diff --git a/source/server/utils/merge/apply.test.ts b/source/server/utils/merge/apply.test.ts
index da11c536..4da64db5 100644
--- a/source/server/utils/merge/apply.test.ts
+++ b/source/server/utils/merge/apply.test.ts
@@ -42,4 +42,16 @@ describe("merge.apply()", function(){
   it("merges special SOURCE_INDEX symbol", function(){
     expect(apply({a:{[SOURCE_INDEX]:0}}, {a:{[SOURCE_INDEX]:1}})).to.deep.equal({a:{[SOURCE_INDEX]:1}});
   });
+
+  it("prevents prototype pollution (create object)", function(){
+    //This is not a strict
+    let result:any = apply({}, {a:{__proto__: {isAdmin: true}, tick: 1} as any});
+    expect(result.a, `Expected prototype pollution to be filtered internally`).to.not.have.property("isAdmin");
+  });
+
+  it("prevents prototype pollution (update object)", function(){
+    let result :any = apply({a: {tick: 0}}, {a:{__proto__: {isAdmin: true}, tick: 1} as any});
+    expect(result).to.deep.equal({a:{tick: 1}});
+    expect(result.a).to.not.have.property("isAdmin");
+  });
 });
\ No newline at end of file
diff --git a/source/server/utils/merge/apply.ts b/source/server/utils/merge/apply.ts
index 196a8396..24c6f42e 100644
--- a/source/server/utils/merge/apply.ts
+++ b/source/server/utils/merge/apply.ts
@@ -11,7 +11,7 @@ import {Diff, DELETE_KEY, SOURCE_INDEX, withIndex} from "./pointers/types.js";
 export default function apply<T extends Record<string, any>>(into :T, ...diffs :Diff<T>[]):T{
   for(const diff of diffs){
     if(SOURCE_INDEX in diff) into = withIndex(into, diff[SOURCE_INDEX] as number);
-    for(const key in diff){
+    for(const key of Object.getOwnPropertyNames(diff) as Extract<keyof Diff<T>, string>[]){
       const value = diff[key] as T[Extract<keyof T, string>];
 
       if(value === DELETE_KEY){
@@ -21,9 +21,7 @@ export default function apply<T extends Record<string, any>>(into :T, ...diffs :
           delete into[key];
         }
   
-      }else if(typeof value !== "object" //primitive
-            ||  typeof into[key] === "undefined" //undefined target
-      ){
+      }else if(typeof value !== "object" /*primitive*/){
         into[key] = value;
       }else if(Array.isArray(value)){
         //Replace arrays without looking.