Conficker (also known as Downup, Downadup e Kido) is one of the oldest and still active malware, according to Check Point researchers it is the most prominent family accounting for 14 percent of recognized attacks Conficker is technically a worm that leverages a vulnerability in the Windows Server Service to spread itself, it targets an already-infected computer by using a specially-crafted RPC request to force a buffer overflow and execute shellcode on the target computer. The threat reached the peak in 2009, when it was estimated to have infected more than three million PCs.”]
Source: https://securityaffairs.co/wordpress/48613/malware/conficker-malware.html