Q: S3 uploads from your app are failing with `403 Forbidden`. What are the possible causes?
1. IAM permissions — the IAM role/user doesn't have s3:PutObject permission on the bucket.
#AWS #S3 & Storage #L2 #Cloud #Infrastructure #S3
🎙️ Candidate Opening & Architectural Context
""In our AWS cloud environment, we managed high-traffic microservices where this exact scenario occurred. When addressing this question, I walk the interviewer through our production incident runbook: isolating the blast radius, checking diagnostic logs and metrics, and applying a safe fix.""
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🛠️ Production Runbook & Step-by-Step Resolution
1️⃣
Initial Diagnostics & Root Cause Analysis
- IAM permissions — the IAM role/user doesn't have
s3:PutObjectpermission on the bucket. - Bucket policy denying access — explicit Deny in bucket policy overrides IAM Allow.
- Block Public Access — if the app is trying to upload with public-read ACL and Block Public Access is enabled, it gets 403.
- Wrong region — bucket is in us-east-1 but app is hitting eu-west-1 endpoint. Use
--regionflag or set correct endpoint.
2️⃣
Remediation & Permanent Safeguards
Execute the resolution runbook and verify workload health:
- STS token expired — if using temporary credentials (IAM role), the session token may have expired.
- KMS encryption — if the bucket enforces KMS encryption, the IAM role needs
kms:GenerateDataKeypermission on the KMS key.
💡 The Senior SRE Gold Nugget (Key Architectural Takeaway)
"Pro-Tip: IAM permissions — the IAM role/user doesn't have s3:PutObject permission on the bucket.."
⚡ 60-Second Elevator Pitch Talking Points
- IAM permissions — the IAM role/user doesn't have s3:PutObject permission on the bucket.
- Bucket policy denying access — explicit Deny in bucket policy overrides IAM Allow.
- Block Public Access — if the app is trying to upload with public-read ACL and Block Public Access...
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