Department of Homeland Security announced newly discovered flaw in open source/free Samba software that provides file and print services to SMB/CIFS clients and interoperability between Linux/Unix servers and Windows-based clients.
Samba says:
All versions of Samba from 3.5.0 onwards are vulnerable to remote code execution allowing a malicious client to upload a shared library to a writable share, and then cause the server to load and execute it."
Go to Samba's Security Update page for available patches to prevent the spread of ransomware on your network. If no patch offered for your version, check out Samba's site for a manual patch that you can easily make by yourself.