The most destructive computer viruses — and how to protect yourself

Computer viruses can strike anyone — and the damage can be serious, from stolen data to total system crashes. To better protect yourself, here’s a look at some of the most destructive viruses ever created, and what you can do to avoid them.

MyDoom

Considered by many to be the most destructive computer virus of all time, MyDoom spread over the now-defunct file-sharing website Kazaa and through email attachments. It would act as a worm, sending out emails to the affected computer’s contact list, with recipients being more likely to open attachments from someone they know. It’s estimated to have caused over US$38 billion in damages through DDoS attacks. MyDoom is still occasionally detected in email attachments, but it is much less widespread than during its initial outbreak. Less experienced users would be wise to learn how to spot compromised emails.

ILOVEYOU

Also known as the love letter virus, this worm acted similarly to MyDoom in that it accessed the user’s email list and spread to others by sending an email titled “ILOVEYOU.” When curious users opened the attachment, the virus would infect their computer as well, and the spread continued. It’s caused billions in damages over the years, including through compromised passwords.

Cyborg – Fake Windows Updates

Disguised as emails and pop-up ads, the Cyborg ransomware virus will trick users into downloading them by suggesting that their computer needs a critical update. It will then encrypt all programs and files in your computer and demand a ransom payment to return the files for you.

Code Red

Named after the flavour of Mountain Dew that two digital security employees were drinking at the time that they discovered it, Code Red is a hard-to-notice virus that operates off the system’s memory and lies mostly dormant until used to launch DDoS attacks. Staying on top of your computer’s security updates can be an effective way to remove this virus that may be lurking in your files.

Melissa

One of the first widespread computer viruses, Melissa was responsible for a number of harmful actions including modifying files, emailing the user’s contact list, and ultimately clogging up servers everywhere. The harm it caused forced the FBI to get involved, with an estimated US$80 million in damages and the creator serving 20 months in prison—reduced from 10 years after he helped law enforcement catch other hackers.

Zeus

Named after the leader of gods, it shouldn’t come as a surprise that Zeus is one of the most damaging viruses of all time, having breached as many as 88% of all Fortune 500 companies. It stole data and caused an estimated US$3 billion in damages, and was responsible for nearly half of all attacks on banks in the year it was created.

Flashback

One of the most notorious viruses to hit Macs, this piece of malware spread rapidly in 2011, during a time when many people believed that Apple products were immune from viruses. Disguising itself as a Flash installation prompt, it would turn the computer into part of its bot network, able to be remotely controlled. At its peak, over 600,000 Macs were infected before Apple pushed out a security update. Flashback is no longer a widespread threat, but it remains an important example of Mac-targeted malware.

Stuxnet

One of the first viruses created for cyberwarfare, Stuxnet was believed by many to be a collaborative project between the United States and Israeli militaries in an effort to disrupt nuclear efforts in Iran. The attack was successful, doing notable damage to the nation’s military programs.

CryptoLocker

This trojan horse targeted Windows computers, spreading through email and other methods of tricking a user into downloading the damaging program. CryptoLocker acts as a piece of ransomware, encrypting computer files and demanding about US$400 in prepaid cash or bitcoin to restore the files.

Sasser

The Sasser worm was notable for targeting unsecured computers that hadn’t updated their security software, which should serve as a lesson in how to stay safe online. The worm would multiply and spread, quickly causing major damage to organizations such as Delta Air Lines and the British Coastguard.

Clop

One of the newer ransomware threats, Clop is capable of blocking over 600 Windows processes and features, demanding payment from the user to unlock them and rendering their PC effectively useless until payment is made. Clop has evolved over the years and is now able to attack entire networks, having done so to the Netherlands’ Maastricht University.

Conficker

Spreading through peer-to-peer networking, Conficker is another worm that turns infected computers into part of a botnet, using them in DDoS attacks directed by a hacker. In total, an estimated 15 million computers were infected at Conficker’s peak.

Sobig

Another worm that spreads through email, Sobig is reported to have caused US$30 billion in damages in a single year. The worm spread so rapidly that people reported receiving hundreds of infected emails a day, with Air Canada even having to suspend flights due to computer traffic getting slowed to a halt.

Klez

Infecting over 7 million computers and causing nearly US$20 billion in damages, the Klez worm is one of the most deadly to ever exist. In circulation since 2001, it has spawned new versions released every few years and spread through emails, continuing to keep security specialists on their toes with new updates.

Swen

Disguising itself to appear as an operating system update, the Swen worm has caused over US$10 billion in damages. Even if users click no on the prompt asking if they want to update their computer, the virus silently installs itself anyway in the background and shuts down a number of computer processes.

Storm

Created in 2007, Storm would go on to become the largest botnet ever created, infecting as many as one million computers, shutting down internet access for large networks including the entirety of Estonia. Cleverly, the virus spread through alerts of a deadly incoming storm, tricking users into clicking the pop-up or link warning them of incoming dangerous weather.

Yaha

Believed to have originated as part of a cyberwar between Pakistan and India, the Yaha virus infects computers and closes down firewalls and antivirus software, making the infected technology more vulnerable to future attacks. It can also send information from the infected PC back to an origin location, making it perfect for collected critical data.

Mimail

This worm was another that spread through infected emails, disguising itself as important information requiring the recipient to open an attachment. Once it had infected the computer, it would harvest data from it and use it as a potential server in a botnet attack. Microsoft released a patch for its email servers that mostly shut this rapidly spreading virus down.

Bugbear

Named after a creature from Dungeons & Dragons, Bugbear is a keylogger that targeted financial institutions and spread to over 150 countries. Keyloggers work by making a record of every keyboard stroke, sending the virus creator a log that can include usernames, passwords, and sensitive data.

ExploreZip

A clever virus that spreads through emails, ExploreZip would not instantly send messages to your entire contact list, but instead spread by waiting to notice when you received an email. It would then reply with “Hi ! I received your email and I shall send you a reply ASAP. Till then, take a look at the attached zipped docs. Bye.” Like most other worms of this type, it’s best prevented by avoiding suspicious attachments and links.