# amazon-s3

Cursor rule covering Amazon S3 best practices. It enforces server-side encryption, disables ACLs, and establishes clean key naming conventions.

- rule · `type=rule` · Cursor · ★ 3580
- Author: sanjeed5
- License: CC0-1.0 · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/sanjeed5/awesome-cursor-rules-mdc/tree/HEAD/rules-mdc/amazon-s3.mdc
- Updated: 2026-05-19
- HTML: https://affpapa.org/en/nevibecode/rules/amazon-s3

## What it does

This rule instructs Cursor to generate secure, structured AWS S3 code using boto3 and Terraform. The rule applies project-wide under the `**/*` glob. The configuration mandates server-side encryption with SSE-S3 or KMS keys, explicitly discouraging unencrypted writes. Access must rely solely on IAM and bucket policies because ACLs remain disabled under the bucket owner enforced model.

Data governance rules require hierarchical key paths segmented by environment and dataset. The guide also mandates bucket versioning alongside metadata tagging for audit trails. To control storage costs, it guides the model to set lifecycle transitions into Infrequent Access and Glacier tiers.

Single PUT requests for files exceeding 100MB are flagged as bad practice. Any payload over that threshold must use multipart uploads via boto3 transfer utilities.

## Requirements
- Works with: AWS

## Install

### Cursor (native platform)

**Manually: put the files in place**
- File: `.cursor/rules/amazon-s3.mdc` — Frontmatter: description, globs, alwaysApply

### Claude Code

**Manually: put the files in place**
- File: `.claude/rules/amazon-s3.md` — The paths frontmatter field limits the files: move globs from the source into paths

### Codex

**AGENTS.md**
- Codex reads AGENTS.md: paste the rule text there

### GitHub Copilot

**Manually: put the files in place**
- File: `.github/instructions/amazon-s3.instructions.md` — The applyTo field sets which files it applies to: move globs from the source into applyTo

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/rules/amazon-s3.md`

### Cline

**Manually: put the files in place**
- File: `.clinerules/amazon-s3.md`

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/sanjeed5/awesome-cursor-rules-mdc/tree/HEAD/rules-mdc/amazon-s3.mdc

## When to use it
- configure server-side encryption with SSE-S3 or KMS keys in boto3
- disable bucket ACLs in favor of IAM and bucket policies
- structure hierarchical object keys for data lakes and app assets
- write lifecycle rules to transition objects into Glacier
- implement multipart uploads for files larger than 100MB

## Example prompts
- Write a boto3 function to upload sensitive reports to S3 using SSE-KMS encryption
- Generate a Terraform configuration for an S3 bucket with versioning and lifecycle rules
- Implement a multipart upload script in Python for a 500MB media file

## Source text · amazon-s3.mdc · CC0-1.0

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: 08951fe576a2e114.

BEGIN THIRD-PARTY FILE 08951fe576a2e114
````text
---
description: This guide provides definitive best practices for interacting with Amazon S3, focusing on security, performance, cost optimization, and maintainable code in modern AWS applications.
globs: **/*
---
# amazon-s3 Best Practices

Amazon S3 is the backbone of object storage in AWS. Adhering to these guidelines ensures your S3 implementations are secure, performant, cost-effective, and maintainable.

## 1. Security First

Security is paramount. Always assume your S3 buckets contain sensitive data and configure them defensively.

### 1.1. Encryption at Rest

Always encrypt objects at rest. Default to AWS-managed keys (SSE-S3) or KMS (SSE-KMS) for stronger control.

❌ **BAD: Unencrypted objects or relying on client-side encryption without server-side enforcement.**
```python
import boto3

s3 = boto3.client('s3')
s3.put_object(Bucket='my-unencrypted-bucket', Key='data.txt', Body=b'sensitive data')
# Data is stored unencrypted by default if bucket policy doesn't enforce it.
```

✅ **GOOD: Enforce server-side encryption (SSE-S3 or SSE-KMS).**
```python
import boto3

s3 = boto3.client('s3')
# Option 1: SSE-S3 (AWS-managed keys)
s3.put_object(
    Bucket='my-secure-bucket',
    Key='data.txt',
    Body=b'sensitive data',
    ServerSideEncryption='AES256' # SSE-S3
)

# Option 2: SSE-KMS (Customer Master Key)
# Ensure 'my-kms-key-id' exists and bucket has permissions.
s3.put_object(
    Bucket='my-secure-bucket',
    Key='more-data.txt',
    Body=b'more sensitive data',
    ServerSideEncryption='aws:kms',
    SSEKMSKeyId='arn:aws:kms:us-east-1:123456789012:key/my-kms-key-id'
)
```
> **Note on SSE-C:** As of April 2026, SSE-C is disabled for new buckets by default. Only enable it via `PutBucketEncryption` API *after* bucket creation if explicitly required for legacy or specific compliance needs. Prefer SSE-S3 or SSE-KMS.

### 1.2. Access Control: Policies over ACLs

Manage access exclusively through IAM and Bucket Policies. Disable ACLs for simplified, auditable permissions.

❌ **BAD: Relying on ACLs for granular object access.**
```python
# Avoid this pattern. ACLs are disabled by default for new buckets.
s3.put_object_acl(
    Bucket='my-bucket',
    Key='object.txt',
    GrantRead='id="some-aws-account-id"'
)
```

✅ **GOOD: Disable ACLs and use IAM/Bucket Policies.**
```python
# Ensure S3 Object Ownership is set to "Bucket owner enforced" to disable ACLs.
# This is the default for new buckets.
# Example Bucket Policy (IaC recommended):
# This policy grants read-only access to a specific IAM role for a specific prefix.
bucket_policy = {
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Principal": {
                "AWS": "arn:aws:iam::123456789012:role/MyReadOnlyRole"
            },
            "Action": ["s3:GetObject"],
            "Resource": ["arn:aws:s3:::my-secure-bucket/data/*"]
        }
    ]
}
# Apply this via IaC (e.g., Terraform, CloudFormation) or boto3.put_bucket_policy
```

### 1.3. Block Public Access

Always enable S3 Block Public Access at the account or organization level. Buckets should never be public unless explicitly required for static website hosting (and even then, use CloudFront with OAC).

✅ **GOOD: Enable S3 Block Public Access.**
```python
# This is typically configured via AWS Console, AWS CLI, or IaC at the account level.
# Example for a specific bucket (via boto3, but IaC is preferred):
s3.put_public_access_block(
    Bucket='my-secure-bucket',
    PublicAccessBlockConfiguration={
        'BlockPublicAcls': True,
        'IgnorePublicAcls': True,
        'BlockPublicPolicy': True,
        'RestrictPublicBuckets': True
    }
)
```

## 2. Code Organization & Data Governance
````
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