diff --git a/openapi3/extension.go b/openapi3/extension.go
index c29959091..37f6b01e5 100644
--- a/openapi3/extension.go
+++ b/openapi3/extension.go
@@ -8,11 +8,19 @@ import (
 )
 
 func validateExtensions(ctx context.Context, extensions map[string]interface{}) error { // FIXME: newtype + Validate(...)
+	allowed := getValidationOptions(ctx).extraSiblingFieldsAllowed
+
 	var unknowns []string
 	for k := range extensions {
-		if !strings.HasPrefix(k, "x-") {
-			unknowns = append(unknowns, k)
+		if strings.HasPrefix(k, "x-") {
+			continue
+		}
+		if allowed != nil {
+			if _, ok := allowed[k]; ok {
+				continue
+			}
 		}
+		unknowns = append(unknowns, k)
 	}
 
 	if len(unknowns) != 0 {
diff --git a/openapi3/issue513_test.go b/openapi3/issue513_test.go
index 332b9226e..442eac848 100644
--- a/openapi3/issue513_test.go
+++ b/openapi3/issue513_test.go
@@ -7,6 +7,71 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+func TestExtraSiblingsInRemoteRef(t *testing.T) {
+	spec := []byte(`
+openapi: 3.0.1
+servers:
+- url: http://localhost:5000
+info:
+  version: v1
+  title: Products api
+  contact:
+    name: me
+    email: me@github.com
+  description: This is a sample
+paths:
+  /categories:
+    get:
+      summary: Provides the available categories for the store
+      operationId: list-categories
+      responses:
+        '200':
+          description: this is a desc
+          content:
+            application/json:
+              schema:
+                $ref: http://schemas.sentex.io/store/categories.json
+`[1:])
+
+	// When that site fails to respond:
+	// see https://github.com/getkin/kin-openapi/issues/495
+
+	// http://schemas.sentex.io/store/categories.json
+	// {
+	//   "$id": "http://schemas.sentex.io/store/categories.json",
+	//   "$schema": "http://json-schema.org/draft-07/schema#",
+	//   "description": "array of category strings",
+	//   "type": "array",
+	//   "items": {
+	//     "allOf": [
+	//       {
+	//         "$ref": "http://schemas.sentex.io/store/category.json"
+	//       }
+	//     ]
+	//   }
+	// }
+
+	// http://schemas.sentex.io/store/category.json
+	// {
+	//   "$id": "http://schemas.sentex.io/store/category.json",
+	//   "$schema": "http://json-schema.org/draft-07/schema#",
+	//   "description": "category name for products",
+	//   "type": "string",
+	//   "pattern": "^[A-Za-z0-9\\-]+$",
+	//   "minimum": 1,
+	//   "maximum": 30
+	// }
+
+	sl := NewLoader()
+	sl.IsExternalRefsAllowed = true
+
+	doc, err := sl.LoadFromData(spec)
+	require.NoError(t, err)
+
+	err = doc.Validate(sl.Context, AllowExtraSiblingFields("$id", "$schema"))
+	require.NoError(t, err)
+}
+
 func TestIssue513OKWithExtension(t *testing.T) {
 	spec := `
 openapi: "3.0.3"
@@ -92,7 +157,7 @@ components:
 	require.ErrorContains(t, err, `extra sibling fields: [schema]`)
 }
 
-func TestIssue513KOMixesRefAlongWithOtherFields(t *testing.T) {
+func TestIssue513KOMixesRefAlongWithOtherFieldsDisallowed(t *testing.T) {
 	spec := `
 openapi: "3.0.3"
 info: