Fixes#8065
Problem:
- CORS headers were only applied after checking bucket existence
- Non-existent buckets returned responses without CORS headers
- This caused CORS preflight failures and information disclosure vulnerability
- Unauthenticated users could infer bucket existence from CORS header presence
Solution:
- Moved CORS evaluation before bucket existence check in middleware
- CORS headers now applied consistently regardless of bucket existence
- Preflight requests succeed for non-existent buckets (matching AWS S3)
- Actual requests still return NoSuchBucket error but with CORS headers
Changes:
- Modified Handler() and HandleOptionsRequest() in middleware.go
- Added comprehensive test suite for non-existent bucket scenarios
- All 39 tests passing (31 existing + 8 new)
Security Impact:
- Prevents information disclosure about bucket existence
- Bucket existence cannot be inferred from CORS header presence/absence
AWS S3 Compatibility:
- Improved compatibility with AWS S3 CORS behavior
- Preflight requests now succeed for non-existent buckets
see https://blog.aqwari.net/xml-schema-go/
1. go get aqwari.net/xml/cmd/xsdgen
2. Add EncodingType element for ListBucketResult in AmazonS3.xsd
3. xsdgen -o s3api_xsd_generated.go -pkg s3api AmazonS3.xsd
4. Remove empty Grantee struct in s3api_xsd_generated.go
5. Remove xmlns: sed s'/http:\/\/s3.amazonaws.com\/doc\/2006-03-01\/\ //' s3api_xsd_generated.go
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