I went through a similar situation a while back, and I can totally relate to the pressure you're under. When a phishing attack occurs, the first step is to contain the incident as quickly as possible to prevent further harm. Start by identifying and isolating the affected accounts or systems. It might involve disconnecting compromised machines from the network or disabling accounts temporarily until you fully understand the scope of the breach.
Next, audit your email systems to identify any additional phishing emails that might still be circulating among your users. It's essential to remove these from mailboxes and block similar suspicious emails from reaching users in the future.
To assess the scope, conduct a thorough investigation into what information could have been accessed and determine which users have interacted with the phishing email. Check logs and monitoring tools for any unusual activities that could provide clues about the attacker’s movements.
Also, consider resetting passwords for affected accounts and any related systems to thwart unauthorized access. This is especially important if multi-factor authentication (MFA) wasn’t in place. Implementing MFA following an incident can significantly reduce the risk of unauthorized access.
For preventive measures, look towards comprehensive user education and training. Regularly educate your staff on how to recognize and report phishing attempts. Incorporate real-world scenario-based training, possibly using platforms like KnowBe4 or Cofense for simulated phishing exercises, to keep everyone vigilant.
Ensure your technical defenses are up to date; this includes maintaining up-to-date security software, patching systems promptly, and deploying advanced email filters. Consider reviewing publications from institutions like the Cybersecurity and Infrastructure Security Agency (CISA) and the National Institute of Standards and Technology (NIST) for up-to-date guidelines and incident response checklists.
Lastly, develop a robust incident response plan if one isn’t already in place, and conduct simulations to test its effectiveness. Remember, the response to phishing attacks should be a continuous cycle of preparation, detection, and improvement.
I hope this helps, and I recommend browsing through CISA’s resources on phishing and incident response for detailed guides and further insights. Good luck, and remember you're taking important steps towards fortifying your organization against these threats.