Ransomware known as WannaCrypt (aka WCry, WannaCry) has already crippled as many as 75,000 unpatched Windows computers in Europe and Asia. So far it hasn’t done much damage in North America, but that could change quickly.

The flaw WannaCrypt uses to infect Windows computers was patched by Microsoft in March, but unpatched computers and those running unsupported versions of Windows were left unprotected.

Microsoft has long since stopped releasing security updates for Windows XP, but WannaCrypt is spreading quickly, and Windows XP computers are essentially defenseless against it. So Microsoft has taken the unprecedented step of publicly releasing an update that protects Windows XP computers from the flaw that WannaCrypt uses to spread.

If you manage any computers that run Windows XP, you should install the update immediately: download update for 32-bit Windows XP Service Pack 3. There’s more information about this from Microsoft.

Techdirt points out that the flaw WannaCrypt exploits was exposed in the recent NSA tool leaks. Which is exactly the problem when security organizations hoard flaws instead of reporting them responsibly.

Update 2017May14: Apparently a security researcher at MalwareTech registered a (previously unregistered) domain used by WannaCrypt as part of his investigation into the ransomware. This is standard practice, because it often allows researchers to gain a better understanding of their subject. Surprisingly, this move stopped WannaCrypt from doing any further damage.

The latest guidance from NCSC.