Email Conversation Takeover: How to Detect Thread Hijacking After Account Compromise

Email conversation takeover is a post-compromise attack in which an adversary controlling a legitimate mailbox hijacks an existing thread to insert fraudulent replies that pass SPF/DKIM/DMARC and exploit recipient trust; this guide explains the attack mechanics, why gateway/native filters miss it, the account/thread/content signals that reveal a hijack, detection methods for Microsoft 365 and Google Workspace, and recommended rapid containment and organization-wide remediation to prevent business email compromise losses.