Conficker was the 2008 computer worm that infected millions of Windows PCs and formed one of the largest botnets of its era.
Conficker began spreading in late 2008 by exploiting a vulnerability in Microsoft Windows Server service. Microsoft had issued a patch before the main outbreak, but systems that remained unpatched were vulnerable. The worm could also spread through removable drives and weak network passwords.
Security researchers estimated that millions of computers became infected. Conficker used several techniques to resist disruption, including generating large numbers of possible domain names that infected machines could contact for instructions. This made it difficult for defenders to predict and block every potential control channel.
The botnet’s feared uses were not fully realized on the scale many expected, partly because researchers and domain registrars coordinated to block or take control of many communication paths. Conficker remains a major case study in patching, coordinated defense and botnet resilience.