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Remediation

If you are using CloudTrails and CloudWatch, perform the following to setup the metric filter, alarm, SNS topic, and subscription:

  1. Create a metric filter based on filter pattern provided which checks for S3 bucket policy changes and the <cloudtrail_log_group_name> taken from audit step 1:
aws logs put-metric-filter --log-group-name <cloudtrail_log_group_name> --filter-name <s3_bucket_policy_changes_metric> --metric-transformations metricName= <s3_bucket_policy_changes_metric>, metricNamespace='CISBenchmark', metricValue=1 --filter-pattern '{ ($.eventSource = s3.amazonaws.com) && (($.eventName = PutBucketAcl) || ($.eventName = PutBucketPolicy) || ($.eventName = PutBucketCors) || ($.eventName = PutBucketLifecycle) || ($.eventName = PutBucketReplication) || ($.eventName = DeleteBucketPolicy) || ($.eventName = DeleteBucketCors) || ($.eventName = DeleteBucketLifecycle) || ($.eventName = DeleteBucketReplication)) }'

Note: You can choose your own metricName and metricNamespace strings. Using the same metricNamespace for all Foundations Benchmark metrics will group them together.

  1. Create an SNS topic that the alarm will notify:
aws sns create-topic --name <sns_topic_name>

Note: you can execute this command once and then re-use the same topic for all monitoring alarms.

  1. Create an SNS subscription to the topic created in step 2:
aws sns subscribe --topic-arn <sns_topic_arn> --protocol <protocol_for_sns> --notification-endpoint <sns_subscription_endpoints>

Note: you can execute this command once and then re-use the SNS subscription for all monitoring alarms.

  1. Create an alarm that is associated with the CloudWatch Logs Metric Filter created in step 1 and an SNS topic created in step 2:
aws cloudwatch put-metric-alarm --alarm-name <s3_bucket_policy_changes_alarm> --metric-name <s3_bucket_policy_changes_metric> --statistic Sum --period 300 --threshold 1 --comparison-operator GreaterThanOrEqualToThreshold --evaluation-periods 1 --namespace 'CISBenchmark' --alarm-actions <sns_topic_arn>