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6. Santy (2004)
A specialised worm that demonstrated quite how subtle attack vectors could be, Santy used search engines — Google at first, then Yahoo and AOL — to find vulnerable sites running phpBB bulletin board software, which it then attacked. It spread worldwide in less than three hours.
The most unusual aspect of the worm was that someone then produced an anti-Santy worm that used the same techniques to find and infect phpBB installations, but then patched the problem and inoculated the sites against further attack.
Photo credit: CNET News
7. Conficker (2008)
This Windows worm, also known as Downadup, hit up to 15 million Microsoft servers, causing operational problems for the British, German and French military among many others. Its use of encryption and stealth code hiding made it very difficult to eradicate, as have its constant revisions: it went through five major updates in six months.
Those revisions have demonstrated that the Conficker writers are closely observing and reacting to industry efforts to eradicate the malware. In response, Microsoft convened a working group of companies across the internet and security markets, and put up a $250,000 bounty for information leading to the conviction of the miscreants.
Photo credit: Gppande
8. Stuxnet (2010)
The most sophisticated malware observed, this is a uniquely targeted worm that propagates via Windows and attacks industrial controller hardware — but only of a certain configuration (such as the Siemens S7-300 controller, above).
It is thought to have been designed to damage the Iranian nuclear programme, and may well have succeeded. When it finds its target system, it reprograms high-frequency motor controllers to operate in an intermittently out-of-specification way. It thereby upsets industrial processes in a manner that's hard to identify.
Although the authors of Stuxnet aren't known, reports earlier this year said that the malware was claimed as a success of the Israeli Defence Force, in a video shown at the retirement party of the force's chief of general staff, Lieutenant General Gabi Ashkenaz.
Photo credit: Ulli1105