The AWS EC2 Startup Script Change detection rule monitors modifications to the startup script of EC2 instances, which can have significant implications for instance security and configuration. This rule specifically triggers when there is a change in the user data associated with an EC2 instance, executed with root privileges each time the instance starts. The rule leverages CloudTrail logs to identify events corresponding to 'ModifyInstanceAttribute', particularly focusing on alterations to user data scripts. Given the critical role of startup scripts in defining instance behavior, the detection of such changes is critical for maintaining instance integrity and security. The rule is classified with high severity due to the potential impact of unauthorized or malicious changes to instance startup configurations. Additionally, the rule includes specific test cases that outline expected behaviors regarding modification actions on instance attributes, ensuring that any unauthorized modifications can be detected efficiently. This rule can be particularly crucial for environments where compliance and strict security posture are necessary, as modifications may indicate potential security incidents or configuration drift.