What is SMB, the file-sharing protocol?

SMB (Server Message Block) is the protocol most office networks use to share files and printers, for example when you open a document on a shared drive. Because it gives access to company data and is also used for remote administration, attackers rely on it heavily once inside: to copy data, to spread to other machines and, in ransomware attacks, to encrypt files on shared drives. SMB should never be reachable from the internet. Inside the network, a workstation suddenly opening SMB sessions to many servers is a classic sign of lateral movement, which NetSenX flags on every plan.

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