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Tens of thousands of Internet threats are currently known to exist, and dozens—sometimes hundreds—of new threats are being released on the Internet every day.
At one time, viruses were the most common problem, but today's Internet threats are often quite sophisticated, designed to spread with extreme rapidity and exploit certain faults or weaknesses in an operating system, browser, or network security system.
Trojan horses
Worms (and droppers, the bits of harmful code often left behind in a worm attack)
Spyware
Network viruses
DoS (Denial of Service attacks)
Mixed threat attacks
Malicious Java code and Applets
VBScript, JavaScript
Joke programs
Executable and Link format
Viruses, including:
HTML viruses
Macro viruses
ActiveX malicious code
.COM and .EXE file infectors
Boot sector viruses
Web mail attachments
Web traffic and sites (disease vectors)
FTP traffic from the Internet (file downloads)
In addition, there are a lot more network vulnerabilities:
Instant Messaging file transfers
Infected laptops that log on to your network
Key-logging spyware that opens an outbound port
Custom hacked legitimate programs, or manipulated counterfeits
Virus Writers