Disable prototype pollution on returned diff object

src/added.js

@@ -1,4 +1,4 @@
-import { isEmpty, isObject, hasOwnProperty } from './utils.js';
+import { isEmpty, isObject, hasOwnProperty, makeObjectWithoutPrototype } from './utils.js';
 
 const addedDiff = (lhs, rhs) => {
 
@@ -19,7 +19,7 @@ const addedDiff = (lhs, rhs) => {
 
     acc[key] = r[key];
     return acc;
-  }, {});
+  }, makeObjectWithoutPrototype());
 };
 
 export default addedDiff;

src/deleted.js

@@ -1,4 +1,4 @@
-import { isEmpty, isObject, hasOwnProperty } from './utils.js';
+import { isEmpty, isObject, hasOwnProperty, makeObjectWithoutPrototype } from './utils.js';
 
 const deletedDiff = (lhs, rhs) => {
   if (lhs === rhs || !isObject(lhs) || !isObject(rhs)) return {};
@@ -18,7 +18,7 @@ const deletedDiff = (lhs, rhs) => {
 
     acc[key] = undefined;
     return acc;
-  }, {});
+  }, makeObjectWithoutPrototype());
 };
 
 export default deletedDiff;

src/diff.js

@@ -1,4 +1,4 @@
-import { isDate, isEmptyObject, isObject, hasOwnProperty } from './utils.js';
+import { isDate, isEmptyObject, isObject, hasOwnProperty, makeObjectWithoutPrototype } from './utils.js';
 
 const diff = (lhs, rhs) => {
   if (lhs === rhs) return {}; // equal return no diff
@@ -15,7 +15,7 @@ const diff = (lhs, rhs) => {
     }
 
     return acc;
-  }, {});
+  }, makeObjectWithoutPrototype());
 
   if (isDate(l) || isDate(r)) {
     if (l.valueOf() == r.valueOf()) return {};

src/updated.js

@@ -1,4 +1,4 @@
-import { isDate, isEmptyObject, isObject, hasOwnProperty } from './utils.js';
+import { isDate, isEmptyObject, isObject, hasOwnProperty, makeObjectWithoutPrototype } from './utils.js';
 
 const updatedDiff = (lhs, rhs) => {
   if (lhs === rhs) return {};
@@ -26,7 +26,7 @@ const updatedDiff = (lhs, rhs) => {
     }
 
     return acc;
-  }, {});
+  }, makeObjectWithoutPrototype());
 };
 
 export default updatedDiff;

src/utils.js

@@ -3,3 +3,4 @@ export const isEmpty = o => Object.keys(o).length === 0;
 export const isObject = o => o != null && typeof o === 'object';
 export const hasOwnProperty = (o, ...args) => Object.prototype.hasOwnProperty.call(o, ...args)
 export const isEmptyObject = (o) => isObject(o) && isEmpty(o);
+export const makeObjectWithoutPrototype = () => Object.create(null);

test/pollution.test.js

@@ -1,85 +1,70 @@
 import addedDiff from "../src/added";
+import updatedDiff from "../src/updated";
+import diff from "../src/diff";
+import deletedDiff from "../src/deleted";
 
 describe("Prototype pollution", () => {
-  test("Demonstrate prototype pollution globally across all objects", () => {
-    const a = {};
-    const b = new Object();
-
-    expect(a.hello).toBeUndefined();
-    expect(b.hello).toBeUndefined();
-    expect({}.hello).toBeUndefined();
-
-    b.__proto__.hello = "world";
-
-    expect(a.hello).toBe("world");
-    expect(b.hello).toBe("world");
-    expect({}.hello).toBe("world");
+  describe("diff", () => {
+    test("should not pollute returned diffs prototype", () => {
+      const l = { role: "user" };
+      const r = JSON.parse('{ "role": "user", "__proto__": { "role": "admin" } }');
+      const difference = diff(l, r);
+
+      expect(l.role).toBe("user");
+      expect(r.role).toBe("user");
+      expect(difference.role).toBeUndefined();
+    });
+
+    test("should not pollute returned diffs prototype on nested diffs", () => {
+      const l = { about: { role: "user" } };
+      const r = JSON.parse('{ "about": { "__proto__": { "role": "admin" } } }');
+      const difference = addedDiff(l, r);
+
+      expect(l.about.role).toBe("user");
+      expect(r.about.role).toBeUndefined();
+      expect(difference.about.role).toBeUndefined();
+    });
   });
 
