Cis AWS Foundations 5.7

Ensure disabling or scheduled deletion of customer created CMKs is 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.

It is recommended that a metric filter and alarm be established for customer-created CMKs that have changed state to disabled or are scheduled for deletion.

Rationale

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.

Data encrypted with disabled or deleted keys will no longer be accessible. Changes in the state of a CMK should be monitored to ensure that the change is intentional.

Impact

Creation, storage, and management of CMK may require additional labor compared to the use of AWS-managed keys.

Audit Procedure

Using AWS CLI

If you are using CloudTrail trails and CloudWatch, perform the following to ensure that there is at least one active multi-region CloudTrail trail with the prescribed metric filters and alarms configured:

1. Identify the log group name that is configured for use with the active multi-region CloudTrail trail:

• List all CloudTrail trails:

aws cloudtrail describe-trails

• Identify multi-region CloudTrail trails: Trails with "IsMultiRegionTrail" set to true

• Note the value associated with "Name":<trail-name>

• Note the <trail-log-group-name> within the value associated with "CloudWatchLogsLogGroupArn"

  • Example: arn:aws:logs:<region>:<account-id>:log-group:<trail-log-group-name>:*

• Ensure the identified multi-region CloudTrail trail is active:

aws cloudtrail get-trail-status --name <trail-name>

• Ensure IsLogging is set to TRUE

• Ensure the identified multi-region CloudTrail trail captures all management events:

aws cloudtrail get-event-selectors --trail-name <trail-name>

• Ensure there is at least one event selector for a trail with IncludeManagementEvents set to true and ReadWriteType set to All

2. Get a list of all associated metric filters for the <trail-log-group-name> captured in step 1:

aws logs describe-metric-filters --log-group-name <trail-log-group-name>

3. Ensure the output from the above command contains the following:

"filterPattern": "{($.eventSource = kms.amazonaws.com) && (($.eventName=DisableKey)||($.eventName=ScheduleKeyDeletion)) }"

4. Note the <disable-or-delete-cmk-changes-metric> value associated with the filterPattern from step 3.

5. Get a list of CloudWatch alarms, and filter on the <disable-or-delete-cmk-changes-metric> captured in step 4:

aws cloudwatch describe-alarms --query 'MetricAlarms[?MetricName==<disable-or-delete-cmk-changes-metric>]'

6. Note the AlarmActions value; this will provide the SNS topic ARN value.

7. Ensure there is at least one active subscriber to the SNS topic:

aws sns list-subscriptions-by-topic --topic-arn <sns-topic-arn>

• At least one subscription should have "SubscriptionArn" with a valid AWS ARN.
  • Example of valid "SubscriptionArn": arn:aws:sns:<region>:<account-id>:<sns-topic-name>:<subscription-id>

Expected Result

A metric filter exists with the filter pattern matching CMK disabling or scheduled deletion events, a CloudWatch alarm is configured for the metric, and the alarm has an active SNS topic with at least one subscriber.

Remediation

Using AWS CLI

If you are using CloudTrail trails and CloudWatch, perform the following steps to set up the metric filter, alarm, SNS topic, and subscription:

1. Create a metric filter based on the provided filter pattern that checks for CMKs that have been disabled or scheduled for deletion and uses the <trail-log-group-name> taken from audit step 1:

aws logs put-metric-filter --log-group-name <trail-log-group-name> --filter-name <disable-or-delete-cmk-changes-metric> --metric-transformations metricName=<disable-or-delete-cmk-changes-metric>,metricNamespace='CISBenchmark',metricValue=1 --filter-pattern '{($.eventSource = kms.amazonaws.com) && (($.eventName=DisableKey)||($.eventName=ScheduleKeyDeletion)) }'

2. Create an SNS topic that the alarm will notify:

aws sns create-topic --name <sns-topic-name>

3. Create an SNS subscription for the topic created in step 2:

aws sns subscribe --topic-arn <sns-topic-arn> --protocol <sns-protocol> --notification-endpoint <sns-subscription-endpoints>

4. 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 <disable-or-delete-cmk-changes-alarm> --metric-name <disable-or-delete-cmk-changes-metric> --statistic Sum --period 300 --threshold 1 --comparison-operator GreaterThanOrEqualToThreshold --evaluation-periods 1 --namespace 'CISBenchmark' --alarm-actions <sns-topic-arn>

Default Value

By default, CloudTrail logs KMS events (DisableKey, ScheduleKeyDeletion), but no CloudWatch metric filters or alarms are set. These changes are recorded but not actively monitored unless configured.

References

1. CCE-79192-1
2. https://docs.aws.amazon.com/awscloudtrail/latest/userguide/cloudwatch-alarms-for-cloudtrail.html
3. https://docs.aws.amazon.com/awscloudtrail/latest/userguide/receive-cloudtrail-log-files-from-multiple-regions.html
4. https://docs.aws.amazon.com/sns/latest/dg/SubscribeTopic.html

CIS Controls

Profile

Level 2 | Manual