Cis AWS Foundations 4.7

Ensure VPC flow logging is enabled in all VPCs

Description

VPC Flow Logs is a feature that enables you to capture information about the IP traffic going to and from network interfaces in your VPC. After you've created a flow log, you can view and retrieve its data in Amazon CloudWatch Logs. It is recommended that VPC Flow Logs be enabled for packet "Rejects" for VPCs.

Rationale

VPC Flow Logs provide visibility into network traffic that traverses the VPC and can be used to detect anomalous traffic or gain insights during security workflows.

Impact

By default, CloudWatch Logs will store logs indefinitely unless a specific retention period is defined for the log group. When choosing the number of days to retain, keep in mind that the average time it takes for an organization to realize they have been breached is 210 days (at the time of this writing). Since additional time is required to research a breach, a minimum retention policy of 365 days allows for detection and investigation. You may also wish to archive the logs to a cheaper storage service rather than simply deleting them. See the following AWS resource to manage CloudWatch Logs retention periods:

1. https://docs.aws.amazon.com/AmazonCloudWatch/latest/DeveloperGuide/SettingLogRetention.html

Audit Procedure

Using AWS Console

1. Sign into the management console.
2. Select Services, then select VPC.
3. In the left navigation pane, select Your VPCs.
4. Select a VPC.
5. In the right pane, select the Flow Logs tab.
6. Ensure a Log Flow exists that has Active in the Status column.

Using AWS CLI

1. Run the describe-vpcs command (OSX/Linux/UNIX) to list the VPC networks available in the current AWS region:

aws ec2 describe-vpcs --region <region> --query Vpcs[].VpcId

The command output returns the VpcId of VPCs available in the selected region.

2. Run the describe-flow-logs command (OSX/Linux/UNIX) using the VPC ID to determine if the selected virtual network has the Flow Logs feature enabled:

aws ec2 describe-flow-logs --filter "Name=resource-id,Values=<vpc-id>"

If there are no Flow Logs created for the selected VPC, the command output will return an empty list [].

3. Repeat step 2 for other VPCs in the same region.
4. Change the region by updating --region, and repeat steps 1-4 for each region.
5. Alternatively, the following command can be used to identify VPCs with and without Flow Logs:

VPCS=$(aws ec2 describe-vpcs --query "Vpcs[].VpcId" --output text)

for VPC in $VPCS; do
  COUNT=$(aws ec2 describe-flow-logs --filter Name=resource-id,Values=$VPC --query "length(FlowLogs)" --output text)

  if [ "$COUNT" -gt 0 ]; then
    echo "$VPC True"
  else
    echo "$VPC False"
  fi
done

Expected Result

All VPCs in all regions have at least one active Flow Log configured.

Remediation

Using AWS Console

1. Sign into the management console.
2. Select Services, then select VPC.
3. In the left navigation pane, select Your VPCs.
4. Select a VPC.
5. In the right pane, select the Flow Logs tab.
6. If no Flow Log exists, click Create Flow Log.
7. For Filter, select Reject.
8. Enter a Role and Destination Log Group.
9. Click Create Log Flow.
10. Click on CloudWatch Logs Group.

Note: Setting the filter to "Reject" will dramatically reduce the accumulation of logging data for this recommendation and provide sufficient information for the purposes of breach detection, research, and remediation. However, during periods of least privilege security group engineering, setting the filter to "All" can be very helpful in discovering existing traffic flows required for the proper operation of an already running environment.

Using AWS CLI

1. Create a policy document, name it role_policy_document.json, and paste the following content:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "test",
      "Effect": "Allow",
      "Principal": {
        "Service": "vpc-flow-logs.amazonaws.com"
      },
      "Action": "sts:AssumeRole"
    }
  ]
}

2. Create another policy document, name it iam_policy.json, and paste the following content:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "logs:CreateLogGroup",
        "logs:CreateLogStream",
        "logs:DescribeLogGroups",
        "logs:DescribeLogStreams",
        "logs:PutLogEvents",
        "logs:GetLogEvents",
        "logs:FilterLogEvents"
      ],
      "Resource": "*"
    }
  ]
}

3. Run the following command to create an IAM role:

aws iam create-role --role-name <aws-support-iam-role> --assume-role-policy-document file://<file-path>role_policy_document.json

4. Run the following command to create an IAM policy:

aws iam create-policy --policy-name <iam-policy-name> --policy-document file://<file-path>iam_policy.json

5. Run the attach-group-policy command, using the IAM policy ARN returned from the previous step to attach the policy to the IAM role:

aws iam attach-group-policy --policy-arn arn:aws:iam::<aws-account-id>:policy/<iam-policy-name> --group-name <group-name>

6. Run the describe-vpcs command to get a list of VPCs in the selected region:

aws ec2 describe-vpcs --region <region>

7. Run the create-flow-logs command to create a flow log for a VPC:

aws ec2 create-flow-logs --resource-type VPC --resource-ids <vpc-id> --traffic-type REJECT --log-group-name <log-group-name> --deliver-logs-permission-arn <iam-role-arn>

8. Repeat step 7 for other VPCs in the selected region.
9. Change the region by updating --region, and repeat the remediation procedure for each region.

Default Value

By default, VPC Flow Logs are not enabled for any newly created VPCs. Logging must be manually configured on a per-VPC basis.

References

1. CCE-79202-8
2. https://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/flow-logs.html

CIS Controls

MITRE ATT&CK Mappings

Profile

Level 2 | Automated