Fix CORS headers not applied to non-existent bucket responses (#8070)

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

weed/s3api/cors/middleware.go

@@ -82,14 +82,9 @@ func (m *Middleware) Handler(next http.Handler) http.Handler {
 			return
 		}
 
-		// Check if bucket exists
-		if err := m.bucketChecker.CheckBucket(r, bucket); err != s3err.ErrNone {
-			// For non-existent buckets, let the normal handler deal with it
-			next.ServeHTTP(w, r)
-			return
-		}
-
-		// Get CORS configuration (bucket-specific or fallback)
+		// Get CORS configuration (bucket-specific or fallback) BEFORE checking bucket existence
+		// This ensures CORS headers are applied consistently regardless of bucket existence,
+		// preventing information disclosure about whether a bucket exists
 		config, found := m.getCORSConfig(bucket)
 		if !found {
 			// No CORS configuration at all, handle based on request type
@@ -117,10 +112,12 @@ func (m *Middleware) Handler(next http.Handler) http.Handler {
 			return
 		}
 
-		// Apply CORS headers
+		// Apply CORS headers early, before bucket existence check
+		// This ensures consistent CORS behavior and prevents information disclosure
 		ApplyHeaders(w, corsResp)
 
-		// Handle preflight requests
+		// Handle preflight requests - return success immediately without checking bucket existence
+		// This matches AWS S3 behavior where preflight requests succeed even for non-existent buckets
 		if corsReq.IsPreflightRequest {
 			// Preflight request should return 200 OK with just CORS headers
 			w.WriteHeader(http.StatusOK)
@@ -128,6 +125,8 @@ func (m *Middleware) Handler(next http.Handler) http.Handler {
 		}
 
 		// For actual requests, continue with normal processing
+		// The handler will check bucket existence and return appropriate errors (e.g., NoSuchBucket)
+		// but CORS headers have already been applied above
 		next.ServeHTTP(w, r)
 	})
 }
@@ -150,14 +149,8 @@ func (m *Middleware) HandleOptionsRequest(w http.ResponseWriter, r *http.Request
 		return
 	}
 
-	// Check if bucket exists
-	if err := m.bucketChecker.CheckBucket(r, bucket); err != s3err.ErrNone {
-		// For non-existent buckets, return OK (let other handlers deal with bucket existence)
-		w.WriteHeader(http.StatusOK)
-		return
-	}
-
-	// Get CORS configuration (bucket-specific or fallback)
+	// Get CORS configuration (bucket-specific or fallback) BEFORE checking bucket existence
+	// This ensures CORS headers are applied consistently regardless of bucket existence
 	config, found := m.getCORSConfig(bucket)
 	if !found {
 		// No CORS configuration at all for OPTIONS request should return access denied

