Original URL: http://www.theregister.co.uk/2011/10/21/stealthy_rootkit_overhauled/

World's stealthiest rootkit gets a makeover

Now, TDL4 harder than ever to eradicate

By Dan Goodin

Posted in Security, 21st October 2011 18:41 GMT

One of the world's more advanced pieces of malware has just gotten a makeover that could make it even more resistant to takedown efforts, security researchers said.

An analysis of recent updates to the TDL4 rootkit, which is also known as TDSS and Alureon, shows that components including its kernel-mode driver and user-mode payload have been rewritten from scratch, researchers from antivirus provider ESET blogged earlier this week. The code overhaul may mean that operators of TDL4, which is used to force keyloggers, adware, and other malicious programs onto compromised machines, may have started providing services to other crimeware groups.

The makeover includes changes to the way TDL4 attempts to remain undetected by antivirus programs and other defenses. Newer versions create a hidden partition at the end of the infected machine's hard disk and set it to active. This ensures that malicious code stashed in it is executed before the Windows operating system is run.

It also protects the code from being removed. The partition is equipped with an advanced file system that checks the integrity of TDL4 components. If any of the files are corrupted, they're removed.

Not that TDL4 wasn't already among the most sophisticated pieces of crimeware available. When it emerged in 2008, it was virtually undetectable by most AV programs, and its use of low-level instructions made it hard for researchers to conduct reconnaissance on it. Its built-in encryption prevents network monitoring tools from monitoring communications sent between infected PCs and command and control servers.

It was among the first rootkits to infect 64-bit versions of Windows by bypassing the OS's kernel mode code signing policy. That protection was introduced into 64-bit versions of Windows and allows drivers to be installed only when they have been digitally signed by a trusted source. In June, researchers at AV provider Kaspersky said TDL4 had infected more than 4.5 million PCs in just three months.

TDL4 also has the ability to communicate over the Kad peer-to-peer network and to infect a the master boot record of a compromised PC's hard drive.

The latest changes suggest that the relentless innovation of those developing TDL4 shows no signs of slowing. ®