diff --git a/docs/upgrade-to-v5.md b/docs/upgrade-to-v5.md
index dec24d3a..9cf7a701 100644
--- a/docs/upgrade-to-v5.md
+++ b/docs/upgrade-to-v5.md
@@ -101,6 +101,18 @@ The following deprecated methods have been removed:
 - `ObjectId.get_inc`
   - The `static getInc()` is private since invoking it increments the next `ObjectId` index, so invoking would impact the creation of subsequent ObjectIds.
 
+### BSON Element names are now fetched only from object's own properties
+
+`BSON.serialize`, `EJSON.stringify` and `BSON.calculateObjectSize` now only inspect own properties and do not consider properties defined on the prototype of the input.
+
+```typescript
+const object = { a: 1 };
+Object.setPrototypeOf(object, { b: 2 });
+BSON.deserialize(BSON.serialize(object));
+// now returns { a: 1 } in v5.0
+// would have returned { a: 1, b: 2 } in v4.x
+```
+
 ### Negative Zero is now serialized to Double
 
 BSON serialize will now preserve negative zero values as a floating point number.
diff --git a/src/extended_json.ts b/src/extended_json.ts
index c2a9671b..9f5f5505 100644
--- a/src/extended_json.ts
+++ b/src/extended_json.ts
@@ -287,7 +287,7 @@ function serializeDocument(doc: any, options: EJSONSerializeOptions) {
   if (typeof bsontype === 'undefined') {
     // It's a regular object. Recursively serialize its property values.
     const _doc: Document = {};
-    for (const name in doc) {
+    for (const name of Object.keys(doc)) {
       options.seenObjects.push({ propertyName: name, obj: null });
       try {
         const value = serializeValue(doc[name], options);
diff --git a/src/parser/calculate_size.ts b/src/parser/calculate_size.ts
index b4a20861..2d56a97f 100644
--- a/src/parser/calculate_size.ts
+++ b/src/parser/calculate_size.ts
@@ -29,7 +29,7 @@ export function calculateObjectSize(
     }
 
     // Calculate size
-    for (const key in object) {
+    for (const key of Object.keys(object)) {
       totalLength += calculateElement(key, object[key], serializeFunctions, false, ignoreUndefined);
     }
   }
diff --git a/src/parser/deserializer.ts b/src/parser/deserializer.ts
index c474539b..dcc1b78b 100644
--- a/src/parser/deserializer.ts
+++ b/src/parser/deserializer.ts
@@ -314,23 +314,16 @@ function deserializeObject(
         (buffer[index + 1] << 8) |
         (buffer[index + 2] << 16) |
         (buffer[index + 3] << 24);
-      let arrayOptions = options;
+      let arrayOptions: DeserializeOptions = options;
 
       // Stop index
       const stopIndex = index + objectSize;
 
       // All elements of array to be returned as raw bson
       if (fieldsAsRaw && fieldsAsRaw[name]) {
-        arrayOptions = {};
-        for (const n in options) {
-          (
-            arrayOptions as {
-              [key: string]: DeserializeOptions[keyof DeserializeOptions];
-            }
-          )[n] = options[n as keyof DeserializeOptions];
-        }
-        arrayOptions['raw'] = true;
+        arrayOptions = { ...options, raw: true };
       }
+
       if (!globalUTFValidation) {
         arrayOptions = { ...arrayOptions, validation: { utf8: shouldValidateKey } };
       }
diff --git a/src/parser/serializer.ts b/src/parser/serializer.ts
index 3cf35e05..e9232ce7 100644
--- a/src/parser/serializer.ts
+++ b/src/parser/serializer.ts
@@ -817,7 +817,7 @@ export function serializeInto(
     }
 
     // Iterate over all the keys
-    for (const key in object) {
+    for (const key of Object.keys(object)) {
       let value = object[key];
       // Is there an override value
       if (typeof value?.toBSON === 'function') {
diff --git a/test/node/extended_json_tests.js b/test/node/extended_json.test.ts
similarity index 94%
rename from test/node/extended_json_tests.js
rename to test/node/extended_json.test.ts
index fd5bae48..c84ed08d 100644
--- a/test/node/extended_json_tests.js
+++ b/test/node/extended_json.test.ts
@@ -1,8 +1,6 @@
-'use strict';
-
-const BSON = require('../register-bson');
+import * as BSON from '../register-bson';
 const EJSON = BSON.EJSON;
-const vm = require('vm');
+import * as vm from 'node:vm';
 
