No 4. Code Red

Above is an image depicting the spread of infection caused by Code Red. Released in 2001, Code Red took advantage of buffer overflow vulnerability in Microsoft IIS servers. It infected upto 359,000 hosts in a single fateful day. It worked by using a long string of the repeated character ‘N’ to overflow a buffer, allowing itself to execute arbitrary code and infect the host machine. The menace of the virus was such that the person who discovered how to block it, Kenneth D. Eichman, was invited to the White House for doing so.

3 months after the release of Code Red, Code Red II was released which essentially was a variant of the original. Both worms combined dealt in damages worth $2 billion at an astonishing rate of $200 million per day.

No 3. Conficker

This worm was first released in 2007 and it targeted the Microsoft Windows operating system. Conficker infection is now amongst the largest virus infections in history with more than seven million government, business and home computers in over 200 countries now under its control. Once infected, the host computer would download malware from sites controlled by the hackers including things like keystroke loggers and remote PC-control software. The Conficker virus has caused a massive $9 billion in damages.