Unusually, Microsoft has provided a patch for systems that are no longer supported, but are vulnerable to the Microsoft Security Bulletin MS17-010 flaw exploited by WannaCryptor (a.k.a. WannaCrypt among other names). These include Windows XP, Windows 8, and Windows Server 2003. A patch for later operating systems (i.e. those versions of Windows still supported) was made available in March 2017.
If you didn’t take advantage of the patch for Windows 8.1 and later at the time, now would be a good time to do so. (A couple of days earlier would have been even better.)
If you’re running one of the unsupported Windows versions mentioned above (and yes, I appreciate that some people have to), I strongly recommend that you either upgrade or take advantage of the new patch.
Microsoft’s announcement is here: Customer Guidance for WannaCrypt attacks, with links to the update and further information. Detection of the threat has also been added to Windows Defender.
Kudos to Microsoft for going the extra mile…
Additional analysis and/or commentary by ESET – Huge ransomware outbreak disrupts IT systems worldwide, WannaCryptor to blame, Malwarebytes – The worm that spreads WanaCrypt0r, and Sophos: Wanna Decrypter 2.0 ransomware attack: what you need to know. Among other vendors, of course. [Added subsequently: Symantec – What you need to know about the WannaCry Ransomware]