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
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