Server Message Block (SMB) is the transport protocol used by Windows machines for a wide variety of purposes such as file sharing, printer sharing, and access to remote Windows services. SMB operates over TCP ports 139 and 445. In April 2017, Shadow Brokers released an SMB vulnerability named “EternalBlue,” which was part of the Microsoft security bulletin MS17-010.

https://www.fireeye.com/blog/threat-research/2017/05/smb-exploited-wannacry-use-of-eternalblue.html