/securing-s3-buckets

name: securing-s3-buckets
description: >
  Create and secure S3 buckets following AWS best practices for access control, encryption,
  monitoring, and remediation of misconfigurations. Use when the user wants to
  secure a new bucket, audit an existing bucket, fix a security finding, configure
  encryption, or enable logging and monitoring. Do NOT use for general S3 data
  operations, S3 Tables setup, or discovering existing data assets.
version: 1

Overview

Implements layered S3 security controls across five workflows: securing new buckets, auditing existing configurations, remediating findings, configuring encryption, and enabling monitoring. Follows AWS Well-Architected security best practices.

Execute commands using the AWS MCP server when connected (sandboxed execution, audit logging, observability). Fall back to AWS CLI or shell otherwise.

Common Tasks

0. Verify Dependencies

Check for required tools before starting.

**Constraints:**

• You MUST inform the user if required tools are missing
• You SHOULD confirm credentials with `aws sts get-caller-identity`

See [references/iam-permissions.md](references/iam-permissions.md) for IAM permissions by workflow.

1. Classify the Request

| User intent | Workflow | |---|---| | Secure a new bucket | A: Secure New Bucket | | Audit / review existing bucket | B: Audit Existing Bucket | | Fix a specific finding | C: Remediate Issue | | Configure encryption | D: Configure Encryption | | Enable logging / monitoring | E: Enable Monitoring |

**Constraints:**

• You MUST ask for all required parameters upfront
• You MUST confirm bucket name and region before any write operation
• You MAY infer region from user context if clearly stated
• You SHOULD run `aws iam simulate-principal-policy` to validate permissions before write operations
• You SHOULD display write commands and wait for confirmation before executing

put-bucket-policy Safety Rules

These rules apply to ALL workflows that call `put-bucket-policy`:

• You MUST attempt to retrieve the existing policy first (`aws s3api get-bucket-policy`) — `put-bucket-policy` replaces the entire policy
• If a policy exists, you MUST back it up before modifying: `aws s3api get-bucket-policy --bucket <name> --output text > backup-policy-$(date +%s).json`
• If `NoSuchBucketPolicy` is returned, proceed with a new policy — no backup is needed
• You MUST merge new statements into the existing policy's Statement array (if one exists)
• You MUST validate merged JSON syntax before applying (e.g. `echo '<policy>' | python3 -m json.tool`)
• You SHOULD display the full `put-bucket-policy` command and wait for confirmation

2. Workflow A — Secure New Bucket

See [references/workflows.md](references/workflows.md) for full CLI steps.

**Required steps (execute in order, do not skip):**

1. Create bucket with `--bucket-namespace account-regional` 2. Enable versioning 3. Enable encryption (SSE-S3 + Bucket Keys + block SSE-C) 4. Enable logging (ask user which option — conditional) 5. Enforce HTTPS-only via `DenyInsecureTransport` bucket policy 6. Enable ABAC

**Constraints:**

• You MUST pass `--bucket-namespace account-regional` on `create-bucket` call — this is REQUIRED, not optional. Example:

  aws s3api create-bucket --bucket <name> --bucket-namespace account-regional --region <region>

• You MUST NOT change Block Public Access — S3 enables it by default on new buckets
• You MUST NOT change ACL ownership controls — S3 disables ACLs (`BucketOwnerEnforced`) by default
• You MUST apply a bucket policy with a `DenyInsecureTransport` statement that denies `s3:*` when `aws:SecureTransport` is `false` — this is REQUIRED, not optional. Example:

  aws s3api put-bucket-policy --bucket <name> --policy '{"Version":"2012-10-17","Statement":[{"Sid":"DenyInsecureTransport","Effect":"Deny","Principal":"*","Action":"s3:*","Resource":["arn:aws:s3:::<name>/*","arn:aws:s3:::<name>"],"Condition":{"Bool":{"aws:SecureTransport":"false"}}}]}'

• You MUST ask the user which logging option they want before step 4
• You MUST follow the [put-bucket-policy safety rules](#put-bucket-policy-safety-rules) for steps 4 and 5
• You SHOULD confirm each step succeeded before proceeding

3. Workflow B — Audit Existing Bucket

See [references/audit-checklist.md](references/audit-checklist.md) for the full checklist.

**Constraints:**

• You MUST run all read-only audit commands before reporting findings
• You MUST NOT execute any write or modify commands during an audit
• You MUST report each control as PASS / FAIL / NOT CONFIGURED with severity
• For logging: report PASS if either S3 server access logging OR CloudTrail data events are enabled; NOT CONFIGURED only if neither

4. Workflow C — Remediate Issue

See [references/remediation.md](references/remediation.md) for fix commands by issue type.

**Constraints:**

• You MUST identify the issue type before applying any fix
• You MUST follow the [put-bucket-policy safety rules](#put-bucket-policy-safety-rules) when modifying policies
• You MUST re-run the relevant audit check after applying the fix to confirm resolution

5. Workflow D — Configure Encryption

See [references/encryption.md](references/encryption.md) for encryption options and commands.

**Constraints:**

• You MUST default to SSE-S3 with S3 Bucket Keys and SSE-C blocked unless the user explicitly requests KMS
• When using SSE-KMS, you MUST use a customer managed key — NEVER the AWS managed `aws/s3` key
• You MUST specify customer-managed KMS keys by full ARN, not alias
• You MUST include `BucketKeyEnabled: true` and `BlockedEncryptionTypes: [SSE-C]` in all configurations
• **Note**: The S3 API accepts `aws/s3` and aliases without error — agent-enforced constraints. Verify with `get-bucket-encryption` after applying.

6. Workflow E — Enable Monitoring

See [references/workflows.md](references/workflows.md) for full CLI steps.

**Constraints:**

• You MUST check wheth