A new version of the malware that crippled Windows PCs last February sidesteps safeguards designed to block rootkits from hijacking machines running 64-bit editions of Windows, according to security researchers.
"A new era has officially dawned; the era of x64 rootkits," said Prevx researcher Marco Giuliani in a post to the company's blog yesterday.
The updated rootkit, which goes by names including Alureon, TDL and Tidserv, is able to infect 64-bit Windows PCs. "TLD3 can be considered as the first x64-compatible kernel mode rootkit infection in the wild," Giuliani said.
Both Prevx and Symantec have found evidence that hackers are actively using the rootkit.
"The infection is spreading on the web, by using both porn websites and exploit kits," said Giuliani, who added that UK-based Prevx had first spotted the new rootkit more than a week ago. Symantec's first sighting was on Wednesday.
A previous version of the rootkit caused serious problems earlier this year after a Microsoft security update crashed 32-bit Windows machines.
Within hours of a February 9, 2010 release of security update MS10-015, users reported that their computers wouldn't restart. Two days later, Microsoft halted automatic distribution of the update and launched an investigation .
MS10-015 patched a 17-year-old Windows kernel bug that was publicly disclosed in January 2010 by Google security engineer Tavis Ormandy.
Microsoft later concluded that only PCs already infected with a rootkit it called Alureon were incapacitated with Blue Screen of Death (BSOD) errors. It didn't restart the distribution of MS10-015 until early March, when it added code to block installation when a rootkit infection was detected. Subsequent kernel patches have included that same detection.
During the BSOD brouhaha, the then-current Alureon was able to successfully infect only 32-bit versions of Windows. That limitation no longer applies.
The new rootkit sidesteps two important anti-rootkit protections Microsoft built into 64-bit Windows, Kernel Mode Code Signing and Kernel Patch Protection, also known as PatchGuard. The pair are designed to make it more difficult for malware to tamper with the operating system's kernel.
"To bypass Kernel Patch Protection and driver signature verification, the rootkit is patching the hard drive's master boot record so that it can intercept Windows' startup routines, own it, and load its driver," Giuliani said.
Rootkits that overwrite the hard drive's master boot record (MBR), where code is stored to bootstrap the operating system after the computer's BIOS does its start-up checks, are essentially invisible to the operating system and security software.
"The main Tidserv components are stored in unused space at the end of the hard drive in encrypted form," said Symantec researchers in a Thursday note on the company's security response blog. "This makes it more difficult to detect and remove once a computer is infected."
Both Prevx and Symantec said that they were continuing to analyse the 64-bit rootkit, and would publish more information when they had it.