diff --git a/x-pack/plugins/timelines/server/search_strategy/timeline/factory/helpers/format_timeline_data.ts b/x-pack/plugins/timelines/server/search_strategy/timeline/factory/helpers/format_timeline_data.ts
index 481b74a802fec..3b83bb0873981 100644
--- a/x-pack/plugins/timelines/server/search_strategy/timeline/factory/helpers/format_timeline_data.ts
+++ b/x-pack/plugins/timelines/server/search_strategy/timeline/factory/helpers/format_timeline_data.ts
@@ -34,7 +34,14 @@ const createBaseTimelineEdges = (): TimelineEdges => ({
 
 function deepMerge(target: EventSource, source: EventSource) {
   for (const key in source) {
-    if (source && source[key] instanceof Object && target && target[key] instanceof Object) {
+    if (
+      !Object.prototype.hasOwnProperty.call(source, key) ||
+      key === '__proto__' ||
+      key === 'constructor'
+    )
+      // eslint-disable-next-line no-continue
+      continue;
+    if (source[key] instanceof Object && target[key] instanceof Object) {
       deepMerge(target[key], source[key]);
     } else {
       target[key] = source[key];