Beware Windows XP, as 74% of rootkit infections can be traced to PCs running that operating system. By comparison, only 17% of rootkit infections were spotted running on Vista, and 12% on Windows 7.

That finding comes from Avast--a free antivirus software developer based in Prague--and is based on a six-month study the company conducted of 630,000 malware samples encountered by its users.

Interestingly, the prevalence of rootkits running on Windows XP, which debuted in 2001, is greater than the operating system's actual market share, which for Avast users was 49%, compared with Windows 7 (38%) and Vista (13%).

Attackers favor rootkits because they can surreptitiously be used to gain admin-level access to a PC, then silently install additional files and steal data. "Because of the way they attack--and stay concealed--deep in the operating system, rootkits are a perfect weapon for stealing private data," said Przemyslaw Gmerek, Avast's lead researcher, in a statement. He plans to detail his rootkit research in full, this week at the Black Hat USA conference, a UBM TechWeb event, in Las Vegas.

Most often, rootkits attack the master boot record (in 62% of attacks), followed by driver infections (27%). In terms of specific rootkits, Avast said that Alureon botnet--aka the TDL3 and TDL4 rootkit family--accounted for nearly three-quarters of all rootkit infections.

What, however, makes Windows XP such a rootkit magnet? "One issue with Windows XP is the high number of pirated versions, especially as users are often unable to properly update them because the software can't be validated by the Microsoft update," said Gmerek. As a result, he said, numerous Windows XP users aren't using the latest version--SP3--but rather earlier versions, such as SP2, which Microsoft ceased supporting--as well as patching--in July 2010. (Microsoft has said that it will cease supporting Windows XP SP3 in April 2014.)

Attackers also favor Windows XP because even SP3 lacks some of the anti-rootkit and other security features that Microsoft has added to more recent operating systems, including user account control, PatchGuard, and driver signing.

Since Windows XP, Microsoft has continued to boost the security of its operating systems and applications, although developer uptake of the enhancements can lag. Earlier this year, notably, Microsoft called on developers to implement more secure lifecycle software practices, as well as to better avail themselves of newer Microsoft-built mitigation technologies, including data execution prevention (to prevent attackers from executing arbitrary code) and address space layout randomization (to make it difficult for malware to locate known files on a PC that may contain vulnerabilities).

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