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The S3 Bucket backing Cloudtrail should be private

Default Severity: critical

Explanation

CloudTrail logs a record of every API call made in your account. These log files are stored in an S3 bucket. CIS recommends that the S3 bucket policy, or access control list (ACL), applied to the S3 bucket that CloudTrail logs to prevents public access to the CloudTrail logs. Allowing public access to CloudTrail log content might aid an adversary in identifying weaknesses in the affected account's use or configuration.

Possible Impact

CloudTrail logs will be publicly exposed, potentially containing sensitive information

Suggested Resolution

Restrict public access to the S3 bucket

Insecure Example

The following example will fail the aws-cloudtrail-no-public-log-access check. 

resource "aws_cloudtrail" "bad_example" {
   s3_bucket_name = "abcdefgh"
   event_selector {
     read_write_type           = "All"
     include_management_events = true

     data_resource {
       type = "AWS::S3::Object"
       values = ["${data.aws_s3_bucket.important-bucket.arn}/"]
     }
   }
}

resource "aws_s3_bucket" "good_example" {
    bucket = "abcdefgh"
    acl = "public-read"
}

Secure Example

The following example will pass the aws-cloudtrail-no-public-log-access check. 

 resource "aws_cloudtrail" "good_example" {
   is_multi_region_trail = true
   s3_bucket_name = "abcdefgh"

   event_selector {
     read_write_type           = "All"
     include_management_events = true

     data_resource {
       type = "AWS::S3::Object"
       values = ["${data.aws_s3_bucket.important-bucket.arn}/"]
     }
   }
 }

resource "aws_s3_bucket" "good_example" {
    bucket = "abcdefgh"
    acl = "private"
}