Cis Aws Foundations 5.11

---
name: cis-aws-foundations-5.11
description: "Ensure Network Access Control List (NACL) changes are monitored"
category: cis-monitoring
version: "7.0.0"
author: cyberstrike-official
tags: [cis, aws, monitoring, cloudwatch, cloudtrail, metric-filters, alarms, nacl, network-acl, vpc]
cis_id: "5.11"
cis_benchmark: "CIS AWS Foundations Benchmark v7.0.0"
tech_stack: [aws]
cwe_ids: []
chains_with: [cis-aws-foundations-4.1, cis-aws-foundations-4.2]
prerequisites: []
severity_boost: {}
---

# Ensure Network Access Control List (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 any changes made to NACLs.

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

## Impact

N/A

## 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:

```bash
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:

```bash
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:

```bash
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:

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

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

```
"filterPattern": "{ ($.eventName = CreateNetworkAcl) || ($.eventName = CreateNetworkAclEntry) || ($.eventName = DeleteNetworkAcl) || ($.eventName = DeleteNetworkAclEntry) || ($.eventName = ReplaceNetworkAclEntry) || ($.eventName = ReplaceNetworkAclAssociation) }"
```

4. Note the `<nacl-changes-metric>` value associated with the `filterPattern` from step 3.

5. Get a list of CloudWatch alarms, and filter on the `<nacl-changes-metric>` captured in step 4:

```bash
aws cloudwatch describe-alarms --query 'MetricAlarms[?MetricName==<nacl-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:

```bash
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 NACL change 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 NACL changes and uses the `<trail-log-group-name>` taken from audit step 1:

```bash
aws logs put-metric-filter --log-group-name <trail-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) }'
```

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

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

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

```bash
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:

```bash
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>
```

## Default Value

By default, CloudTrail logs NACL events (e.g., CreateNetworkAcl, ReplaceNetworkAclEntry), but no CloudWatch metric filters or alarms exist. These changes are captured but not actively monitored unless configured.

## References

1. CCE-79196-2
2. https://docs.aws.amazon.com/awscloudtrail/latest/userguide/receive-cloudtrail-log-files-from-multiple-regions.html
3. https://docs.aws.amazon.com/awscloudtrail/latest/userguide/cloudwatch-alarms-for-cloudtrail.html
4. https://docs.aws.amazon.com/sns/latest/dg/SubscribeTopic.html

## CIS Controls

| Controls Version | Control                                                                              | IG 1 | IG 2 | IG 3 |
| ---------------- | ------------------------------------------------------------------------------------ | ---- | ---- | ---- |
| v8               | 8.5 Collect Detailed Audit Logs                                                      |      | x    | x    |
| v8               | 8.11 Conduct Audit Log Reviews                                                       |      | x    | x    |
| v7               | 6.3 Enable Detailed Logging                                                          |      | x    | x    |
| v7               | 11.3 Use Automated Tools to Verify Standard Device Configurations and Detect Changes |      | x    | x    |

## Profile

Level 2 | Manual