The note.
WannaCry was a ransomware attack by the WannaCry ransomware worm. It was able to spread through EternalBlue (a exploit that uses a bug to run commands on a PC without the owner knowing). Previously, Microsoft had patched up the exploit however many companies either had not applied the patch due to various reasons or used operating systems that were out of date and were not supported by Microsoft.
Despite Microsoft releasing a patch for the vulnerability in March 2017, many organizations and users had not applied the update. This was due to various reasons, including organizational negligence, the complexity of updating large networks, and reliance on legacy systems that were no longer supported by Microsoft. Consequently, many systems remained vulnerable to the attack.