FTP does not provide encryption or authentication, which makes it vulnerable to eavesdropping, spoofing, and tampering attacks. Simple mail transfer protocol (SMTP), simple object access protocol (SOAP), and hypertext transfer protocol (HTTP) are also services that can be used to exchange data over a network, but they have more security features than FTP, such as encryption, authentication, or validation. References: CISA Review Manual (Digital Version)