1. MyDoom

MyDoom was first unearthed in 2004, the worm was designed to target Microsoft Windows Operating System. When infecting a machine, it was able to open random programs and create network openings allowing hackers to then control the computer. The worm was spread through email attachments which contained the text “Andy; I’m just doing my job, nothing personal, sorry“, yet the author of the worm is unknown. MyDoom is still the fastest-spreading worm yet, and certainly the most expensive.

Estimated cost of the malware: £29.1 billion ($38 billion)


ILOVEYOU spoofed itself as a love confession email attachment, peaking users’ curiosity. The ‘love confession’ posed as a text file, but was instead a script that would replicate, send itself to everyone in the user’s contact list and overwrite files until the computer would no longer boot up. Due to its heavy open rate, the virus spread quickly.  ILOVEYOU infected computers all over the word (approximately 10% of the worlds connected computers). As ILOVEYOU deployed back in 2000, it can be considered the first socially engineered virus.

Estimated cost of the malware: £7.7 billion ($10 billion)

3. Conficker

Conficker launched in 2008, the virus took advantage of an exploit in Windows operating systems, that meant an unauthenticated file would be installed. The virus could affect servers with firewalls as long as print & file sharing was enabled. Conficker was spread by infected USB drives and over networks. Later variants of Conficker could disable anti-malware programs, create backdoors in firewalls and communicate with other infected machines via peer-to-peer networks. The virus caused major disruption by infecting computers and medical devices in UK & US hospitals, French fighter planes were unable to download their flight plans, it infected 75% of the UK Royal Navy fleet & shut down the UK’s Manchester City Council IT system.

Estimated cost of the malware: £6.97 billion ($9.1 billion)

4. WannaCry

WannaCry emerged in May 2017 and spread throughout networks across the globe. The ransomware exploited a vulnerability in Microsoft Window’s operation system, meaning hackers could encrypt organisations’ files and demand a ransom for them to be released. Avast detected 250,000 instances of WannaCry across 116 countries, with one of the most notable attacks being the NHS.

Estimated cost of the malware: £3.1 billion ($4 billion)

5.Code Red

Code Red is a worm that infects devices through a basic internet connection, no human action needed. Code Red transpired in 2001, infecting devices by scanning networks for a host. Code Red overwhelmed computers with random data requests to the point that it began to overwrite its own memory. Code Red invaded numerous PCs, including devices in The White House.

Estimated cost of the malware: £2.11 billion ($2.75 billion)

6. Melissa

Melissa is a Microsoft Word macro virus that first appeared in 1999. A macro is a series of commands or instructions that are carried out automatically, Melissa was one of the first email-activated viruses. The virus shut down the safeguards in Microsoft Word 97 & 2000, lowered the security settings and disabled the macro security. The infected computers with Microsoft Outlook would send the infected document to the top 50 contacts in the user’s address book, and if the day of the month matched the minute, it would insert a quote from The Simpsons. Melissa affected hundreds of websites, and The Microsoft Corporation had to disable all incoming and outgoing emails.

Estimated cost of malware £919 million ($1.2 billion)


7. Slammer

Slammer is a worm that reared its ugly head in 2003, and again in 2016. Slammer was the fastest spreading worm of its time, it managed to shut down South Korea’s internet for 12 hours! The worm caused a denial of service (DoS) on some Internet hosts and dramatically slowed down general Internet traffick. Slammer took 15 minutes to spread worldwide, it spread to over 90% of all vulnerable hosts in 10 minutes and infected approximately 359,000 computers. Slammer did plenty of damage around the world with airlines having to delay and cancel flights, newspaper releases being delayed, USA’s 911 Emergency Network being shut down and crashing Portugal’s internet network.

Esimated cost of the malware: £766.3 million ($1 billion)

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