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The way in was described as phishing that stole an employee's credentials; from there the internal network was breached and ransomware ran, taking down many of the group's services (including Niconico) for months and leaking ~250,000 people's data. Reporting and analysis attribute the company-wide spread to systems of very different criticality sharing the same network — reportedly with too little segmentation. Defend by segmenting the network by criticality, using phishing-resistant authentication, and preparing business continuity (BCP) and recovery.