Control: 4.11 Ensure Network Access Control Lists (NACL) changes are monitored

Description

Real-time monitoring of API calls can be achieved by directing CloudTrail Logs to CloudWatch Logs, or an external Security information and event management (SIEM) environment, and establishing corresponding metric filters and alarms. NACLs are used as a stateless packet filter to control ingress and egress traffic for subnets within a VPC. It is recommended that a metric filter and alarm be established for changes made to NACLs.

CloudWatch is an AWS native service that allows you to observe and monitor resources and applications. CloudTrail Logs can also be sent to an external Security information and event management (SIEM) environment for monitoring and alerting.

Monitoring changes to NACLs will help ensure that AWS resources and services are not unintentionally exposed.

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 NACL 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 `<nacl_changes_metric>` --metric-transformations metricName= `<nacl_changes_metric>` ,metricNamespace='CISBenchmark',metricValue=1 -- filter-pattern '{ ($.eventName = CreateNetworkAcl) || ($.eventName = CreateNetworkAclEntry) || ($.eventName = DeleteNetworkAcl) || ($.eventName = DeleteNetworkAclEntry) || ($.eventName = ReplaceNetworkAclEntry) || ($.eventName = ReplaceNetworkAclAssociation) }'

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

2. 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.

3. 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.

4. 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 `<nacl_changes_alarm>` -- metric-name `<nacl_changes_metric>` --statistic Sum --period 300 -- threshold 1 --comparison-operator GreaterThanOrEqualToThreshold --evaluation- periods 1 --namespace 'CISBenchmark' --alarm-actions <sns_topic_arn>