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The 10 Worst Computer Viruses to Ever Infect Networks

Some computer viruses have been playful, like the original computer virus known as the Elk Cloner. It infected the Apple II machines way back in 1982, and was mostly for a laugh. Computer viruses of the modern age, however, have been much more damaging and costly. Some of the worst computer viruses cause millions in damages.

Let’s look at 10 computer viruses that were costly, rather than annoying, and maybe even educate ourselves about computer security a little.

10 Worst Computer Viruses

1. Conficker causes massive damage

The Konficker virus took advantage of an exploit in Windows 2000, XP, and 2003. It spread via USBs and over networks, and could even infect servers with print and file sharing enabled.

worst-computer-viruses-bombWith the millions of computers that were infected Konficker could:

  • Disable Anti-malware programs, leading to even more infections.
  • Create backdoors in firewalls, opening computers up to all kinds of problems.
  • Possibly start an enormous botnet.
  • Launch a huge DDoS attack.
  • Copy a search engine and steal private information from those who use it, and then sell that information. (like Google does, except …no that’s exactly what Google does)

It is estimated that Konficker caused, get ready for this, $9.1 billion in damages to computers and networks worldwide. This number was pushed high as a number of military systems in England were infected, as well as medical systems in both the UK and USA.

2. Zeus throws his thunderbolts

Zeus was a Trojan horse virus which infected Windows computers and controlled them to commit digital crimes. This was mostly man in the browser keylogging, and form grabbing. How computers were infected varies. Some were via downloads, others were via phishing scams.

It was first discovered in 2009, and had gone on to infect thousands of FTP accounts for major corporations like:

  • Amazon
  • Oracle
  • Cisco
  • Bank of America

The primary use of Zeus by its owners were to steal login details for social networks, email accounts, and online banking customers.

It has been estimated that $70 million were stolen by the controllers of Zeus, easily making it one of the worst computer viruses. On the bright side, over 100 arrests were made in connection with Zeus.

3. Cryptolocker adds encryption where you don’t want it

Everyone should love encryption. It’s what makes your online banking secure, makes a VPN network work to protect your privacy, and protects your sensitive files. Cryptolocker was an example of encryption gone bad. It would infect computers, find files on it that were considered of value, and then encrypt them without giving the decryption key …unless you paid up.

Cryptolocker is an example of ransomware, a type of virus which is becoming popular again. Worse still, Cryptolocker was associated with the same people operating Zeus. It fell apart when the Zeus arrests were made, but it is estimated that they made over $3 million off this encryption as extortion tool.

4. ILOVEYOU: When love gives you a worm

Getting a letter from a secret admirer in the Philippines should always be met with a bit of skepticism if you’ve never been there. If it’s via email, and you’ve never met them before, you’re getting scammed. The ILOVEYOU worm was a type of email trojan that:

  • Replicated itself in folders all over your computer.
  • Some replications replaced your own files.
  • Sent out registry keys.
  • Used your email contacts and IRC to forward itself.
  • Stole your passwords and emailed the data back to the hacker.

This one doesn’t have a happy ending. The hacker, who launched this back in 2000, has not been caught. Over $10 billion in damages were done.

5. The many faces of Klez

worst-computer-viruses-wormKlez was one of those computer viruses that seemed to want to do everything. It was:

  • A worm that infected your computer through email.
  • A trojan horse.
  • A fake virus scanning program.
  • An email spoofer.

It really did it all, and it got worse as more hackers got hold of it and altered it repeatedly. They had labeled it all the way up to a G class variant, starting at A! Sure, a simple anti-virus program could stop it, but with all those changes it was allowed to keep on growing for far too long.

6. A code red backdoor emergency

The original Code Red worm started a DDoS attack on the servers of just one house. Unfortunately, that house was The White House. Every computer that was infected by Code Red was programmed to send an email to the White House at exactly the same time, completely overwhelming the servers of the White House.

That was a bad time, but just as bad was Code Red II. This virus created a backdoor into the machine that was infected. This backdoor was used by the hacker to remotely control the victim’s computer. Computer owners found themselves helpless to use their machines as they were no longer their own.

7. Admin spelled backwards is Nimda

Nimda was the fastest spreading computer virus of its time, and possibly all time. It went from nothing to the top of the reported attacks list in 22 minutes. With a spread that fast, you can be sure it had a big goal: To bring down the average speed of the entire Internet.

computer-bugThis attack crashed a number of networks, such as:

  • Dell
  • Demon
  • Australia’s Parliament

With an estimated cost of over $531 million it was, essentially, one of the largest DDoS attacks of all time, only paling in comparison to the recent attack on Brian Krebs. The 2 million machines that were infected played a large role in how quickly, and how effectively, it spread and reached its goal.

8. SQL Slammer worm causes worldwide havoc

This is one of those viruses that you may have heard of, especially if you were a Bank of America customer at the time. It shut down your ATMs for a period of time in 2003. It also:

  • Crashed the e-ticketing service of Continental Airlines.
  • Caused an outage to part of Seattle’s 911 service.
  • Slowed Internet traffic across the world.

The reason this all was allowed to happen was many people didn’t patch a known issue on Microsoft computers. Microsoft made a patch available, but people didn’t download it. Too many of the worst computer viruses exist simply because people don’t take steps freely available to them. $1 billion in damages could have been avoided if more people just installed the patch!

9. Sasser & Netsky: Two viruses, one man

These were 2 very similar viruses which were designed by one person. The Sasser virus was very different in that it didn’t use emails to spread, but instead scanned random IP addresses and told systems it found to be vulnerable to download it. Those who downloaded it had to unplug their computers to power them down in most cases. Victims included:

  • The AFP news agency having satellite communications blocked.
  • Delta Air Lines cancelling many transatlantic flights due to their systems being infected and slowed down.
  • Sampo Bank having to close down completely, while Goldman Sachs, Deutsche Post, and the European Commission also had to move their business around the virus.
  • The X-ray department at Lund University Hospital lost use of their X-ray machines.
  • The British Coastguard having losses to their electronic maps.

Some pretty serious stuff!

Netsky was a bit more typical as it spread primarily through spoofed email accounts. Its point was to spread large files rapidly across the Internet, causing serious speed delays all over the world. Both viruses all happened because of some 18 year old in Russia who was celebrating his birthday…

10. The perfect/worst Storm Worm

email-spoofing-worst-computer-virusThe Storm Worm got its name as it spread via email initially with an email subject of “230 dead as storm batters Europe.” Those who were curious enough to open the email had their computers infect, and the machines were taken over by the hackers as part of a huge botnet.

How were users to protect themselves from this virus? All they had to do …was not open the email. Seriously: Go to CNN for your news, not your email! So many computer viruses would have never spread if more people simply didn’t open emails they weren’t sure of. Upwards of 50 million computers could have been infected, all because someone couldn’t stop themselves from being too curious.

If you have learned anything by reading through all of this, I hope that it is that you need to invest in some computer security software. You can click the link to learn about five exceptional tools that will help protect you against the worst computer viruses, such as the ones we just looked at.