S3 Bucket Policy Project 2: Allow Only CloudFront via OAC

By Ravindra Bagale, trainer · LinkedIn · Last updated: 28 October 2026

1. Everyday project story

Singapore edtech lead Mei Lin and Hyderabad course platform owner Suresh Reddy both stream course videos. Viewers must use CloudFront URLs only — never the raw S3 REST URL.

Builds on the public-website project: production usually flips to private bucket + OAC instead of Principal *.

2. Why OAC-only access?

Why do we need this?

Public GetObject ignores CloudFront signed URLs, WAF, and geo restrictions. OAC makes CloudFront the only reader.

• Keep bucket private (BPA fully on)
• CloudFront signs origin requests with OAC
• Bucket policy Allow only cloudfront.amazonaws.com with SourceArn
• Direct S3 URL returns 403

3. What is the policy?

What is it?

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "AllowCloudFrontServicePrincipalRead",
      "Effect": "Allow",
      "Principal": { "Service": "cloudfront.amazonaws.com" },
      "Action": "s3:GetObject",
      "Resource": "arn:aws:s3:::rb-ott-origin-ap-south-1/*",
      "Condition": {
        "StringEquals": {
          "AWS:SourceArn": "arn:aws:cloudfront::111122223333:distribution/EDFDVBD6EXAMPLE"
        }
      }
    }
  ]
}

Line-by-line

1. Principal Service cloudfront.amazonaws.com — not an IAM user.
2. Action GetObject — origin fetch only.
3. Resource objects in the origin bucket.
4. Condition AWS:SourceArn — only YOUR distribution.
5. No Principal * — anonymous direct access stays denied.
6. Replace distribution id before apply.

4. How to do it (CLI with output)

How to do it (step by step)

# Private bucket
aws s3api create-bucket --bucket rb-ott-origin-ap-south-1 --region ap-south-1 \
  --create-bucket-configuration LocationConstraint=ap-south-1
aws s3api put-public-access-block --bucket rb-ott-origin-ap-south-1 --public-access-block-configuration \
  BlockPublicAcls=true,IgnorePublicAcls=true,BlockPublicPolicy=true,RestrictPublicBuckets=true

# Create OAC, attach to distribution origin, note Distribution ARN, then:
aws s3api put-bucket-policy --bucket rb-ott-origin-ap-south-1 --policy file://oac.json
# (empty success)

echo demo-video-bytes > demo.mp4
aws s3 cp demo.mp4 s3://rb-ott-origin-ap-south-1/videos/demo.mp4

5. Test: allowed and denied

# DENY — direct S3 (no CF)
curl -s -o /dev/null -w "%{http_code}\n" \
  https://rb-ott-origin-ap-south-1.s3.ap-south-1.amazonaws.com/videos/demo.mp4
# 403

# ALLOW — via CloudFront domain (after OAC attached)
curl -s -o /dev/null -w "%{http_code}\n" \
  https://d111111abcdef8.cloudfront.net/videos/demo.mp4
# 200

6. OAC vs legacy OAI

7. Multi-industry

OTT (Mumbai — Vikram Joshi), edtech video (Hyderabad — Suresh Reddy), news archives (London — Emily Carter), hospital training films (Kochi — Dr. Neha Nair).

8. AccessDenied debugging and Policy Simulator

When S3 returns AccessDenied, check in order: (1) Block Public Access, (2) matching bucket policy Deny, (3) missing IAM Allow, (4) KMS key policy, (5) VPC endpoint policy, (6) legacy ACL. IAM Policy Simulator can miss BPA/VPC/KMS — always re-test with real CLI from the real network path.

# Decode
aws s3api get-bucket-policy --bucket BUCKET --query Policy --output text | python3 -m json.tool
aws s3api get-public-access-block --bucket BUCKET
aws iam simulate-principal-policy --policy-source-arn ARN \
  --action-names s3:GetObject --resource-arns arn:aws:s3:::BUCKET/key.txt
aws cloudtrail lookup-events --lookup-attributes AttributeKey=EventName,AttributeValue=GetObject --max-items 5

9. Lab: private origin + OAC policy

Do this in ap-south-1 (Mumbai) unless your org forbids it. Set a budget alert first.

How to do it (step by step)

1. Create private bucket; BPA all true.
2. Create CloudFront distribution with S3 origin + OAC.
3. Put SourceArn-scoped bucket policy; upload a test object.
4. curl direct S3 → 403; curl CF domain → 200.

Cleanup (stop billing)

1. Disable distribution; delete OAC; empty and delete bucket.
2. Remove lab DNS records if any.

10. Common errors and fixes

Each row is a real mistake students and juniors hit. Read the fix before you panic.