 // BSON types
 const Binary = BSON.Binary;
@@ -30,6 +28,7 @@ function getOldBSON() {
   try {
     // do a dynamic resolve to avoid exception when running browser tests
     const file = require.resolve('bson');
+    // eslint-disable-next-line @typescript-eslint/no-var-requires
     const oldModule = require(file).BSON;
     const funcs = new oldModule.BSON();
     oldModule.serialize = funcs.serialize;
@@ -49,7 +48,7 @@ describe('Extended JSON', function () {
 
   before(function () {
     const buffer = Buffer.alloc(64);
-    for (var i = 0; i < buffer.length; i++) buffer[i] = i;
+    for (let i = 0; i < buffer.length; i++) buffer[i] = i;
     const date = new Date();
     date.setTime(1488372056737);
     doc = {
@@ -80,7 +79,7 @@ describe('Extended JSON', function () {
 
   it('should correctly extend an existing mongodb module', function () {
     // TODO(NODE-4377): doubleNumberIntFit should be a double not a $numberLong
-    var json =
+    const json =
       '{"_id":{"$numberInt":"100"},"gh":{"$numberInt":"1"},"binary":{"$binary":{"base64":"AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUmJygpKissLS4vMDEyMzQ1Njc4OTo7PD0+Pw==","subType":"00"}},"date":{"$date":{"$numberLong":"1488372056737"}},"code":{"$code":"function() {}","$scope":{"a":{"$numberInt":"1"}}},"dbRef":{"$ref":"tests","$id":{"$numberInt":"1"},"$db":"test"},"decimal":{"$numberDecimal":"100"},"double":{"$numberDouble":"10.1"},"int32":{"$numberInt":"10"},"long":{"$numberLong":"200"},"maxKey":{"$maxKey":1},"minKey":{"$minKey":1},"objectId":{"$oid":"111111111111111111111111"},"objectID":{"$oid":"111111111111111111111111"},"oldObjectID":{"$oid":"111111111111111111111111"},"regexp":{"$regularExpression":{"pattern":"hello world","options":"i"}},"symbol":{"$symbol":"symbol"},"timestamp":{"$timestamp":{"t":0,"i":1000}},"int32Number":{"$numberInt":"300"},"doubleNumber":{"$numberDouble":"200.2"},"longNumberIntFit":{"$numberLong":"7036874417766400"},"doubleNumberIntFit":{"$numberLong":"19007199250000000"}}';
 
     expect(json).to.equal(EJSON.stringify(doc, null, 0, { relaxed: false }));
@@ -88,7 +87,7 @@ describe('Extended JSON', function () {
 
   it('should correctly deserialize using the default relaxed mode', function () {
     // Deserialize the document using non strict mode
-    var doc1 = EJSON.parse(EJSON.stringify(doc, null, 0));
+    let doc1 = EJSON.parse(EJSON.stringify(doc, null, 0));
 
     // Validate the values
     expect(300).to.equal(doc1.int32Number);
@@ -109,23 +108,23 @@ describe('Extended JSON', function () {
 
   it('should correctly serialize, and deserialize using built-in BSON', function () {
     // Create a doc
-    var doc1 = {
+    const doc1 = {
       int32: new Int32(10)
     };
 
     // Serialize the document
-    var text = EJSON.stringify(doc1, null, 0, { relaxed: false });
+    const text = EJSON.stringify(doc1, null, 0, { relaxed: false });
     expect(text).to.equal('{"int32":{"$numberInt":"10"}}');
 
