Cursor
storage
S3 Access Denied Errors in Cursor-Generated AWS Code
Your Cursor-generated code that interacts with Amazon S3 fails with 403 Access Denied errors when attempting to upload, download, or list objects. The AWS SDK throws AccessDenied exceptions even though you've configured credentials and created the bucket.
S3 permissions are notoriously complex, involving the intersection of IAM policies, bucket policies, ACLs, and encryption settings. Cursor often generates syntactically correct S3 code but with incorrect assumptions about the permission model — using wrong region configurations, missing required permissions in the IAM policy, or assuming public access that's been blocked by default.
The frustration compounds because the same code might work with one AWS account's permissions but fail with another, or work for reads but not writes, or work for small files but fail for multipart uploads.
Error Messages You Might See
AccessDenied: Access Denied
An error occurred (403) when calling the PutObject operation: Access Denied
SignatureDoesNotMatch: The request signature we calculated does not match
AccessDenied: User: arn:aws:iam::123456:user/app is not authorized to perform s3:PutObject
KMS.AccessDeniedException: The ciphertext refers to a customer master key that does not exist
Common Causes
- IAM policy too restrictive — The IAM user/role only has s3:GetObject but the code also needs s3:PutObject, s3:ListBucket, or s3:DeleteObject
- Bucket policy blocks access — The bucket has a restrictive bucket policy that overrides IAM permissions
- S3 Block Public Access enabled — Default S3 settings block all public access, but Cursor's code tries to set objects as public-read
- Wrong region configuration — The SDK is configured for us-east-1 but the bucket is in eu-west-1, causing signature mismatches
- Incorrect ARN in IAM policy — The IAM policy references the bucket ARN without the /* suffix for object-level operations
- KMS encryption key permissions — The bucket uses KMS encryption and the IAM role doesn't have kms:Decrypt or kms:GenerateDataKey permissions
How to Fix It
- Verify IAM permissions — Ensure your IAM policy includes all required actions: s3:PutObject, s3:GetObject, s3:ListBucket, s3:DeleteObject. Use the ARN format
arn:aws:s3:::bucket-namefor bucket-level andarn:aws:s3:::bucket-name/*for object-level operations - Check bucket region — Verify the region in your SDK config matches the actual bucket region. Find it in the S3 console under bucket Properties
- Review Block Public Access settings — If your code uses public-read ACLs, either disable Block Public Access or change the code to use signed URLs instead
- Test with AWS CLI first — Run
aws s3 cp test.txt s3://your-bucket/to verify credentials and permissions before debugging code - Check CloudTrail logs — Look at CloudTrail S3 data events to see the exact API call and which policy denied it
- Add KMS permissions if encrypted — If the bucket uses SSE-KMS, add kms:Decrypt and kms:GenerateDataKey to the IAM policy for the KMS key ARN
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Get HelpFrequently Asked Questions
Why does my S3 code work locally but fail in production?
Your local AWS CLI likely uses a different IAM user with broader permissions than the production IAM role. Check the production role's permissions in the IAM console and compare them to what the code requires.
Should I make my S3 bucket public to fix Access Denied?
Almost never. Instead, use pre-signed URLs to grant temporary access to specific objects. Generate them server-side with a short expiration (15 minutes to 1 hour) and pass the URL to the client.