-  test("addedDiff does not pollute global prototype when running diff with added `__proto__` key", () => {
-    const a = { role: "user" };
-    const b = JSON.parse('{ "__proto__": { "role": "admin" } }');
-
-    expect(a.role).toBe("user");
-    expect(a.__proto__.role).toBeUndefined();
-    expect(b.role).toBeUndefined();
-    expect(b.__proto__.role).toBe("admin");
-    expect({}.role).toBeUndefined();
-    expect({}.__proto__role).toBeUndefined();
-
-    const difference = addedDiff(a, b);
-
-    expect(a.role).toBe("user");
-    expect(a.__proto__.role).toBeUndefined();
-    expect(b.__proto__.role).toBe("admin");
-    expect(b.role).toBeUndefined();
-    expect({}.role).toBeUndefined();
-    expect({}.__proto__role).toBeUndefined();
-
-    expect(difference).toEqual({ __proto__: { role: "admin" } });
+  describe("addedDiff", () => {
+    test("addedDiff should not pollute returned diffs prototype", () => {
+      const l = { role: "user" };
+      const r = JSON.parse('{ "__proto__": { "role": "admin" } }');
+      const difference = addedDiff(l, r);
+
+      expect(l.role).toBe("user");
+      expect(r.role).toBeUndefined();
+      expect(difference.role).toBeUndefined();
+    });
+
+    test("should not pollute returned diffs prototype on nested diffs", () => {
+      const l = { about: { role: "user" } };
+      const r = JSON.parse('{ "about": { "__proto__": { "role": "admin" } } }');
+      const difference = addedDiff(l, r);
+
+      expect(l.about.role).toBe("user");
+      expect(r.about.role).toBeUndefined();
+      expect(difference.about.role).toBeUndefined();
+    });
   });
 
-  test("addedDiff does not pollute global prototype when running diff with added `__proto__` key generated from JSON.parse and mutating original left hand object", () => {
-    let a = { role: "user" };
-    // Note: Don't trust `JSON.parse`!!!
-    const b = JSON.parse('{ "__proto__": { "role": "admin" } }');
-
-    expect(a.role).toBe("user");
-    expect(a.__proto__.role).toBeUndefined();
-    expect(b.role).toBeUndefined();
-    expect(b.__proto__.role).toBe("admin");
-    expect({}.role).toBeUndefined();
-    expect({}.__proto__role).toBeUndefined();
-
-    // Note: although this does not pollute the global proto, it does pollute the original object. (Don't mutate kids!)
-    a = addedDiff(a, b);
+  test("updatedDiff should not pollute returned diffs prototype", () => {
+    const l = { role: "user" };
+    const r = JSON.parse('{ "role": "user", "__proto__": { "role": "admin" } }');
+    const difference = updatedDiff(l, r);
 
-    expect(a.role).toBe("admin");
-    expect(a.__proto__.role).toBe("admin");
-    expect(b.__proto__.role).toBe("admin");
-    expect(b.role).toBeUndefined();
-    expect({}.role).toBeUndefined();
-    expect({}.__proto__role).toBeUndefined();
+    expect(l.role).toBe("user");
+    expect(r.role).toBe("user");
+    expect(difference.role).toBeUndefined();
   });
 
-  test("addedDiff does not pollute global prototype or original object when running diff with added `__proto__` key", () => {
-    let a = { role: "user" };
-    const b = { __proto__: { role: "admin" } };
-
-    expect(a.role).toBe("user");
-    expect(a.__proto__.role).toBeUndefined();
-    expect(b.role).toBe("admin");
-    expect(b.__proto__.role).toBe("admin");
-    expect({}.role).toBeUndefined();
-    expect({}.__proto__role).toBeUndefined();
-
-    a = addedDiff(a, b);
+  test("deletedDiff should not pollute returned diffs prototype", () => {
+    const l = { role: "user" };
+    const r = JSON.parse('{ "__proto__": { "role": "admin" } }');
+    const difference = deletedDiff(l, r);
 
-    expect(a.role).toBeUndefined();
-    expect(a.__proto__.role).toBeUndefined();
-    expect(b.role).toBe("admin");
-    expect(b.__proto__.role).toBe("admin");
-    expect({}.role).toBeUndefined();
-    expect({}.__proto__role).toBeUndefined();
+    expect(l.role).toBe("user");
+    expect(r.role).toBeUndefined();
+    expect(difference.role).toBeUndefined();
   });
 });