11. Troubleshooting checklist

1. Confirm region and endpoint hostname for the bucket.
2. Print get-bucket-policy and get-public-access-block.
3. Reproduce with a least-privilege lab IAM user.
4. Binary-search Denies by commenting one Sid at a time in lab.
5. Read CloudTrail errorCode and additionalEventData.
6. If KMS involved, test key permissions separately.
7. From VPC path: route to gateway prefix list + endpoint policy.
8. Cleanup versioning/Object Lock labs carefully.

12. FAQ

13. Interview Q&A

14. Tips in English, Marathi and Hindi

15. Policy design checklist before you save

1. Write the business rule in one sentence (who can do what from where).
2. Choose Allow vs Deny-unless consciously; remember explicit Deny wins.
3. List every Resource ARN shape you need (bucket vs bucket/*).
4. Name every Condition key exactly (case-sensitive).
5. Decide BPA settings before put-bucket-policy.
6. Plan the allow test and the deny test with expected HTTP/CLI codes.
7. Plan cleanup (versioning, Object Lock, KMS schedules).
8. Paste JSON through python3 -m json.tool before apply.

16. Policy Simulator walkthrough

IAM Policy Simulator is great for identity policies and a first pass on resource policies, but it can miss Block Public Access, VPC endpoint policies, and some KMS denials. Use it to catch typos, then always re-test with AWS CLI from the real network path (office IP, VPC, or CloudFront).

1. Open IAM → Policy simulator → pick the user/role.
2. Add action s3:GetObject / PutObject and the object ARN.
3. If you use resource policies, include the bucket policy text where the tool allows.
4. Note Allow/Deny; then run the matching CLI from the real path.
5. If Simulator Allow but CLI Deny, check BPA, VPCe, KMS, and explicit Denies next.

17. Copy-paste runbook (print this)

# 0) Region + names
export AWS_REGION=ap-south-1
export BUCKET=rb-lab-$(date +%s)-ap-south-1

# 1) Create + show BPA
aws s3api create-bucket --bucket $BUCKET --region $AWS_REGION \
  --create-bucket-configuration LocationConstraint=$AWS_REGION
aws s3api get-public-access-block --bucket $BUCKET || true

# 2) Validate + apply policy
python3 -m json.tool policy.json >/dev/null
aws s3api put-bucket-policy --bucket $BUCKET --policy file://policy.json
aws s3api get-bucket-policy --bucket $BUCKET --query Policy --output text | python3 -m json.tool | head

# 3) Allow test + Deny test (fill per project)
# ... project-specific curls / aws s3 cp ...

# 4) Cleanup
aws s3 rm s3://$BUCKET --recursive || true
aws s3api delete-bucket-policy --bucket $BUCKET || true
aws s3api delete-bucket --bucket $BUCKET

18. Industry story bank (reuse the pattern)

Each project above uses one industry story. Use the same policy pattern in these other industries without rewriting the JSON from scratch — only change the bucket name, principal, and Condition values.

• Healthcare (Priya Nair, Apollo-style clinic): patient PDFs — prefer SSE-KMS + OrgID + MFA delete.
• Fintech (Arjun Mehta, UPI wallet): statements — office IP + SecureTransport + CloudFront OAC for the customer portal.
• EdTech (Sofia Alvarez, online course): course videos — CloudFront OAC only; never public ACLs.
• Logistics (Kenji Watanabe, fleet app): GPS dumps — VPC endpoint only from the private subnet runners.
• Media (Fatima Khan, newsroom): raw footage — Object Lock COMPLIANCE + deny delete.
• Retail (Lucas Meyer, D2C brand): product images — public read for /images/* only; deny everything else.
• SaaS (Ananya Iyer, B2B CRM): tenant exports — ${aws:username} or ${aws:PrincipalTag/tenant} prefixes.
• Government (Rahul Deshmukh, state portal): RTI archives — OrgID + MFA + Object Lock.

19. AccessDenied debugging flowchart

1. Read the error code and the request ID. Save both.
2. Confirm you are calling the right bucket and region.
3. Check Block Public Access if the principal is * or a service that looks public.
4. Decode the policy: is there an explicit Deny that matches? Deny wins.
5. Check Condition keys: SecureTransport, SourceVpce, SourceIp, PrincipalOrgID, encryption headers.
6. For KMS: does the key policy allow the caller? Bucket policy alone is not enough.
7. For CloudFront: is OAC present and the distribution id matching?
8. For VPC: are you really going through the endpoint (no public IP leak)?
9. Simulate in IAM Policy Simulator, then re-test with CLI from the real path.
10. Only then open a support case with the request id and a redacted policy.

20. What good looks like in a PR review

• Policy JSON validates and is indented.
• Every Statement has Sid, Effect, Action, Resource.
• No Principal:* with write actions unless BPA and a Deny fence exist.
• Allow test and Deny test are documented with expected output.
• Cleanup steps will not strand Object Lock or KMS keys.

Revision, interview and comparison cards

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More cards: Ravi Sir's Cards.