A new and improved botnet that has infected more than four million PCs is “practically indestructible,” security researchers say.

“TDL-4,” the name for both the bot Trojan program that infects machines and the ensuing collection of compromised computers, is “the most sophisticated threat today,” said Kaspersky Labs researcher Sergey Golovanov in a detailed analysis Monday.

TDL-4 infects the MBR, or master boot record, of the PC with a rootkit — malware that hides by subverting the operating system. The master boot record is the first sector of the hard drive, where code is stored to bootstrap the operating system after the computer’s BIOS does its start-up checks.

What makes the botnet indestructible is the combination of its advanced encryption and the use of a public peer-to-peer (P2P) network for the instructions issued to the malware by command-and-control (C&C) servers.