weed/s3api/cors/middleware_nonexistent_bucket_test.go

@@ -0,0 +1,240 @@
+package cors
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/gorilla/mux"
+	"github.com/seaweedfs/seaweedfs/weed/s3api/s3err"
+)
+
+// TestMiddlewareNonExistentBucket tests that CORS headers are applied even for non-existent buckets
+// This prevents information disclosure about bucket existence and ensures consistent CORS behavior
+func TestMiddlewareNonExistentBucket(t *testing.T) {
+	tests := []struct {
+		name                 string
+		fallbackConfig       *CORSConfiguration
+		requestOrigin        string
+		requestMethod        string
+		isOptions            bool
+		expectedStatus       int
+		expectedOriginHeader string
+		description          string
+	}{
+		{
+			name: "Preflight request to non-existent bucket with global CORS config",
+			fallbackConfig: &CORSConfiguration{
+				CORSRules: []CORSRule{
+					{
+						AllowedOrigins: []string{"*"},
+						AllowedMethods: []string{"GET", "POST", "PUT", "DELETE", "HEAD"},
+						AllowedHeaders: []string{"*"},
+					},
+				},
+			},
+			requestOrigin:        "https://example.com",
+			requestMethod:        "OPTIONS",
+			isOptions:            true,
+			expectedStatus:       http.StatusOK,
+			expectedOriginHeader: "https://example.com",
+			description:          "Preflight to non-existent bucket should succeed with CORS headers",
+		},
+		{
+			name: "Actual request to non-existent bucket with global CORS config",
+			fallbackConfig: &CORSConfiguration{
+				CORSRules: []CORSRule{
+					{
+						AllowedOrigins: []string{"*"},
+						AllowedMethods: []string{"GET", "POST", "PUT", "DELETE", "HEAD"},
+						AllowedHeaders: []string{"*"},
+					},
+				},
+			},
+			requestOrigin:        "https://example.com",
+			requestMethod:        "GET",
+			isOptions:            false,
+			expectedStatus:       http.StatusNotFound, // Handler returns NoSuchBucket
+			expectedOriginHeader: "https://example.com",
+			description:          "GET to non-existent bucket should have CORS headers even with NoSuchBucket error",
+		},
+		{
+			name: "Preflight to non-existent bucket with specific origin",
+			fallbackConfig: &CORSConfiguration{
+				CORSRules: []CORSRule{
+					{
+						AllowedOrigins: []string{"https://allowed.com"},
+						AllowedMethods: []string{"GET", "POST"},
+						AllowedHeaders: []string{"*"},
+					},
+				},
+			},
+			requestOrigin:        "https://allowed.com",
+			requestMethod:        "OPTIONS",
+			isOptions:            true,
+			expectedStatus:       http.StatusOK,
+			expectedOriginHeader: "https://allowed.com",
+			description:          "Preflight to non-existent bucket with matching origin should succeed",
+		},
+		{
+			name: "Preflight to non-existent bucket with non-matching origin",
+			fallbackConfig: &CORSConfiguration{
+				CORSRules: []CORSRule{
+					{
+						AllowedOrigins: []string{"https://allowed.com"},
+						AllowedMethods: []string{"GET", "POST"},
+						AllowedHeaders: []string{"*"},
+					},
+				},
+			},
+			requestOrigin:        "https://notallowed.com",
+			requestMethod:        "OPTIONS",
+			isOptions:            true,
+			expectedStatus:       http.StatusForbidden,
+			expectedOriginHeader: "",
+			description:          "Preflight to non-existent bucket with non-matching origin should fail",
+		},
+		{
+			name:                 "Preflight to non-existent bucket without CORS config",
+			fallbackConfig:       nil,
+			requestOrigin:        "https://example.com",
+			requestMethod:        "OPTIONS",
+			isOptions:            true,
+			expectedStatus:       http.StatusForbidden,
+			expectedOriginHeader: "",
+			description:          "Preflight to non-existent bucket without CORS config should fail",
+		},
+		{
+			name: "Bucket listing request to non-existent bucket with CORS",
+			fallbackConfig: &CORSConfiguration{
+				CORSRules: []CORSRule{
+					{
+						AllowedOrigins: []string{"*"},
+						AllowedMethods: []string{"GET", "POST", "PUT", "DELETE", "HEAD"},
+						AllowedHeaders: []string{"*"},
+					},
+				},
+			},
+			requestOrigin:        "https://mydomain.com",
+			requestMethod:        "OPTIONS",
+			isOptions:            true,
+			expectedStatus:       http.StatusOK,
+			expectedOriginHeader: "https://mydomain.com",
+			description:          "Bucket listing preflight to non-existent bucket should have CORS headers (issue #8065)",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			// Setup mocks - bucket does NOT exist
+			bucketChecker := &mockBucketChecker{bucketExists: false}
+			configGetter := &mockCORSConfigGetter{
+				config:  nil, // No bucket-specific config
+				errCode: s3err.ErrNone,
+			}
+
+			// Create middleware with fallback config
+			middleware := NewMiddleware(bucketChecker, configGetter, tt.fallbackConfig)
+
+			// Create request with mux variables
+			req := httptest.NewRequest(tt.requestMethod, "/nonexistent-bucket/testobject", nil)
+			req = mux.SetURLVars(req, map[string]string{
+				"bucket": "nonexistent-bucket",
+				"object": "testobject",
+			})
+			if tt.requestOrigin != "" {
+				req.Header.Set("Origin", tt.requestOrigin)
+			}
+			if tt.isOptions {
+				req.Header.Set("Access-Control-Request-Method", "GET")
+			}
+
+			// Create response recorder
+			w := httptest.NewRecorder()
+
+			// Create a handler that returns 404 for non-existent buckets
+			nextHandler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				// Simulate NoSuchBucket error
+				w.WriteHeader(http.StatusNotFound)
+			})
+
+			// Execute middleware
+			if tt.isOptions {
+				middleware.HandleOptionsRequest(w, req)
+			} else {
+				middleware.Handler(nextHandler).ServeHTTP(w, req)
+			}
+
+			// Check status code
+			if w.Code != tt.expectedStatus {
+				t.Errorf("%s: expected status %d, got %d", tt.description, tt.expectedStatus, w.Code)
+			}
+
+			// Check CORS header
+			actualOrigin := w.Header().Get("Access-Control-Allow-Origin")
+			if actualOrigin != tt.expectedOriginHeader {
+				t.Errorf("%s: expected Access-Control-Allow-Origin='%s', got '%s'",
+					tt.description, tt.expectedOriginHeader, actualOrigin)
+			}
+		})
+	}
+}
+
+// TestMiddlewareConsistentBehavior tests that CORS headers are consistent regardless of bucket existence
+// This is a security test to ensure bucket existence cannot be inferred from CORS header presence
+func TestMiddlewareConsistentBehavior(t *testing.T) {
+	fallbackConfig := &CORSConfiguration{
+		CORSRules: []CORSRule{
+			{
+				AllowedOrigins: []string{"*"},
+				AllowedMethods: []string{"GET", "POST", "PUT", "DELETE", "HEAD"},
+				AllowedHeaders: []string{"*"},
+			},
+		},
+	}
+
+	testCases := []struct {
+		bucketExists bool
+		description  string
+	}{
+		{bucketExists: true, description: "existing bucket"},
+		{bucketExists: false, description: "non-existent bucket"},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.description, func(t *testing.T) {
+			// Setup mocks
+			bucketChecker := &mockBucketChecker{bucketExists: tc.bucketExists}
+			configGetter := &mockCORSConfigGetter{
+				config:  nil,
+				errCode: s3err.ErrNone,
+			}
+
+			middleware := NewMiddleware(bucketChecker, configGetter, fallbackConfig)
+
+			// Create preflight request
+			req := httptest.NewRequest("OPTIONS", "/testbucket/testobject", nil)
+			req = mux.SetURLVars(req, map[string]string{
+				"bucket": "testbucket",
+				"object": "testobject",
+			})
+			req.Header.Set("Origin", "https://example.com")
+			req.Header.Set("Access-Control-Request-Method", "GET")
+
+			w := httptest.NewRecorder()
+			middleware.HandleOptionsRequest(w, req)
+
+			// Both existing and non-existent buckets should return same CORS headers
+			actualOrigin := w.Header().Get("Access-Control-Allow-Origin")
+			if actualOrigin != "https://example.com" {
+				t.Errorf("%s: expected Access-Control-Allow-Origin='https://example.com', got '%s'",
+					tc.description, actualOrigin)
+			}
+
+			// Both should return 200 OK for preflight
+			if w.Code != http.StatusOK {
+				t.Errorf("%s: expected status 200, got %d", tc.description, w.Code)
+			}
+		})
+	}
+}