More than four million PCs have been enrolled in a botnet security experts say is almost 'indestructible'
The botnet, known as TDL, targets Windows PCs and tries hard to avoid detection and even harder to shut down.
Code that hijacks a PC hides in places security software rarely looks and the botnet is controlled using custom-made encryption.
Security researchers said recent botnet shutdowns had made TDL's controllers harden it against investigation.
The 4.5 million PCs have become victims over the last three months following the appearance of the fourth version of the TDL virus.
The changes introduced in TDL-4 made it the "most sophisticated threat today," wrote Kaspersky Labs security researchers Sergey Golovanov and Igor Soumenkov in a detailed analysis of the virus.
"The owners of TDL are essentially trying to create an 'indestructible' botnet that is protected against attacks, competitors, and anti-virus companies," wrote the researchers. (read more)