Virus attacks are not a shocking news anymore. But here is the list of the worst of those attacks which shocked many at that time in history.

1. Melissa

Melissa was created by David L. Smith in 1999 and is based on a Microsoft Word macro. He intended to spread the virus through e-mail messages. The virus prompts the recipient to open a document and by doing so the virus gets activated. The activated virus replicates itself and will be transferred to 50 persons whose address is present in the recipient’s e-mail address book. The virus was spread rapidly after it was unleashed by Smith. The increase in e-mail traffic due to the virus forced some companies to block e-mail programs until the virus attack was controlled.

2.  MyDoom

The MyDoom creates a backdoor in the OS of the victim’s computer. The MyDoom virus had two triggers. One of them began a denial of service (DoS) attack on Feb. 1, 2004. In Feb. 12, 2004 the second trigger was initiated which stopped the virus distributing itself. Later that year, MyDoom virus outbreak occurred for a second time, which targeted several search engine companies. The virus would send a search request to a search engine and will use e-mail addresses obtained in the search results. Such a type of attack slowed down search engine services and caused some website crash.

3. ILOVEYOUILOVEYOU

virus threat was discovered a year after the release of the Melissa virus and it emerged from Philippines. Unlike the Melissa virus, ILOVEYOU was a standalone program which was capable of replicating itself. The virus initially traveled through the e-mail, same way as Melissa virus. The email had a subject which says that the message was a love letter from the secret admirer. Attachment with this e-mail caused all the trouble. The file LOVE-LETTER-FOR-YOU.TXT.vbs contained the worm. As the name suggests Visual Basic Scripting was used for creating this virus. The copied itself several times and made victim’s several folders hidden, it added several new files to the victim’s computer registry keys and replaced several files with copies of itself.

4. Nimda

Nimda was spread through the Internet rapidly and became one of the fastest propagating computer virus. The Nimda worms aimed on the Internet servers and its real purpose was to slow down the Internet traffic. Nimda could travel through the Internet in multiple methods which included the email. The Nimda worm was able to create a backdoor into the victim’s OS. If the victim was logged in as the administrator for the machine, then the worm would provide the attacker the full control over the system. The Nimda virus caused several network systems to crash as the system’s resources were taken away by the worm. The Nimda worm was one of the dreaded distributed denial of service (DDoS) attack virus.

5. The Klez Virus

The Klez virus appeared in late 2001 and infected a victim’s computer through an e-mail message. The virus replicated itself and was sent itself to all the contacts in the victim’s address book. The virus could disable virus-scanning software and could falsely act as a virus-removal tool. The modified version of this virus could take any name from the contact list of the victim and can place that address in the “From” field. This technique is called spoofing. By spoofing the e-mail appears to come from a source when it’s actually coming from somewhere else. Spoofing will prevent the user’ s chance to block email from a suspected recipient.

6. SQL Slammer/Sapphire SQL

Slammer/Sapphire virus caused a damage of more than $1 billion and the affected networks included Bank of America’s ATM service, Continental Airlines etc . A few minutes after the infection of the first Internet server, the number of victims of the Slammer virus doubled every few seconds. After Fifteen minutes of the first attack, half of the servers that act as the pillars of the Internet were affected by the virus.

7. Sasser and Netsky

The Sasser worm exploited Microsoft Windows vulnerability. The infected system will look for other vulnerable systems and instruct those systems to download the virus. A random scan of the IP addresses was done to find potential victims. The virus made it difficult to shut down the computer without turning OFF the system. The Netsky virus spread through e-mail and Windows networks. The virus spoofs e-mail addresses and is propagated through a 22,016-byte file attachment. The virus causes a denial of service (DoS) attack on the affected system.

8. Leap-A/Oompa-A

Mac computers are generally protected from virus attacks due to a concept called security through obscurity. Apple keeps its OS and hardware a closed. This has not completely stopped the attack on the Mac systems. Oompa-A, was one of the viruses which aimed at Mac systems. The viruses used the iChat instant messaging program for its propagation among vulnerable Mac computers. The Leap-A virus was not able to cause much harm to computers, but showed that even a Mac computer can be affected by malicious softwares.

9. Code Red and Code Red II

Code Red and Code Red II exploited operating system vulnerability found in Windows 2000 and Windows NT machines. A buffer overflow problem was the vulnerability. Due to this if the OS receives more information than its buffers handling capacity; the adjacent memory will be overwritten. The original worm initiated a distributed denial of service attack to the White House website. That means all the infected computers with Code Red try to contact the Web servers at the same time, thereby overloading the machines. The infected machine no longer obeys the owner, allowing a remote user to control and access the machine.

10. Storm Worm

The Storm Worm got this particular name because of the fact that the e-mail messages which carry the virus carried a subject “230 dead as storm batters Europe.” The virus is a Trojan horse program and its payload is another program. Some versions of this Worm turn computers into bots or Zombies. The infected computers become vulnerable to further attack by the person behind the attack.