diff --git a/server/router/api/v1/attachment_service.go b/server/router/api/v1/attachment_service.go
index e7bec035f..f2218da41 100644
--- a/server/router/api/v1/attachment_service.go
+++ b/server/router/api/v1/attachment_service.go
@@ -246,6 +246,12 @@ func (s *APIV1Service) GetAttachment(ctx context.Context, request *v1pb.GetAttac
 	if attachment == nil {
 		return nil, status.Errorf(codes.NotFound, "attachment not found")
 	}
+
+	// Check access permission based on linked memo visibility.
+	if err := s.checkAttachmentAccess(ctx, attachment); err != nil {
+		return nil, err
+	}
+
 	return convertAttachmentFromStore(attachment), nil
 }
 
@@ -257,10 +263,24 @@ func (s *APIV1Service) UpdateAttachment(ctx context.Context, request *v1pb.Updat
 	if request.UpdateMask == nil || len(request.UpdateMask.Paths) == 0 {
 		return nil, status.Errorf(codes.InvalidArgument, "update mask is required")
 	}
+	user, err := s.fetchCurrentUser(ctx)
+	if err != nil {
+		return nil, status.Errorf(codes.Internal, "failed to get current user: %v", err)
+	}
+	if user == nil {
+		return nil, status.Errorf(codes.Unauthenticated, "user not authenticated")
+	}
 	attachment, err := s.Store.GetAttachment(ctx, &store.FindAttachment{UID: &attachmentUID})
 	if err != nil {
 		return nil, status.Errorf(codes.Internal, "failed to get attachment: %v", err)
 	}
+	if attachment == nil {
+		return nil, status.Errorf(codes.NotFound, "attachment not found")
+	}
+	// Only the creator or admin can update the attachment.
+	if attachment.CreatorID != user.ID && !isSuperUser(user) {
+		return nil, status.Errorf(codes.PermissionDenied, "permission denied")
+	}
 
 	currentTs := time.Now().Unix()
 	update := &store.UpdateAttachment{
@@ -549,6 +569,44 @@ func (s *APIV1Service) validateAttachmentFilter(ctx context.Context, filterStr s
 	return nil
 }
 
+// checkAttachmentAccess verifies the user has permission to access the attachment.
+// For unlinked attachments (no memo), only the creator can access.
+// For linked attachments, access follows the memo's visibility rules.
+func (s *APIV1Service) checkAttachmentAccess(ctx context.Context, attachment *store.Attachment) error {
+	user, _ := s.fetchCurrentUser(ctx)
+
+	// For unlinked attachments, only the creator can access.
+	if attachment.MemoID == nil {
+		if user == nil {
+			return status.Errorf(codes.Unauthenticated, "user not authenticated")
+		}
+		if attachment.CreatorID != user.ID && !isSuperUser(user) {
+			return status.Errorf(codes.PermissionDenied, "permission denied")
+		}
+		return nil
+	}
+
+	// For linked attachments, check memo visibility.
+	memo, err := s.Store.GetMemo(ctx, &store.FindMemo{ID: attachment.MemoID})
+	if err != nil {
+		return status.Errorf(codes.Internal, "failed to get memo: %v", err)
+	}
+	if memo == nil {
+		return status.Errorf(codes.NotFound, "memo not found")
+	}
+
+	if memo.Visibility == store.Public {
+		return nil
+	}
+	if user == nil {
+		return status.Errorf(codes.Unauthenticated, "user not authenticated")
+	}
+	if memo.Visibility == store.Private && memo.CreatorID != user.ID && !isSuperUser(user) {
+		return status.Errorf(codes.PermissionDenied, "permission denied")
+	}
+	return nil
+}
+
 // shouldStripExif checks if the MIME type is an image format that may contain EXIF metadata.
 // Returns true for formats like JPEG, TIFF, WebP, HEIC, and HEIF which commonly contain
 // privacy-sensitive metadata such as GPS coordinates, camera settings, and device information.
diff --git a/server/router/api/v1/memo_attachment_service.go b/server/router/api/v1/memo_attachment_service.go
index fead95d7d..153f1aa80 100644
--- a/server/router/api/v1/memo_attachment_service.go
+++ b/server/router/api/v1/memo_attachment_service.go
@@ -98,6 +98,24 @@ func (s *APIV1Service) ListMemoAttachments(ctx context.Context, request *v1pb.Li
 	if err != nil {
 		return nil, status.Errorf(codes.Internal, "failed to get memo: %v", err)
 	}
+	if memo == nil {
+		return nil, status.Errorf(codes.NotFound, "memo not found")
+	}
+
+	// Check memo visibility.
+	if memo.Visibility != store.Public {
+		user, err := s.fetchCurrentUser(ctx)
+		if err != nil {
+			return nil, status.Errorf(codes.Internal, "failed to get user: %v", err)
+		}
+		if user == nil {
+			return nil, status.Errorf(codes.Unauthenticated, "user not authenticated")
+		}
+		if memo.Visibility == store.Private && memo.CreatorID != user.ID && !isSuperUser(user) {
+			return nil, status.Errorf(codes.PermissionDenied, "permission denied")
+		}
+	}
+
 	attachments, err := s.Store.ListAttachments(ctx, &store.FindAttachment{
 		MemoID: &memo.ID,
 	})
diff --git a/server/router/api/v1/memo_service.go b/server/router/api/v1/memo_service.go
index b73bd5403..f5d250a16 100644
--- a/server/router/api/v1/memo_service.go
+++ b/server/router/api/v1/memo_service.go
@@ -551,6 +551,21 @@ func (s *APIV1Service) CreateMemoComment(ctx context.Context, request *v1pb.Crea
 	if err != nil {
 		return nil, status.Errorf(codes.Internal, "failed to get memo")
 	}
+	if relatedMemo == nil {
+		return nil, status.Errorf(codes.NotFound, "memo not found")
+	}
+
+	// Check memo visibility before allowing comment.
+	user, err := s.fetchCurrentUser(ctx)
+	if err != nil {
+		return nil, status.Errorf(codes.Internal, "failed to get user")
+	}
+	if user == nil {
+		return nil, status.Errorf(codes.Unauthenticated, "user not authenticated")
+	}
+	if relatedMemo.Visibility == store.Private && relatedMemo.CreatorID != user.ID && !isSuperUser(user) {
+		return nil, status.Errorf(codes.PermissionDenied, "permission denied")
+	}
 
