As the clock winds down to the sun-setting of support of Microsoft's 12-year-old XP operating system on April 8 of next year, Microsoft today released new data that demonstrates the dangers of sticking with the older OS, which accounts for about one-fifth of all operating systems in use today.

Case in point: While 17 percent of Windows machines worldwide running the latest versions of software and real-time security tools from Microsoft faced malware in the first half of this year, XP machines were exploited the most. Windows 8 and Windows XP actually encountered nearly the same volume of malware, but XP machines were six times more likely to get infected from those malware threats, according to new data in Microsoft's Security Intelligence Report Volume 15, which was published today. And XP is still prevalent out there, accounting for 21 percent of all OSes in use today, according to data from StatCounter, Microsoft said.

Meanwhile, the number of all operating system vulnerability disclosures increased for the first time in a while, by nearly 40 percent in the first half of 2013, surpassing the number of browser vuln disclosures, the report says. Some 22.2 percent of all vulnerability disclosures were in operating systems. Browser bug disclosures dropped by 18.3 percent during the period, accounting for some 14.3 percent of all vuln disclosures.

Microsoft also revealed that disclosures of bugs in its own products increased 7.4 percent after a period of decline. There was a 3.1 increase in those disclosures over the same period in 2012.

Industrywide bug disclosures dropped 1.3 percent in the first half of this year from the second half of 2012, and dropped 10.1 percent since the first half of 2012.

"An increase in application vulnerability disclosures in 1H12 interrupted a trend of consistent period-over-period decreases dating back to 2H09. It remains to be seen whether the decrease in 2H12 marks a return to this trend," Microsoft said in its report, which gathers data from more than one billion Windows machines around the globe as well as from other vulnerability reports. "Overall, however, vulnerability disclosures remain significantly lower than they were prior to 2009, when totals of 3,500 disclosures or more per half-year period were not uncommon."

But the big message in Microsoft's new SIR report is the risk associated with sticking with XP. "It's still at 21 percent, and it's a 12-year-old OS, which is crazy," says Holly Stewart, senior program manager at Microsoft's Malware Protection Center. "A lot of [XP users] feel XP provides them with everything they want, and they are comfortable with it. It's not about functionality. It's about increased susceptibility" to threats with XP, she says.

Fiberlink, which manages laptops and desktops for enterprises, recently found that nearly half of the machines it manages for companies are still running XP. Chuck Brown, director of product management at Fiberlink, says he thinks the holdouts are doing so for financial reasons. He's already seeing machines getting updated to Windows 7 or 8, but fully expects at least 3 percent of his customers to still be XP as of the April 8 deadline.

[New data shows nearly half of XP machines still alive and well among 1 million machines managed by one vendor. See Windows XP Holdouts Hold On.]

XP is being targeted most by three major malware families today, Microsoft says: Sality, which steals personal information and can dial down the security settings in an infected machine; Ramnit, which infects Windows executable files, Microsoft Office files, and HTML files; and Vobfus, a worm that can download other malware onto a PC and can be spread via USB flash drives, for example.

"These are all worms, and fast-spreading ones," Stewart says. "They incorporate exploits from old vulnerabilities, and they rely on the interconnected world ... They exploit victims and use that computer as a launching pad to outreach others with the same loopholes -- friends, family, etc."

In the past year, 30 Microsoft Security Bulletins included patches for Windows 7, 8, and XP. "Imagine if XP had not been supported over the last year," Stewart says.

Microsoft won't completely cut off businesses that still plan to run XP on April 8, however. The software giant will offer a premiere support contract that will provide critical updates to XP. But the service must be purchased, and it only covers new threats to XP, Stewart says.

The main conduit for threats detected against Microsoft's own network in the first half of 2013 were Web browsers, the report says. Microsoft IT systems detected some 369,000 malicious JavaScript infection attempts on corporate machines, and some 220,000 malicious URL infection attempts. "Because web browsing was the most frequently used transmission vector for infection attempts at Microsoft in 1H13 ... the prevalence of HTML (.htm) and JavaScript (.js) files among threat detections is unsurprising," the report says. "Malicious program files (.exe) and malware disguised as temporary files (.tmp, .temp) were also detected relatively frequently."

Other findings in the report: High-severity vulnerability disclosures jumped nearly 13 percent across the industry in the first half of the year after decreasing in the second half of last year. High-severity vulnerabilities comprised 36.7 percent of total disclosures in the first half of 2013.

The full Microsoft SIRv15 report is available here for download.

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