Description

Ensure that AWS CloudTrail events are sent to Amazon CloudWatch Logs and that the trail's log group has a metric filter with an associated CloudWatch alarm for activity by the AWS account root user. It matches actions performed directly by the root user, excluding actions taken by AWS services on the account's behalf. The alarm notifies responders whenever a matching event is recorded.

Rationale

The root user has unrestricted access to every resource in the account and should only be used for the few tasks that require it. Without monitoring, root user activity, including use of stolen root credentials, can go unnoticed, allowing data access, configuration tampering or deletion of resources. An alarm on root activity lets the security team confirm every use quickly. The root user should also be protected with MFA, have no access keys, and be kept for break-glass use only.

Impact

  • Provides near real-time alerts on every use of the root user

  • No effect on running workloads; the filter and alarm only read CloudTrail events

  • Adds a small cost for the CloudWatch alarm, and for CloudWatch Logs ingestion if CloudTrail was not already sending events to CloudWatch Logs

Default Value

No metric filters or CloudWatch alarms exist by default. A CloudTrail trail does not send events to CloudWatch Logs unless this is configured on the trail.

Pre-Requisites

  • IAM permissions: cloudtrail:DescribeTrails, cloudtrail:UpdateTrail, logs:DescribeMetricFilters, logs:PutMetricFilter, cloudwatch:DescribeAlarmsForMetric, cloudwatch:PutMetricAlarm, plus logs:DeleteMetricFilter and cloudwatch:DeleteAlarms for the backout plan

  • A CloudTrail trail, preferably multi-region, that sends its events to a CloudWatch Logs log group

  • An Amazon SNS topic with confirmed subscribers (for example, the security team's email address) to receive the alarm notifications

Remediation

Test Plan

Console:

  1. Sign in to the AWS Management Console at https://console.aws.amazon.com.

  2. Open the AWS CloudTrail console and select Trails.

  3. Open each trail and, in the CloudWatch Logs section, note the log group the trail sends events to.

  4. Open the Amazon CloudWatch console in the region of that log group, choose Log groups and select the log group.

  5. Open the Metric filters tab and check for a filter whose pattern includes these conditions, in this order: $.userIdentity.type = "Root", $.userIdentity.invokedBy NOT EXISTS and $.eventType != "AwsServiceEvent".

  6. Note the metric name of that filter, then choose Alarms, All alarms and check that an alarm exists for that metric.

  7. If no trail's log group has such a metric filter with an alarm, follow the implementation steps.

CLI:

List the trails and the CloudWatch Logs log group each one sends events to:

aws cloudtrail describe-trails --region <REGION> --query "trailList[].[Name,HomeRegion,IsMultiRegionTrail,CloudWatchLogsLogGroupArn]" --output table

List the metric filters of the log group (the name after log-group: in the ARN), in the region of the log group:

aws logs describe-metric-filters --region <REGION> --log-group-name <LOG_GROUP_NAME> --query "metricFilters[].[filterName,filterPattern,metricTransformations[0].metricName,metricTransformations[0].metricNamespace]" --output table

List the alarms on the metric of the matching filter:

aws cloudwatch describe-alarms-for-metric --region <REGION> --metric-name <METRIC_NAME> --namespace <METRIC_NAMESPACE> --query "MetricAlarms[].[AlarmName,StateValue,ActionsEnabled]" --output table

The account fails if no trail sends events to a log group, if no metric filter on a trail's log group has a pattern that includes all of the conditions listed above, or if no alarm exists for the metric of that filter.

Implementation Steps

Console:

  1. Sign in to the AWS Management Console at https://console.aws.amazon.com.

  2. If no trail sends events to CloudWatch Logs, open the AWS CloudTrail console, open a multi-region trail, choose Edit in the CloudWatch Logs section, enable it, select or create a log group and an IAM role, and save the changes.

  3. Open the Amazon CloudWatch console in the region of the trail's log group, choose Log groups and select the log group.

  4. Open the Metric filters tab and choose Create metric filter.

  5. In Filter pattern, enter { $.userIdentity.type = "Root" && $.userIdentity.invokedBy NOT EXISTS && $.eventType != "AwsServiceEvent" } and choose Next.

  6. Enter the metric details and choose Next:

    • Filter name: root_usage

    • Metric namespace: CISBenchmark

    • Metric name: root_usage_metric

    • Metric value: 1

  7. Choose Create metric filter.

  8. On the Metric filters tab, select the new filter and choose Create alarm.

  9. Set the metric and conditions, then choose Next:

    • Statistic: Sum

    • Period: 5 minutes

    • Threshold type: Static, Greater/Equal, 1

  10. Under Notification, select the SNS topic that alerts the security team and choose Next.

  11. Enter the alarm name root_usage_alarm, choose Next, then Create alarm.

Backout Plan

  1. Sign in to the AWS Management Console at https://console.aws.amazon.com.

  2. Open the Amazon CloudWatch console, choose Alarms, All alarms, select the root_usage_alarm alarm and delete it.

  3. Choose Log groups, select the trail's log group, open the Metric filters tab, select the root_usage filter and delete it.

CLI:

aws cloudwatch delete-alarms --region <REGION> --alarm-names root_usage_alarm
aws logs delete-metric-filter --region <REGION> --log-group-name <LOG_GROUP_NAME> --filter-name root_usage

Note: removing the metric filter and alarm stops alerts for activity by the AWS account root user, so these events can again go unnoticed.

Reference