The Shadow File Modification rule monitors changes to the Linux shadow file, which is critical for system security as it stores hashed user passwords. Modifications to this file can indicate potential unauthorized actions such as password changes or user creations by threat actors seeking to maintain access to a system. This rule uses EQL (Event Query Language) to detect file changes specifically when the file path is '/etc/shadow' and the event type is 'change' with an action of 'rename'. The integration requires Elastic Defend, providing comprehensive endpoint protection, and is designed to trigger alerts for suspicious shadow file modifications for further investigation. Response recommendations include isolating the affected system, verifying file integrity, and auditing user activities.