CIS AWS Foundations v1.2 3.8: Ensure a log metric filter and alarm exist for S3 bucket policy changes

To meet CIS AWS Foundations v1.2 3.8, you must detect S3 bucket policy changes by creating a CloudWatch Logs metric filter on CloudTrail logs and a CloudWatch alarm that notifies your security/operations channel when those events occur. Operationalize it by standardizing the filter pattern, alarm actions, and evidence collection across all AWS accounts and regions.

Key takeaways:

• You need two things: a CloudWatch Logs metric filter for S3 bucket policy change events and an alarm wired to an on-call notification path.
• The control only works if CloudTrail management events are logged to a CloudWatch Logs log group (or equivalent pipeline you can prove).
• Auditors will ask for proof of end-to-end detection: event → metric → alarm → notification → ticket/response.

The target keyword control, “cis aws foundations v1.2 3.8: ensure a log metric filter and alarm exist for s3 bucket policy changes requirement,” is a detection and response baseline. It focuses on one high-impact failure mode: a change to an S3 bucket policy that makes data accessible to the wrong principals, either accidentally or by an attacker with AWS credentials. Bucket policies are a primary control plane for S3 authorization; unauthorized edits can expose sensitive data, enable exfiltration paths, or break segregation of duties.

This requirement is mapped in AWS Security Hub under the CIS benchmark mapping, which is useful because it gives you a standardized way to assess and report on the control across accounts ¹. The operational goal is not “have CloudTrail.” The operational goal is alerting: create a measurable signal for bucket policy changes and route it to people and systems that can triage quickly, with records you can retain for audit.

What follows is requirement-level guidance you can hand to a cloud security engineer and also defend in an audit: scope, build steps, evidence, common hangups, and a practical execution plan.

Regulatory text

Excerpt (as provided): “Implement CIS AWS Foundations Benchmark v1.2 requirement 3.8 as mapped in AWS Security Hub.” ²

Operator interpretation: You must implement the CIS AWS Foundations v1.2 control that checks for a log metric filter and alarm that detects S3 bucket policy changes. In practice, that means:

1. CloudTrail events must be present in a log stream you can query (commonly a CloudWatch Logs log group),
2. a metric filter must match the relevant S3 API events, and
3. a CloudWatch alarm must trigger and notify a response path.
   ²

Plain-English interpretation (what the requirement is “really” asking)

You are proving that any change to an S3 bucket policy generates an alert that reaches the right team. Bucket policy edits are low-frequency but high-risk; the benchmark expects you to treat them like a security-relevant configuration change, not a routine log entry.

This is a detective control. It does not prevent the change. It reduces time-to-detection and improves your ability to investigate who changed what, when, and from where.

Who it applies to

Entities

• Any organization operating AWS accounts where S3 is used, including production, staging, and shared services accounts ³.

Operational context (where auditors look)

• Multi-account AWS Organizations environments (central logging plus workload accounts).
• Regulated workloads where S3 stores customer data, logs, backups, or analytics datasets.
• Teams using IaC (Terraform/CloudFormation/CDK) where changes are frequent and need change attribution.

What you actually need to do (step-by-step)

Below is the most audit-friendly path because it aligns to the CIS mapping and the way Security Hub evaluates the control ¹.

Step 1: Confirm prerequisites (CloudTrail to CloudWatch Logs)

1. Verify CloudTrail is enabled for management events (S3 bucket policy changes are management/control plane events).
2. Verify CloudTrail delivers to CloudWatch Logs, not only to S3. Your metric filter needs a CloudWatch Logs log group as the source.
3. Ensure the log group is in a security-controlled account or logging account where possible, with restricted write permissions.

Audit note: Many teams fail this control because CloudTrail exists but is only written to S3, so there is no CloudWatch Logs metric filter source.

Step 2: Identify the exact event names to detect

Bucket policy changes typically surface in CloudTrail with an event name such as:

• PutBucketPolicy
• DeleteBucketPolicy

Your detection should match these. Keep the scope tight. Auditors prefer precise patterns over “match everything with s3.amazonaws.com.”

