diff --git a/Backend/src/services/ChaptersLesson.service.ts b/Backend/src/services/ChaptersLesson.service.ts
index 5483ff81..57462d69 100644
--- a/Backend/src/services/ChaptersLesson.service.ts
+++ b/Backend/src/services/ChaptersLesson.service.ts
@@ -182,6 +182,10 @@ export class ChaptersLessonService {
             if (!currentChapter) {
                 throw new NotFoundError('Chapter not found');
             }
+            // Validate chapter belongs to the specified course
+            if (currentChapter.course_id !== course_id) {
+                throw new NotFoundError('Chapter not found in this course');
+            }
             const oldSortOrder = currentChapter.sort_order;
 
             // If same position, no need to reorder