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 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] is practically indestructible,” Golovanov said.

“I wouldn’t say it’s perfectly indestructible, but it is pretty much indestructible,” said Joe Stewart, director of malware research at Dell SecureWorks and an internationally-known botnet expert, in an interview today. “It does a very good job of maintaining itself.”

Because TDL-4 installs its rootkit on the MBR, it is invisible to both the operating system and more, importantly, security software designed to sniff out malicious code.

But that’s not TDL-4’s secret weapon.

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. “The way peer-to-peer is used for TDL-4 will make it extremely hard to take down this botnet”.