     // Deserialize the json in strict and non strict mode
-    var doc2 = EJSON.parse(text, { relaxed: false });
+    let doc2 = EJSON.parse(text, { relaxed: false });
     expect(doc2.int32._bsontype).to.equal('Int32');
     doc2 = EJSON.parse(text);
     expect(doc2.int32).to.equal(10);
   });
 
   it('should correctly serialize bson types when they are values', function () {
-    var serialized = EJSON.stringify(new ObjectId('591801a468f9e7024b6235ea'), { relaxed: false });
+    let serialized = EJSON.stringify(new ObjectId('591801a468f9e7024b6235ea'), { relaxed: false });
     expect(serialized).to.equal('{"$oid":"591801a468f9e7024b6235ea"}');
     serialized = EJSON.stringify(new ObjectID('591801a468f9e7024b6235ea'), { relaxed: false });
     expect(serialized).to.equal('{"$oid":"591801a468f9e7024b6235ea"}');
@@ -183,8 +182,8 @@ describe('Extended JSON', function () {
     expect(EJSON.parse('null')).to.be.null;
     expect(EJSON.parse('[null]')[0]).to.be.null;
 
-    var input = '{"result":[{"_id":{"$oid":"591801a468f9e7024b623939"},"emptyField":null}]}';
-    var parsed = EJSON.parse(input);
+    const input = '{"result":[{"_id":{"$oid":"591801a468f9e7024b623939"},"emptyField":null}]}';
+    const parsed = EJSON.parse(input);
 
     expect(parsed).to.deep.equal({
       result: [{ _id: new ObjectId('591801a468f9e7024b623939'), emptyField: null }]
@@ -334,14 +333,14 @@ describe('Extended JSON', function () {
   it('should work for function-valued and array-valued replacer parameters', function () {
     const doc = { a: new Int32(10), b: new Int32(10) };
 
-    var replacerArray = ['a', '$numberInt'];
-    var serialized = EJSON.stringify(doc, replacerArray, 0, { relaxed: false });
+    const replacerArray = ['a', '$numberInt'];
+    let serialized = EJSON.stringify(doc, replacerArray, 0, { relaxed: false });
     expect(serialized).to.equal('{"a":{"$numberInt":"10"}}');
 
     serialized = EJSON.stringify(doc, replacerArray);
     expect(serialized).to.equal('{"a":10}');
 
-    var replacerFunc = function (key, value) {
+    const replacerFunc = function (key, value) {
       return key === 'b' ? undefined : value;
     };
     serialized = EJSON.stringify(doc, replacerFunc, 0, { relaxed: false });
@@ -352,11 +351,13 @@ describe('Extended JSON', function () {
   });
 
   if (!usingOldBSON) {
-    it.skip('skipping 4.x/1.x interop tests', () => {});
+    it.skip('skipping 4.x/1.x interop tests', () => {
+      // ignore
+    });
   } else {
     it('should interoperate 4.x with 1.x versions of this library', function () {
       const buffer = Buffer.alloc(64);
-      for (var i = 0; i < buffer.length; i++) {
+      for (let i = 0; i < buffer.length; i++) {
         buffer[i] = i;
       }
       const [oldBsonObject, newBsonObject] = [OldBSON, BSON].map(bsonModule => {
@@ -454,7 +455,9 @@ describe('Extended JSON', function () {
     // by mongodb-core, then remove this test case and uncomment the MinKey checks in the test case above
     it('should interop with MinKey 1.x and 4.x, except the case that #310 breaks', function () {
       if (!usingOldBSON) {
-        it.skip('interop tests', () => {});
+        it.skip('interop tests', () => {
+          // ignore
+        });
         return;
       }
 
@@ -516,7 +519,7 @@ describe('Extended JSON', function () {
     const serialized = EJSON.stringify(original);
     expect(serialized).to.equal('{"__proto__":{"a":42}}');
     const deserialized = EJSON.parse(serialized);
-    expect(deserialized).to.have.deep.ownPropertyDescriptor('__proto__', {
+    expect(deserialized).to.have.ownPropertyDescriptor('__proto__', {
       configurable: true,
       enumerable: true,
       writable: true,
@@ -527,7 +530,8 @@ describe('Extended JSON', function () {
 
   context('circular references', () => {
     it('should throw a helpful error message for input with circular references', function () {
-      const obj = {
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      const obj: any = {
         some: {
           property: {
             array: []
@@ -542,7 +546,8 @@ Converting circular structure to EJSON:
     });
 