 	// Create the memo comment first.
 	memoComment, err := s.CreateMemo(ctx, &v1pb.CreateMemoRequest{
diff --git a/server/router/api/v1/reaction_service.go b/server/router/api/v1/reaction_service.go
index 872ececde..a7c7cc3bd 100644
--- a/server/router/api/v1/reaction_service.go
+++ b/server/router/api/v1/reaction_service.go
@@ -15,6 +15,33 @@ import (
 )
 
 func (s *APIV1Service) ListMemoReactions(ctx context.Context, request *v1pb.ListMemoReactionsRequest) (*v1pb.ListMemoReactionsResponse, error) {
+	// Extract memo UID and check visibility.
+	memoUID, err := ExtractMemoUIDFromName(request.Name)
+	if err != nil {
+		return nil, status.Errorf(codes.InvalidArgument, "invalid memo name: %v", err)
+	}
+	memo, err := s.Store.GetMemo(ctx, &store.FindMemo{UID: &memoUID})
+	if err != nil {
+		return nil, status.Errorf(codes.Internal, "failed to get memo: %v", err)
+	}
+	if memo == nil {
+		return nil, status.Errorf(codes.NotFound, "memo not found")
+	}
+
+	// Check memo visibility.
+	if memo.Visibility != store.Public {
+		user, err := s.fetchCurrentUser(ctx)
+		if err != nil {
+			return nil, status.Errorf(codes.Internal, "failed to get user")
+		}
+		if user == nil {
+			return nil, status.Errorf(codes.Unauthenticated, "user not authenticated")
+		}
+		if memo.Visibility == store.Private && memo.CreatorID != user.ID && !isSuperUser(user) {
+			return nil, status.Errorf(codes.PermissionDenied, "permission denied")
+		}
+	}
+
 	reactions, err := s.Store.ListReactions(ctx, &store.FindReaction{
 		ContentID: &request.Name,
 	})
@@ -40,6 +67,25 @@ func (s *APIV1Service) UpsertMemoReaction(ctx context.Context, request *v1pb.Ups
 	if user == nil {
 		return nil, status.Errorf(codes.Unauthenticated, "user not authenticated")
 	}
+
+	// Extract memo UID and check visibility before allowing reaction.
+	memoUID, err := ExtractMemoUIDFromName(request.Reaction.ContentId)
+	if err != nil {
+		return nil, status.Errorf(codes.InvalidArgument, "invalid memo name: %v", err)
+	}
+	memo, err := s.Store.GetMemo(ctx, &store.FindMemo{UID: &memoUID})
+	if err != nil {
+		return nil, status.Errorf(codes.Internal, "failed to get memo: %v", err)
+	}
+	if memo == nil {
+		return nil, status.Errorf(codes.NotFound, "memo not found")
+	}
+
+	// Check memo visibility.
+	if memo.Visibility == store.Private && memo.CreatorID != user.ID && !isSuperUser(user) {
+		return nil, status.Errorf(codes.PermissionDenied, "permission denied")
+	}
+
 	reaction, err := s.Store.UpsertReaction(ctx, &store.Reaction{
 		CreatorID:    user.ID,
 		ContentID:    request.Reaction.ContentId,
diff --git a/server/router/fileserver/fileserver.go b/server/router/fileserver/fileserver.go
index bc7581829..0aaffd42f 100644
--- a/server/router/fileserver/fileserver.go
+++ b/server/router/fileserver/fileserver.go
@@ -469,8 +469,19 @@ func calculateThumbnailDimensions(width, height int) (int, int) {
 
 // checkAttachmentPermission verifies the user has permission to access the attachment.
 func (s *FileServerService) checkAttachmentPermission(ctx context.Context, c echo.Context, attachment *store.Attachment) error {
+	// For unlinked attachments, only the creator can access.
 	if attachment.MemoID == nil {
-		return nil // Unlinked attachments are accessible.
+		user, err := s.getCurrentUser(ctx, c)
+		if err != nil {
+			return echo.NewHTTPError(http.StatusInternalServerError, "failed to get current user").SetInternal(err)
+		}
+		if user == nil {
+			return echo.NewHTTPError(http.StatusUnauthorized, "unauthorized access")
+		}
+		if user.ID != attachment.CreatorID && user.Role != store.RoleAdmin {
+			return echo.NewHTTPError(http.StatusForbidden, "forbidden access")
+		}
+		return nil
 	}
 
 	memo, err := s.Store.GetMemo(ctx, &store.FindMemo{ID: attachment.MemoID})
@@ -493,7 +504,7 @@ func (s *FileServerService) checkAttachmentPermission(ctx context.Context, c ech
 		return echo.NewHTTPError(http.StatusUnauthorized, "unauthorized access")
 	}
 
-	if memo.Visibility == store.Private && user.ID != attachment.CreatorID {
+	if memo.Visibility == store.Private && user.ID != memo.CreatorID && user.Role != store.RoleAdmin {
 		return echo.NewHTTPError(http.StatusForbidden, "forbidden access")
 	}