AWS IAM Principal Enumeration via UpdateAssumeRolePolicy

Detects repeated failed attempts to update an IAM role’s trust policy in an AWS account, consistent with role and user enumeration techniques. In this technique, an attacker who controls credentials in the current account repeatedly calls UpdateAssumeRolePolicy on a single role, cycling through guessed cross-account role or user ARNs as the principal. When those principals are invalid, IAM returns MalformedPolicyDocumentException, producing a burst of failed UpdateAssumeRolePolicy events. This rule alerts on that brute-force pattern originating from this account, which may indicate that the account is being used as attack infrastructure or that offensive tooling (such as Pacu) is running here. Note: this rule does not detect other accounts enumerating roles, because those API calls are logged in the caller’s account, not the target account.