Step 3: Create a CloudWatch Logs metric filter

1. Go to the CloudWatch Logs log group that receives CloudTrail events.
2. Create a metric filter that matches bucket policy change events.
3. Emit a custom metric (namespace like CISBenchmark or Security, metric name like S3BucketPolicyChanges).
4. Set the metric value to 1 for each match.

Implementation guidance (pattern intent): Filter where:

• eventSource equals s3.amazonaws.com, and
• eventName equals PutBucketPolicy or DeleteBucketPolicy.

Keep a copy of the final filter pattern text as an artifact (see Evidence section).

Step 4: Create an alarm on the metric

1. Create a CloudWatch alarm on the custom metric.
2. Alarm condition should trigger when the metric is greater than or equal to 1 within your chosen evaluation window.
3. Configure alarm actions to notify a response channel, commonly:
   • SNS topic feeding email, chat, or incident tooling
   • Pager/on-call integration (through SNS or EventBridge rules in some environments)

Operator tip: Route alarms to a ticket queue if you need audit-proof tracking. If you only send email, make sure you can retain the alerts and show triage evidence.

Step 5: Validate end-to-end (tabletop + technical test)

Run a controlled test in a non-production account:

1. Change a test bucket policy (or apply/delete it).
2. Confirm the event appears in CloudTrail.
3. Confirm the metric increments.
4. Confirm the alarm transitions to ALARM state.
5. Confirm the notification is received and results in a ticket or recorded triage.

Capture screenshots or exported logs from each step.

Step 6: Deploy consistently across accounts and regions

To operationalize quickly and keep it consistent:

• Prefer IaC (Terraform/CloudFormation) for the metric filter and alarm.
• Use an AWS Organizations rollout pattern (StackSets or pipeline) to ensure every account has the same control.
• If you centralize CloudTrail logs in one logging account, confirm the filter/alarm is created where the CloudTrail logs land.

Step 7: Operational ownership and runbook

Assign:

• Control owner: Cloud Security / GRC (accountable for benchmark alignment)
• Technical owner: Platform/Cloud Engineering (responsible for implementation)
• Responder: SOC/on-call engineer

Write a short runbook:

• How to confirm if the change was authorized (change ticket, deployment record)
• How to identify the actor (CloudTrail userIdentity, source IP, assumed role)
• Containment steps (revert policy, block role/session, enable SCP guardrails if applicable)

Required evidence and artifacts to retain

Keep artifacts that prove design, implementation, and operation:

Design evidence (what you intended)

• Control statement mapped to this requirement, referencing CIS v1.2 3.8 and Security Hub mapping ².
• Logging architecture diagram (where CloudTrail logs go; where metric filters live).

Implementation evidence (what you built)

• Screenshot or exported configuration of:
  • CloudTrail configuration showing CloudWatch Logs delivery
  • CloudWatch Logs metric filter (pattern + metric namespace/name)
  • CloudWatch alarm configuration (threshold, evaluation, actions)
• IaC code snippets or repository links (preferred).
• List of accounts/regions covered (export from your deployment tool).

Operating evidence (proof it works)

• Test record showing a bucket policy change caused an alarm and notification.
• Incident/ticket sample with timestamps and triage notes.
• Periodic verification record (for example, a monthly check that the alarm still exists and is enabled).

Common exam/audit questions and hangups

Auditor question	What they’re probing	How to answer cleanly
“Show me the metric filter and alarm.”	Existence and configuration	Provide CloudWatch screenshots/IaC output for filter + alarm.
“Where do the underlying logs come from?”	CloudTrail pipeline completeness	Show CloudTrail → CloudWatch Logs integration.
“Does this cover all accounts?”	Scope and drift management	Show org-wide rollout method and account inventory.
“How do you respond to alerts?”	Operational maturity	Provide runbook + ticket examples.
“What if Security Hub flags CloudWatch.8 failing?”	Continuous compliance	Show your remediation workflow tied to findings 1.

Frequent implementation mistakes (and how to avoid them)

1. CloudTrail logs only to S3, not CloudWatch Logs.
   Fix: Ensure CloudTrail is configured to send to a CloudWatch Logs log group used for metric filters.

