An estimated 200 million people still using Microsoft's mothballed operating system Windows XP are at risk of cyberattack, experts warned.
The U.S. technology giant stopped support for XP in April, meaning users will not get critical security updates, leaving the OS open to hacking and attack.
Research firm IDC outlined the large number of consumers and businesses under threat, showing that 200 million users are still on XP, while the OS has 26.2 percent market share, according to Netmarketshare numbers.
"You're talking about 200 million computers running XP, simply put, they are open," Ernest Hilbert, former FBI agent and head of cyber investigations for EMEA at risk consultancy Kroll, told CNBC in a phone interview.
Heartbleed, IE threat
This comes amid a series of cybersecurity concerns over the past few weeks. The so-called "Heartbleed bug" was discovered by researchers last month in OpenSSL software—an encryption service used by around two-thirds of websites to protect information sent to and from web pages.
Microsoft also rushed to fix a bug in its popular Internet Explorer web browser after a computer security firm revealed the flaw, and it said in a blog post on Thursday that XP users would receive the update on a one-off basis.
"Just because this update is out now doesn't mean you should stop thinking about getting off Windows XP and moving to a newer version of Windows and the latest version of Internet Explorer," Adrienne Hall, general manager of Trustworthy Computing at Microsoft, said in an official blog post on Thursday.
"If you are on Windows 7, upgrade to Internet Explorer 11. Our modern operating systems provide more safety and security than ever before."
Despite this, Hilbert warned that other aspects of Windows XP are still are risk despite the Internet Explorer fix.
"There are still people out there looking at XP to find a way to manipulate it and now they know that nobody is going to try to fix it."
XP not 'mission critical'
Earlier this year, concern over the end of support for XP spread to the banks as the majority of their ATM machines ran the OS. Several of the major banks told CNBC they had signed extended deals with Microsoft.
But not all security experts agree the end of XP support could be fatal due to the outdated nature of the software.
"A lot of them will be old PCs and not being used for critical stuff. They are grannies and granddads keeping in touch with distant grandchildren," Bob Tarzey, analyst and director at Quocirca, an IT analysis firm.
"The bottom line is XP doesn't have the horse power to do a lot of the things people want to do on PCs nowadays. I doubt that computers running Windows XP are mission-critical."