A developer has implemented a new feature in an application that involves uploading large files (up to 5GB) to an S3 bucket using presigned URLs from an EC2 instance. Users are reporting occasional 'Access Denied' errors during the upload process, even though the IAM role attached to the EC2 instance has `s3:PutObject` permissions for the target bucket. The presigned URLs are generated correctly with an expiration of 15 minutes. What is the MOST probable cause of the 'Access Denied' errors?
- AThe S3 bucket policy explicitly denies uploads from the EC2 instance's IP address range.
- BThe presigned URL is being used after its 15-minute expiration period.
- CThe Content-Type header used during the upload does not match the 'Content-Type' specified when generating the presigned URL.
- DThe S3 bucket has versioning enabled, which conflicts with direct uploads via presigned URLs.
Show answer & explanationAnswer & explanation
Correct answer: C. The Content-Type header used during the upload does not match the 'Content-Type' specified when generating the presigned URL.
When generating a presigned URL for an S3 upload, if certain request headers (like `Content-Type`) are specified, then the client must use *exactly* those headers (and values) when making the actual PUT request. A mismatch will result in an 'Access Denied' error, as the signature no longer matches the request.
Why the other options are wrong
- A. If a bucket policy denied access by IP, it would likely be a consistent failure, not occasional, and would affect all uploads from that instance.
- B. While using an expired URL would cause 'Access Denied', the question states 'occasional' errors, and the URLs are generated with a 15-minute expiration, implying most are used within time. This is less probable for 'occasional' errors compared to header mismatches.
- D. Versioning enabled does not conflict with presigned URL uploads; it simply keeps multiple versions of the object.
S3 Presigned URL Header Matching
When generating an S3 presigned URL, if you include specific HTTP headers (such as `Content-Type`, `Content-Disposition`, `x-amz-meta-*`) in the signing process, the client must include those *exact* headers with their *exact* values in the subsequent actual HTTP request to S3. Any mismatch will result in an 'Access Denied' error.
- Ensures the signed request matches the actual request.
- Crucial for security and integrity of the signed operation.
- Content-Type mismatch is a very common pitfall.
Memory trick: Presigned URL's 'Access Denied': Check the Headers first, then Expiration.