Error: 403 AccessDenied — When Public Access Blocks Your S3 Goals

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Error: 403 AccessDenied — When Public Access Blocks Your S3 Goals

#aws#s3#devops#cloud

Why S3 isn’t broken — it’s protecting you from yourself.





Problem

It’s a normal afternoon deploy. You upload your files to S3, confident everything is wired correctly. Then you hit this:

An error occurred (AccessDenied) when calling the PutObject operation: Access Denied

You check the bucket policy. You have full permissions. The IAM role looks right. Everything should work — but it doesn’t.

This is one of the most common (and most frustrating) S3 errors: 403 AccessDenied. The truth is, S3 isn’t rejecting you out of spite. It’s enforcing layers of protection you might not even realize exist.

Clarifying the Issue

When you get a 403 from S3, it usually means one of three things:

  1. IAM or bucket policy denies access. The request is explicitly denied or lacks the required permission. In AWS’s policy evaluation logic, explicit deny > explicit allow > implicit deny. That means even if your IAM policy allows an action, a single explicit deny or higher-level restriction overrides it.
  2. Public Access Block (BPA) is enabled. Even if your policy allows public access, BPA overrides it with a global deny.
  3. Cross-account confusion. Sometimes the request originates from another account — for example, a Lambda function in Account A trying to write to a bucket owned by Account B. Unless Account B’s bucket policy explicitly grants access to the principal in Account A, the request will fail with AccessDenied.

Remember: AWS evaluates explicit denies before allows. So even if your policy looks correct, a single deny statement or BPA configuration can shut it all down.

Why It Matters

This error isn’t just a nuisance — it’s a signal that AWS’s security layers are doing their job.

  • Prevents data exposure: BPA stops accidental public leaks.
  • Protects compliance boundaries: Ensures internal buckets stay private even when policies drift.
  • Reduces human error: Forces engineers to explicitly enable public access when they truly need it.

Understanding where the deny comes from helps you move faster without weakening security.

Key Terms

  • 403 AccessDenied: Generic response indicating the caller is not authorized for the requested action.
  • Block Public Access (BPA): Account- or bucket-level setting that overrides permissions to prevent public access.
  • Explicit Deny: A rule that always overrides any allow in AWS policy evaluation.
  • Caller Identity: The IAM principal (user or role) whose credentials were used in the request.

Steps at a Glance

  1. Confirm who the caller really is.
  2. Check if Public Access Block is enabled.
  3. Review bucket and IAM policies for denies.
  4. Validate cross-account permissions.
  5. Test again with BPA adjustments.

Detailed Steps

1. Confirm the Caller Identity
Ensure you’re using the credentials you think you are:

aws sts get-caller-identity

Look for the Arn and Account values. Many 403s trace back to an unexpected role or profile.

2. Check for Public Access Block (BPA)
Retrieve the BPA configuration:

aws s3api get-public-access-block --bucket my-bucket

If you see any true values (like BlockPublicAcls or RestrictPublicBuckets), public access is denied regardless of your bucket policy.

3. Review Bucket Policy and IAM Permissions
Look for explicit denies or missing actions:

aws s3api get-bucket-policy --bucket my-bucket

If the policy includes "Effect": "Deny" statements, they override everything else. Even an allow in IAM can’t bypass it.

4. Validate Cross-Account Access
If your app or another AWS account needs to access the bucket, make sure both sides trust each other:

aws s3api get-bucket-policy-status --bucket my-bucket

A result showing "IsPublic": false means BPA or policy restrictions are active. Double-check that the principal (e.g., Lambda function in another account) is explicitly allowed in the bucket policy.

5. Test Again with BPA Adjustments
To safely allow public or cross-account access:

aws s3api delete-public-access-block --bucket my-bucket

Then reapply a restrictive bucket policy to allow only the needed actions. Never disable BPA globally unless you’re working in a sandbox.

Conclusion

The 403 AccessDenied error isn’t an outage — it’s a security feature doing its job. By verifying the caller, understanding how BPA and policies interact, and applying precise permissions, you turn a frustrating block into a safety net.

S3 isn’t broken. It’s guarding your data until you prove you can be trusted with it.

Aaron Rose is a software engineer and technology writer at tech-reader.blog and the author of Think Like a Genius.

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