An organization has experienced a security incident involving an advanced persistent threat (APT) that has bypassed existing security controls and established a foothold on the network. As part of the incident response activities, what is the MOST appropriate immediate action to take once the threat is confirmed?
Start documenting the incident details for the after-action report and notify external stakeholders.
Begin forensic analysis on all systems to understand all the methods the APT used to infiltrate the network.
Isolate the affected systems from the network to prevent the APT from further establishing its presence or causing additional damage.
Immediately shutdown the entire network to eradicate the APT's foothold and start the recovery process.