Based on further research, the new Bad Rabbit Ransomware Worm uses NSA EternalRomance exploit patched by Microsoft in March 2017

https://arstechnica.com/information-technology/2017/10/bad-rabbit-used-nsa-eternalromance-exploit-to-spread-researchers-say/

Despite early reports that there was no use of National Security Agency-developed exploits in this week’s crypto-ransomware outbreak, research released by Cisco Talos suggests that the ransomware worm known as “Bad Rabbit” did in fact use a stolen Equation Group exploit  revealed by Shadowbrokers to spread across victims’ networks. The attackers used EternalRomance, an exploit that bypasses security over Server Message Block (SMB) file-sharing connections, enabling remote execution of instructions on Windows clients and servers. NotPetya also leveraged this exploit.

Arriving disguised as an Adobe Flash update, Bad Rabbit has multiple ways of spreading itself across networks. It can exploit open SMB connections on the infected Windows system, and it can also exploit the Windows Management Instrumentation Command-line (WMIC) scripting interface to execute code remotely on other Windows systems on the network, according to analysis by EndGame’s Amanda Rousseau. And the malware has a collection of hard-coded usernames and passwords, as Rousseau and researcher Kevin Beaumont noted.