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."
Golovanov and Stewart based their judgments on a variety of TDL-4's traits, all which make it an extremely tough character to detect, delete, suppress or eradicate.
For one thing, said Golovanov, 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 -- sector 0 -- of the hard drive, where code is stored to bootstrap the operating system after the computer's BIOS does its start-up checks.
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," said Roel Schouwenberg, senior malware researcher at Kaspersky, in an email reply Tuesday to follow-up questions. "The TDL guys are doing their utmost not to become the next gang to lose their botnet."
Schouwenberg cited several high-profile botnet take-downs -- which have ranged from a coordinated effort that crippled Conficker last year to 2011's FBI-led take-down of Coreflood -- as the motivation for hackers to develop new ways to keep their armies of hijacked PCs in the field.
"Each time a botnet gets taken down it raises the bar for the next time," noted Schouwenberg. "The truly professional cyber criminals are watching and working on their botnets to make them more resilient against takedowns or takeovers."
TDL-4's makers created their own encryption algorithm, Kaspersky's Golovanov said in his analysis, and the botnet uses the domain names of the C&C servers as the encryption keys.
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