Cis AWS Foundations 5.1

Ensure unauthorized API calls 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.

It is recommended that a metric filter and alarm be established for unauthorized API calls.

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.

Monitoring unauthorized API calls will help reduce the time it takes to detect malicious activity and can alert you to potential security incidents.

Impact

This alert may be triggered by normal read-only console activities that attempt to opportunistically gather optional information but gracefully fail if they lack the necessary permissions.

If an excessive number of alerts are generated, then an organization may wish to consider adding read access to the limited IAM user permissions solely to reduce the number of alerts.

In some cases, doing this may allow users to actually view some areas of the system; any additional access granted should be reviewed for alignment with the original limited IAM user intent.

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": "{ ($.errorCode ="*UnauthorizedOperation") || ($.errorCode ="AccessDenied*") && ($.sourceIPAddress!="delivery.logs.amazonaws.com") && ($.eventName!="HeadBucket") }",

4. Note the <unauthorized-api-calls-metric> value associated with the filterPattern from step 3.

5. Get a list of CloudWatch alarms, and filter on the <unauthorized-api-calls-metric> captured in step 4:

aws cloudwatch describe-alarms --query "MetricAlarms[?MetricName == <unauthorized-api-calls-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 unauthorized API calls, 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 unauthorized API calls 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 <unauthorized-api-calls-metric> --metric-transformations metricName=unauthorized_api_calls_metric,metricNamespace=CISBenchmark,metricValue=1 --filter-pattern "{ ($.errorCode ="*UnauthorizedOperation") || ($.errorCode ="AccessDenied*") && ($.sourceIPAddress!="delivery.logs.amazonaws.com") && ($.eventName!="HeadBucket") }"

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 reuse the same topic for all monitoring alarms.

Note: Capture the TopicArn that is displayed when creating the SNS topic in step 2.

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>

Note: You can execute this command once and then reuse the same subscription for all monitoring alarms.

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 "unauthorized_api_calls_alarm" --metric-name "unauthorized_api_calls_metric" --statistic Sum --period 300 --threshold 1 --comparison-operator GreaterThanOrEqualToThreshold --evaluation-periods 1 --namespace "CISBenchmark" --alarm-actions <sns-topic-arn>

Default Value

By default, no CloudWatch metric filters or alarms exist for unauthorized API calls.

References

1. https://aws.amazon.com/sns/
2. CCE-79186-3
3. https://docs.aws.amazon.com/awscloudtrail/latest/userguide/receive-cloudtrail-log-files-from-multiple-regions.html
4. https://docs.aws.amazon.com/awscloudtrail/latest/userguide/cloudwatch-alarms-for-cloudtrail.html
5. https://docs.aws.amazon.com/sns/latest/dg/SubscribeTopic.html

CIS Controls

Profile

Level 2 | Manual