In the post-close phase of mergers and acquisitions (M&A), ensuring cybersecurity becomes crucial as the integration of networks exposes both the acquiring and acquired companies to potential risks. Effective post-close cybersecurity involves engaging a trusted partner to handle network security while integration occurs, conducting thorough IT hygiene assessments to uncover vulnerabilities, and preparing for potential breaches with an incident response playbook. Red Team/Blue Team exercises can help test the security perimeter and readiness for real-world cyberattacks. Failure to adequately address cybersecurity during this phase can lead to significant risks, including undetected breaches that could have been lurking for months, thereby jeopardizing the success of the integration. A worst-case scenario could involve a ransomware attack affecting both parties if networks are inadequately separated or protected, highlighting the importance of comprehensive cybersecurity planning and execution.