2. Filtering on the wrong event names.
   Fix: Validate with an actual test bucket policy change and confirm the CloudTrail eventName fields you see match your pattern.

3. Alarm exists but has no actions.
   Fix: Require at least one alarm action (SNS or integrated incident path). Audit teams treat “silent alarms” as nonfunctional.

4. Central logging exists, but the filter is created in workload accounts.
   Fix: Put the metric filter where the CloudTrail log group lives. If CloudTrail is centralized, the detection must be centralized too.

5. No evidence of ongoing operation.
   Fix: Schedule periodic tests and retain artifacts. A control that worked once but has no ongoing verification fails in many audits.

Enforcement context and risk implications

No public enforcement cases were provided in the supplied source catalog for this specific CIS requirement, so you should not position this as “required by regulators” unless another applicable regulation in your environment explicitly says so.

Risk-wise, bucket policy changes are a common precursor to:

• accidental public access,
• cross-account access expansions,
• data exfiltration paths created through overly broad principals or conditions.

This control reduces dwell time by making those changes visible to responders.

A practical 30/60/90-day execution plan

Use phased execution without promising specific completion times.

First 30 days (Immediate baseline)

• Inventory where CloudTrail is enabled and where it sends logs.
• Pick the standard filter pattern and alarm behavior.
• Implement in a pilot account; perform and document an end-to-end test.
• Define ownership and the triage runbook aligned to your incident process.

By 60 days (Scale and stabilize)

• Roll out via IaC across accounts and regions (or across your centralized logging account if you aggregate).
• Wire alarm actions to ticketing/on-call, not personal inboxes.
• Set up a lightweight compliance check: confirm metric filter/alarm exists and is enabled across your account inventory.

By 90 days (Operationalize and audit-proof)

• Add periodic testing (scheduled or change-driven) and retain results.
• Integrate Security Hub findings into your remediation workflow and evidence binder ¹.
• Run an internal audit: sample multiple accounts, verify logs, filter, alarm action, and response record.

Where Daydream fits (without changing your architecture)

If you’re managing this across many accounts, the hard part is not creating one alarm. The hard part is proving coverage and retaining evidence over time. Daydream can track the requirement, map it to your AWS account controls, and keep periodic verification artifacts organized for audit readiness, aligned to the CIS mapping you report through Security Hub ¹.

Frequently Asked Questions

Do I need this in every region?

Scope depends on where CloudTrail management events are captured and where logs are stored. If CloudTrail is multi-region and centralized to one CloudWatch Logs log group, the filter and alarm can live centrally; document that architecture clearly.

What if we send CloudTrail to S3 and query it with Athena instead of CloudWatch Logs?

CIS 3.8 is specifically about a log metric filter and alarm, which is the CloudWatch Logs + CloudWatch alarm pattern in AWS implementations ¹. If you use an alternative detection pipeline, expect audit friction unless you can show equivalence and how Security Hub evaluates the requirement.

Which events should the filter match for “bucket policy changes”?

Match the CloudTrail events that represent policy edits, commonly PutBucketPolicy and DeleteBucketPolicy. Confirm by testing in a non-production account and comparing your pattern to the CloudTrail event record.

Can we alert through EventBridge instead of CloudWatch alarms?

You can build strong detections with EventBridge, but this specific benchmark requirement expects a metric filter and alarm construct as commonly assessed through Security Hub ¹. If you diverge, document compensating control rationale and be prepared to show equivalent alerting evidence.

How do we prove the alarm is “working” without changing real production policies?

Test in a dedicated sandbox account or a non-sensitive bucket with a controlled policy change. Retain the CloudTrail record, the metric datapoint evidence, the alarm state transition, and the notification/ticket artifact.

What’s the minimum evidence set an auditor will accept?

Provide (1) CloudTrail-to-CloudWatch Logs configuration, (2) the metric filter definition, (3) the alarm definition with actions, and (4) one end-to-end test record showing the alarm notified the response path ¹.

Footnotes

1. AWS Security Hub CIS AWS Foundations mapping table

2. CIS AWS Foundations Benchmark; AWS Security Hub CIS AWS Foundations mapping table

3. CIS AWS Foundations Benchmark