A developer has implemented a new feature in an application that involves uploading large files directly to an S3 bucket using presigned URLs. Users are reporting that uploads intermittently fail with a '403 Forbidden' error, even though the presigned URL was generated correctly with appropriate permissions. The application code generating the presigned URL does not specify any 'Content-Type' or 'Content-MD5' during generation, but the client-side upload includes a 'Content-Type' header. Which of the following is the MOST likely cause of the 403 Forbidden errors?
- AThe IAM role or user generating the presigned URL lacks the 's3:PutObject' permission on the target S3 bucket.
- BThe S3 bucket policy is implicitly denying the upload operation due to a condition that is not being met.
- CThe client-side upload request includes HTTP headers that were not specified or matched when the presigned URL was generated.
- DThe presigned URL's expiration time is too short, causing it to expire before the large file upload completes.
Show answer & explanationAnswer & explanation
Correct answer: C. The client-side upload request includes HTTP headers that were not specified or matched when the presigned URL was generated.
When generating an S3 presigned URL, if certain HTTP headers (like `Content-Type`, `Content-Disposition`, `Content-Encoding`, `x-amz-meta-*`) are expected to be sent by the client during the upload, they *must* be included in the presigned URL generation request. If the client sends headers that were not specified during URL generation (or sends different values for specified headers), S3 will return a 403 Forbidden error because the signature calculated on the server-side won't match the actual request.
Why the other options are wrong
- A. The question states the presigned URL was generated 'correctly with appropriate permissions', so this is less likely to be the direct cause of the 403 specific to this scenario.
- B. While a bucket policy could deny, the scenario points to a client-side discrepancy with headers, which is a more direct cause for 403s with presigned URLs when permissions are otherwise fine.
- D. An expired URL would typically result in a '403 Request has expired' specific error, not just a generic '403 Forbidden' related to header mismatch.
S3 Presigned URL Header Matching
When generating an S3 presigned URL for upload, any HTTP headers (e.g., Content-Type, Content-MD5, x-amz-meta-*) that the client is expected to send with the PUT request *must* be included in the presigning request. If the client sends headers that do not match those used during presigning, the upload will fail with a 403 Forbidden error due to signature mismatch.
- Client-side headers must match presigned URL generation.
- Common headers: Content-Type, Content-MD5, x-amz-meta-.
- Mismatch results in 403 Forbidden error.
Memory trick: The 'Presigned' URL is a signed contract; every 'Header' must match perfectly.