SMB

SMB is the protocol Windows uses for file and printer sharing across a network, running on TCP port 445. It has a long history of serious vulnerabilities, most famously the EternalBlue exploit in SMBv1 that powered the WannaCry and NotPetya outbreaks in 2017. Defenders disable SMBv1, block port 445 at the perimeter, and require SMB signing to prevent relay attacks.