The rule 'GCP KMS Bulk Encryption by GCS Service Account' is designed to monitor and detect bulk KMS encryption operations carried out by Google Cloud Storage (GCS) service accounts. It operates on the premise that large-scale encryption activities (threshold set to 10 or more operations) may indicate a malicious event, specifically a ransomware attack. In such scenarios, adversaries utilize the KMS Encrypt API to encrypt data extensively, potentially rendering it inaccessible. The detection leverages GCP Audit Logs to identify these operations, and the thresholds are set to differentiate between regular operational behavior and possible attacks. The runbook provides a systematic approach to investigate such incidents by querying audit logs, identifying the source and nature of encrypted data, and examining recent IAM policy changes. This rule falls under those referencing the MITRE ATT&CK framework, particularly TA0040:T1486, indicating its alignment with well-known attack strategies.