Final answer:
Hijacking a telnet session without sniffing the traffic involves methods such as IP session hijacking or exploiting network vulnerabilities. It's a complex task requiring significant skills and understanding of network security protocols. Secure alternatives like SSH should be used to prevent such attacks.
Step-by-step explanation:
If Trudy cannot sniff the traffic but knows there is a telnet connection, she could still attempt to hijack the session by employing other attack methods such as IP session hijacking or sidejacking. Since telnet is not a secure protocol and does not encrypt its communications, once Trudy gains control over one end of the session, for example by taking over a user's session ID, she can effectively insert her own commands.
To hijack a telnet session without sniffing the traffic, Trudy might exploit vulnerabilities in the network infrastructure, use prediction of sequence numbers to spoof packets, or trick the user into connecting to a proxy she controls. The difficulty of carrying out such an attack varies depending on the knowledge of the attacker, the security measures in place, and the specific situation, but generally speaking, it is considered a complex task that requires significant skills in network security and a good understanding of how the TCP/IP protocol works.
That said, using more secure communication protocols such as SSH (Secure Shell) which provides encryption would be a recommended solution to prevent such attacks.