Security researchers are warning that the sophisticated Alureon -- aka TDL -- malware has gotten an upgrade, enabling it to take down even 64-bit versions of Windows Vista and Windows 7.
According to Chandra Prakash, technical fellow at GFI Labs, "the TDL4 rootkit bypasses driver signing policy on 64-bit machines by changing the boot options of Microsoft boot programs that will allow an unsigned driver to load."
The rootkit, which itself arrives as an unsigned driver, also disables debuggers, which "makes reverse engineering this rootkit very difficult," said Prakash.
Versions of Alureon that target 32-bit Windows first began appearing in 2006. According to Microsoft, "these Trojans allow an attacker to intercept incoming and outgoing Internet traffic in order to gather confidential information such as user names, passwords, and credit card data," and may also enable attackers to pass malicious data to a PC.
The 32-bit version made a large splash after Microsoft's February 2010 Patch Tuesday, when many Windows users found themselves unable to boot their machines. Microsoft traced the cause not to its security update directly, but rather to the effect that its security update had on machines already infected with Alureon.
At the time, Microsoft noted that 64-bit versions of Windows were immune from Alureon thanks to kernel patch protection, aka "PatchGuard."
Now, instead of taking down PatchGuard, Alureon has simply routed around it. According to Microsoft, "normally, 64-bit Windows has several protections against untrusted modifications to the kernel, including a requirement that all drivers be signed, and PatchGuard, which prevents tampering of certain system structures." Accordingly, Alureon taps a feature built into 64-bit Windows -- intended to support full disk encryption and software compression -- which allows unsigned drivers to be loaded.
Microsoft's statement on the matter is that Alureon "does not actually violate the guarantees PatchGuard provides about system integrity." That said, it will exploit 64-bit Windows.