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Description

Real-time monitoring of API calls can be achieved by directing CloudTrail logs to CloudWatch logs and establishing corresponding metric filters and alarms. It is recommended that a metric filter and alarm be established for changes to S3 bucket policies.

Rationale

Monitoring changes to S3 bucket policies may reduce the time to detect and correct permissive policies on sensitive S3 buckets.

Remediation

Perform the following to set up the metric filter, alarm, SNS topic, and subscription:

From the command line

1. Retrieve cloudtrail log group name

   aws cloudtrail describe-trails
   

2. Create a metric filter based on the filter pattern provided, which checks for IAM policy changes and the <cloudtrail_log_group_name>.

   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))}'
   

   • Ensure CloudTrail is set to multi-region and isLogging is set True.
   • Ensure there is at least one Event Selector with IncludeManagementEvents set to True and ReadWriteType set to All.

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

3. Create an SNS topic that the alarm notifies.

   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.

4. 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.

5. Create an alarm that is associated with the CloudWatch logs metric filter created in step 1 and the 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>
   

References

1. https://docs.aws.amazon.com/awscloudtrail/latest/userguide/cloudwatch-alarms-for-cloudtrail.html
2. https://docs.aws.amazon.com/awscloudtrail/latest/userguide/receive-cloudtrail-log-files-from-multiple-regions.html
3. https://docs.aws.amazon.com/sns/latest/dg/SubscribeTopic.html

Additional information

Configuring log metric filter and alarm on multi-region (global) CloudTrail logs provides the following benefits:

• Ensures that activities from all regions (used as well as unused) are monitored.
• Ensures that activities on all supported global services are monitored.
• Ensures that all management events across all regions are monitored.