In networking, black holes refer to places in the network where incoming or outgoing traffic is silently discarded (or "dropped"), without informing the source that the data did not reach its intended recipient.
When examining the topology of the network, the black holes themselves are invisible, and can only be detected by monitoring the lost traffic; hence the name.
The attacker has made a remote connection using anonymous ftp connection and is now uploading files.
User 1 has a secure system and is using OpenSSH to connect to a remote system
User 1 and Attacker 2: User 1 has a secure system and Attacker2 is unable to connect
Attacker 4 has connected to the remote system using a password-free log in. The attack is now deleting system files.
User2 has connected to the remote system using telnet
User 2 and Attacker 3: Attacker 3 has connected to User 2's system using telnet and can now connect to the remote system