A threat actor has crafted and sent a spear-phishing email with what appears to be a trustworthy link to the site of a conference that an employee recently attended. The employee clicked the link and was redirected to a malicious site through which the employee downloaded a PDF attachment infected with ransomware. The employee opened the attachment, which exploited vulnerabilities on the desktop. The ransomware is now installed and is calling back to its command and control server. Which security solution is needed at this stage to mitigate the attack?