Error: 405 Method Not Allowed — When Wrong HTTP Methods Derail S3 Requests

#aws#s3#devops#cloud

Why PUT, POST, and DELETE can’t always do what you think they can.

Problem

It’s late, your integration pipeline is failing, and your logs are littered with:

<Error>
  <Code>MethodNotAllowed</Code>
  <Message>The specified method is not allowed against this resource.</Message>
</Error>

You try to upload, copy, or delete an object — and instead of S3 complying, you hit a wall. The bucket exists, the key looks right, but S3 refuses the request.

It feels like an outage. In reality, it’s a mismatch between what you asked S3 to do and what S3 actually supports on that endpoint or resource.

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Clarifying the Issue

The 405 Method Not Allowed error in Amazon S3 isn’t a service failure. It happens when:

• Wrong HTTP verb is used. For example, sending POST to a bucket when only PUT or GET are valid.
• Signed URL misuse: Pre-signed URLs are tied to a specific HTTP method (e.g., a PUT pre-sign won’t allow a DELETE).
• ACL or policy mismatch: Even if the method is valid, permissions may restrict it for that user or resource.
• Operation against wrong resource: Some methods work on objects but not buckets, and vice versa.

In short: your request type doesn’t match what S3 expects for that resource.

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Why It Matters

• Broken automation: CI/CD pipelines fail when deployments can’t upload or delete artifacts.
• Integration outages: Apps depending on signed URLs or object writes break in production.
• Wasted troubleshooting time: Teams chase phantom “S3 outages” when the problem is really an invalid verb.
• Security implications: Overly broad pre-signed URLs or sloppy method use can unintentionally grant access.
• Design principles: HTTP verbs matter. PUT is idempotent (running the same request multiple times has no additional effect), which is why S3 uses it for uploads. POST is not idempotent, and S3 restricts it to avoid inconsistent states.

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Key Terms

• HTTP Verb (Method): The action requested by an HTTP client (e.g., GET, PUT, POST, DELETE).
• Pre-signed URL: A temporary, signed link that allows limited access to an S3 object with a specific method.
• ACL (Access Control List): Defines who can perform certain operations on S3 objects.
• Idempotency: A design principle where repeating an operation has the same effect as doing it once. PUT is idempotent, POST is not.
• 405 Method Not Allowed: HTTP error code indicating that the method is not supported on the target resource.

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Steps at a Glance

1. Confirm the method used in the failing request.
2. Check if the method is supported for that resource.
3. Verify IAM policies and ACLs.
4. Inspect pre-signed URLs for method mismatches.
5. Update code, policies, or URLs to align with supported methods.

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Detailed Steps

1. Confirm the Method Used in the Request
Enable verbose logging to capture the exact HTTP verb:

aws s3 cp file.txt s3://my-bucket/ --debug

Look for PUT, POST, or DELETE in the debug output.

Example: If you attempt this with POST:

curl -X POST https://my-bucket.s3.amazonaws.com/file.txt -d @file.txt

…it will fail with a 405. The correct PUT request is:

curl -X PUT https://my-bucket.s3.amazonaws.com/file.txt -d @file.txt

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2. Check Method Support for the Resource
Remember:

• Buckets: support GET (list), PUT (create), and DELETE (delete bucket).
• Objects: support GET, PUT, DELETE, but not POST directly.

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3. Verify IAM Policies and ACLs
Ensure the caller has permissions for the method:

aws iam simulate-principal-policy \
  --policy-source-arn arn:aws:iam::123456789012:user/dev \
  --action-names s3:PutObject s3:DeleteObject

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4. Inspect Pre-Signed URLs
Check the method encoded in the URL. A URL signed for PUT will fail if used for DELETE:

aws s3 presign s3://my-bucket/file.txt --method PUT

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5. Update Code, Policies, or URLs

• Fix client SDK calls to use correct verbs.
• Regenerate pre-signed URLs with the intended method.
• Adjust bucket policies or ACLs if the method should be allowed.

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Conclusion

The 405 Method Not Allowed error isn’t an S3 outage — it’s a mismatch between request and resource. By double-checking the HTTP method, confirming what’s valid for buckets vs. objects, verifying IAM policies, and ensuring pre-signed URLs align, you can quickly resolve it.

Think of it as S3 saying: “I understand what you’re asking, but I don’t allow that here.” Align the method with the resource, and your pipelines flow again.

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