Telnet
Purpose: Provides bidirectional interactive text-oriented communication using a virtual terminal connection.
Vulnerabilities: Weak authentication (e.g., sending credentials in plaintext), Eavesdropping (as data is transmitted in plaintext), Man-in-the-middle attacks, Session hijacking, Brute force attacks on login credentials.
Tools: Nmap, Hydra, Wireshark, Telnet clients.