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
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
Real developers can help you.
Matthew Butler
Systems Development Engineer @ Amazon Web Services
Yovel Cohen
I got a lot of experience in building Long-horizon AI Agents in production, Backend apps that scale to millions of users and frontend knowledge as well.
Richard McSorley
Full-Stack Software Engineer with 8+ years building high-performance applications for enterprise clients. Shipped production systems at Walmart (4,000+ stores), Cigna (20M+ users), and Arkansas Blue Cross. 5 patents in retail/supply chain tech. Currently focused on AI integrations, automation tools, and TypeScript-first architectures.
Taufan
I’m a product-focused engineer and tech leader who builds scalable systems and turns ideas into production-ready platforms.
Over the past years, I’ve worked across startups and fast-moving teams, leading backend architecture, improving system reliability, and shipping products used by thousands of users. My strength is not just writing code — but connecting product vision, technical execution, and business impact.
legrab
I'll fill this later
hanson1014
Full-stack developer experienced in fixing and deploying AI-generated apps from Lovable, Bolt.new, Cursor, and Replit. I specialize in debugging Supabase integration issues (auth flows, RLS policies, database connections), fixing broken deployments, resolving routing/blank screen problems, and cleaning up messy React/Vite codebases. I also build production apps with the Claude API and have shipped a Mac desktop dev tool (Nexterm from scratch. Based in Hong Kong, fast turnaround.
Mehdi Ben Haddou
- Founder of Chessigma (1M+ users) & many small projects
- ex Founding Engineer @Uplane (YC F25)
- ex Software Engineer @Amazon and @Booking.com
Matt Butler
Software Engineer @ AWS
Meïr Ankri
Full-stack developer specializing in React / Next.js / Node.js with 6+ years of experience. I've worked across various sectors including automotive (Reezocar/Société Générale), healthcare (Medical Link SaaS), and e-commerce (Glasman). I build web apps end-to-end, from architecture to production, with a focus on scalability, performance, and code quality. I also mentor junior developers and contribute to technical decisions and code reviews.
Nam Tran
10 years as fullstack developer
You don't need to be technical. Just describe what's wrong and a verified developer will handle the rest.
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.