Internet-connected system threats

A system connected to the Internet can be reached by anyone in the world, so it faces the following major threats.

1. Viruses and worms: malicious code spreading through e-mail attachments, downloads and network services. The Morris worm and later Internet worms spread through vulnerable services without user action.
2. Trojan horses and spyware: programs that look useful but steal data or open the system to attackers.
3. Denial of Service (DoS/DDoS): flooding a server with many requests (often from a botnet) so legitimate users cannot be served.
4. Eavesdropping/sniffing: capturing passwords and data travelling in plain text across the network.
5. Spoofing and masquerading: forging IP addresses, e-mail senders or web sites (phishing) to trick users.
6. Buffer overflow and exploiting software bugs: attackers send specially crafted input to run their own code.
7. Man-in-the-middle and replay attacks: the attacker intercepts and alters or resends messages.
8. Password attacks: guessing, dictionary attacks and brute force.
9. Malicious mobile code: harmful ActiveX controls, Java applets or JavaScript downloaded from web pages.
10. Social engineering: deceiving users into disclosing information.

Countermeasures

Firewalls, antivirus, patching, encryption (SSL/TLS, IPsec, VPN), strong authentication, intrusion detection systems, regular backups and user awareness.