S3 Bucket Policy Project 3: Restrict Access to a VPC Endpoint

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

1. Everyday project story

Bengaluru bank engineer Arjun Mehta and Toronto fintech lead Sophie Martin store KYC scans in S3. Only app subnets via vpce-xxxxxxxx should read them.

2. Why VPC-endpoint-only?

Why do we need this?

Even private EC2 can reach S3 via public AWS endpoints. A restrictive bucket policy forces the gateway endpoint path and blocks stolen keys used from the internet.

• Gateway endpoint for S3 (no interface ENI hourly)
• Route table prefix list to S3
• Bucket Deny unless aws:SourceVpce matches
• Optional endpoint policy least privilege

3. What is the policy?

What is it?

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "DenyUnlessFromMyVpce",
      "Effect": "Deny",
      "Principal": "*",
      "Action": "s3:*",
      "Resource": [
        "arn:aws:s3:::rb-payroll-ap-south-1",
        "arn:aws:s3:::rb-payroll-ap-south-1/*"
      ],
      "Condition": {
        "StringNotEquals": {
          "aws:SourceVpce": "vpce-0a1b2c3d4e5f67890"
        }
      }
    }
  ]
}

Line-by-line

1. Effect Deny + StringNotEquals — deny if not from endpoint.
2. Principal * — all callers including your IAM.
3. Action s3:* — broad lock; IAM still must Allow needed actions.
4. Resource bucket and bucket/* — List + object APIs.
5. aws:SourceVpce = endpoint id vpce-...
6. Deny does not grant; IAM Allow still required.

4. CLI with output

How to do it (step by step)

# Gateway endpoint
aws ec2 create-vpc-endpoint --vpc-id vpc-0123456789abcdef0 \
  --service-name com.amazonaws.ap-south-1.s3 \
  --route-table-ids rtb-aaa rtb-bbb --vpc-endpoint-type Gateway
# { "VpcEndpoint": { "VpcEndpointId": "vpce-0a1b2c3d4e5f67890", ... } }

aws s3api put-bucket-policy --bucket rb-payroll-ap-south-1 --policy file://vpce.json

5. Test allow and deny

# ALLOW — EC2 in VPC with endpoint route
aws s3 cp s3://rb-payroll-ap-south-1/payslip.pdf - --region ap-south-1 | wc -c
# >0

# DENY — laptop on internet, same IAM key
aws s3 cp s3://rb-payroll-ap-south-1/payslip.pdf /tmp/x --region ap-south-1
# An error occurred (AccessDenied) when calling the GetObject operation: Access Denied

6. Multi-industry

Payroll (Mumbai — Fatima Khan), banking KYC (Bengaluru — Arjun Mehta), hospital records (Toronto — Sophie Martin), govt file share (Delhi — Kabir Singh).

7. 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

8. Lab: SourceVpce lock

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 lab bucket + gateway endpoint on a private route table.
2. Apply DenyUnlessFromMyVpce policy.
3. From private EC2: GetObject succeeds.
4. From internet laptop with same keys: AccessDenied.

Cleanup (stop billing)

1. Delete bucket policy; delete endpoint; delete bucket.
2. Remove lab routes if dedicated.

9. Common errors and fixes

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

10. 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.
9. Verify route to S3 prefix list on the instance.

11. FAQ

12. Interview Q&A

13. Tips in English, Marathi and Hindi

14. 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.

15. 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.

16. 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

17. 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.

18. 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.

19. 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.