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Amazon EC2 Auto Scaling group has Capacity Rebalancing enabled

autoscaling_group_capacity_rebalance_enabled
Service: autoscalingSeverity: medium
OfficialBuilt-inVerified
Categories:resilience
Resource Type:AwsAutoScalingAutoScalingGroup

EC2 Auto Scaling groups use Capacity Rebalancing to act on EC2 rebalance recommendations by launching replacement Spot instances and terminating at-risk ones after they are healthy.

Assesses whether this proactive replacement behavior is enabled.

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Risk

Without Capacity Rebalancing, Spot interruptions can drop targets and reduce capacity, causing timeouts, 5xx spikes, and backlog growth. The two-minute notice is often insufficient, reducing service availability and increasing the chance of cascading failures and slow recovery.

Run this check with Prowler

Run in Prowler Cloud
Run
prowler aws --checks autoscaling_group_capacity_rebalance_enabled

Recommendation

Enable Capacity Rebalancing for ASGs that use Spot.

Apply resilience practices:

  • Prefer price-capacity-optimized allocation
  • Keep headroom below MaxSize
  • Use lifecycle hooks to drain/deregister
  • Design stateless, interruption-tolerant workloads (least privilege and defense-in-depth for dependencies)

Remediation

CLI

aws autoscaling update-auto-scaling-group --auto-scaling-group-name <example_resource_name> --capacity-rebalance

Terraform

# Terraform: Enable Capacity Rebalancing on an Auto Scaling group
resource "aws_autoscaling_group" "<example_resource_name>" {
  name               = "<example_resource_name>"
  min_size           = 1
  max_size           = 1
  desired_capacity   = 1
  availability_zones = ["<example_az>"]

  launch_template {
    id    = "<example_resource_id>"
    version = "$Latest"
  }

  capacity_rebalance = true  # CRITICAL: Turns on Capacity Rebalancing
}

NativeIaC

# CloudFormation: Enable Capacity Rebalancing on an Auto Scaling group
Resources:
  <example_resource_name>:
    Type: AWS::AutoScaling::AutoScalingGroup
    Properties:
      MinSize: "1"
      MaxSize: "1"
      AvailabilityZones: ["<example_az>"]
      LaunchTemplate:
        LaunchTemplateName: <example_resource_name>
        Version: "$Default"
      CapacityRebalance: true  # CRITICAL: Enables proactive replacement of at-risk Spot instances

Other

1In the AWS Console, go to EC2 > Auto Scaling Groups
2Select <example_resource_name> and open the Details tab
3Click Allocation strategies > Edit, check Capacity rebalancing
4Click Update/Save

Check Code

from prowler.lib.check.models import Check, Check_Report_AWS
from prowler.providers.aws.services.autoscaling.autoscaling_client import (
    autoscaling_client,
)


class autoscaling_group_capacity_rebalance_enabled(Check):
    def execute(self):
        findings = []
        for group in autoscaling_client.groups:
            if group.load_balancers and group.target_groups:
                report = Check_Report_AWS(metadata=self.metadata(), resource=group)

                report.status = "FAIL"
                report.status_extended = f"Autoscaling group {group.name} does not have capacity rebalance enabled."
                if group.capacity_rebalance:
                    report.status = "PASS"
                    report.status_extended = f"Autoscaling group {group.name} has capacity rebalance enabled."

                findings.append(report)

        return findings

References