Kaspersky Lab Weblog:
Support for Windows XP is ending: after today there will be no new security updates, non-security hotfixes, free or paid assisted support options or online technical content updates.
Is this a problem? After all, it's a 12-year old operating system.
It wouldn't be, if it weren't for the fact that there are still a lot of people running Windows XP - our data indicate that around 18 per cent of our customers are still running Windows XP. That's a lot of people wide open to attack once the security patches dry up: effectively, every vulnerability discovered from now will become a zero-day vulnerability - that is, one for which there is no chance of a patch.
The problem will be compounded once application vendors stop developing updates for Windows XP - every un-patched application will become another potential point of compromise, further increasing the potential attack surface.
Continued : http://www.securelist.com/en/blog/208213056/End_of_the_line_for_Windows_XP
Related:
Rest in Peace, Windows XP
The XPocalypse is upon us: Windows XP support has ended
XPocalypse: Experts Warn of Attackers Hoarding Windows XP 'Forever Days'

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