     it('should throw a helpful error message for input with circular references, one-level nested', function () {
-      const obj = {};
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      const obj: any = {};
       obj.obj = obj;
       expect(() => EJSON.serialize(obj)).to.throw(`\
 Converting circular structure to EJSON:
@@ -551,7 +556,8 @@ Converting circular structure to EJSON:
     });
 
     it('should throw a helpful error message for input with circular references, one-level nested inside base object', function () {
-      const obj = {};
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      const obj: any = {};
       obj.obj = obj;
       expect(() => EJSON.serialize({ foo: obj })).to.throw(`\
 Converting circular structure to EJSON:
@@ -560,7 +566,8 @@ Converting circular structure to EJSON:
     });
 
     it('should throw a helpful error message for input with circular references, pointing back to base object', function () {
-      const obj = { foo: {} };
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      const obj: any = { foo: {} };
       obj.foo.obj = obj;
       expect(() => EJSON.serialize(obj)).to.throw(`\
 Converting circular structure to EJSON:
@@ -785,4 +792,13 @@ Converting circular structure to EJSON:
       expect(parsedUUID).to.deep.equal(expectedResult);
     });
   });
+
+  it('should only enumerate own property keys from input objects', () => {
+    const input = { a: 1 };
+    Object.setPrototypeOf(input, { b: 2 });
+    const string = EJSON.stringify(input);
+    expect(string).to.not.include(`"b":`);
+    const result = JSON.parse(string);
+    expect(result).to.deep.equal({ a: 1 });
+  });
 });
diff --git a/test/node/parser/calculate_size.test.ts b/test/node/parser/calculate_size.test.ts
new file mode 100644
index 00000000..0afda66c
--- /dev/null
+++ b/test/node/parser/calculate_size.test.ts
@@ -0,0 +1,10 @@
+import * as BSON from '../../register-bson';
+import { expect } from 'chai';
+
+describe('calculateSize()', () => {
+  it('should only enumerate own property keys from input objects', () => {
+    const input = { a: 1 };
+    Object.setPrototypeOf(input, { b: 2 });
+    expect(BSON.calculateObjectSize(input)).to.equal(12);
+  });
+});
diff --git a/test/node/parser/deserializer.test.ts b/test/node/parser/deserializer.test.ts
new file mode 100644
index 00000000..7f98690e
--- /dev/null
+++ b/test/node/parser/deserializer.test.ts
@@ -0,0 +1,18 @@
+import * as BSON from '../../register-bson';
+import { expect } from 'chai';
+
+describe('deserializer()', () => {
+  describe('when the fieldsAsRaw options is present and has a value that corresponds to a key in the object', () => {
+    it('ignores non-own properties set on the options object', () => {
+      const bytes = BSON.serialize({ someKey: [1] });
+      const options = { fieldsAsRaw: { someKey: true } };
+      Object.setPrototypeOf(options, { promoteValues: false });
+      const result = BSON.deserialize(bytes, options);
+      expect(result).to.have.property('someKey').that.is.an('array');
+      expect(
+        result.someKey[0],
+        'expected promoteValues option set on options object prototype to be ignored, but it was not'
+      ).to.not.have.property('_bsontype', 'Int32');
+    });
+  });
+});
diff --git a/test/node/parser/serializer.test.ts b/test/node/parser/serializer.test.ts
new file mode 100644
index 00000000..10b839ed
--- /dev/null
+++ b/test/node/parser/serializer.test.ts
@@ -0,0 +1,17 @@
+import * as BSON from '../../register-bson';
+import { bufferFromHexArray } from '../tools/utils';
+import { expect } from 'chai';
+
+describe('serialize()', () => {
+  it('should only enumerate own property keys from input objects', () => {
+    const input = { a: 1 };
+    Object.setPrototypeOf(input, { b: 2 });
+    const bytes = BSON.serialize(input);
+    expect(bytes).to.deep.equal(
+      bufferFromHexArray([
+        '106100', // type int32, a\x00
+        '01000000' // int32LE = 1
+      ])
+    );
+  });
+});