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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.
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