diff --git a/.size-limit.js b/.size-limit.js
index f7763abbdf..8a1527a7fc 100644
--- a/.size-limit.js
+++ b/.size-limit.js
@@ -4,21 +4,21 @@ module.exports = [
 	{
 		name: 'CJS',
 		path: 'dist/lightweight-charts.production.cjs',
-		limit: '49.32 KB',
+		limit: '49.37 KB',
 	},
 	{
 		name: 'ESM',
 		path: 'dist/lightweight-charts.production.mjs',
-		limit: '49.25 KB',
+		limit: '49.29 KB',
 	},
 	{
 		name: 'Standalone-ESM',
 		path: 'dist/lightweight-charts.standalone.production.mjs',
-		limit: '50.97 KB',
+		limit: '51.02 KB',
 	},
 	{
 		name: 'Standalone',
 		path: 'dist/lightweight-charts.standalone.production.js',
-		limit: '51.00 KB',
+		limit: '51.06 KB',
 	},
 ];
diff --git a/src/helpers/strict-type-checks.ts b/src/helpers/strict-type-checks.ts
index 7832fa980a..40f3882cba 100644
--- a/src/helpers/strict-type-checks.ts
+++ b/src/helpers/strict-type-checks.ts
@@ -14,7 +14,11 @@ export function merge(dst: Record<string, any>, ...sources: Record<string, any>[
 	for (const src of sources) {
 		// eslint-disable-next-line no-restricted-syntax
 		for (const i in src) {
-			if (src[i] === undefined) {
+			if (
+				src[i] === undefined ||
+				!Object.prototype.hasOwnProperty.call(src, i) ||
+				['__proto__', 'constructor', 'prototype'].includes(i)
+			) {
 				continue;
 			}
 
diff --git a/tests/unittests/helpers.spec.ts b/tests/unittests/helpers.spec.ts
new file mode 100644
index 0000000000..8a2b79642a
--- /dev/null
+++ b/tests/unittests/helpers.spec.ts
@@ -0,0 +1,173 @@
+import { expect } from 'chai';
+import { describe, it } from 'mocha';
+
+import { clone, merge } from '../../src/helpers/strict-type-checks';
+
+describe('Helpers', () => {
+	/* eslint-disable
+		@typescript-eslint/unbound-method,
+		@typescript-eslint/no-explicit-any,
+		@typescript-eslint/no-unsafe-member-access,
+		@typescript-eslint/no-unused-expressions,
+	*/
+	describe('merge', () => {
+		it('should perform deep merge of objects', () => {
+			const dst = {
+				a: 1,
+				b: {
+					c: 2,
+					d: {
+						e: 3,
+					},
+				},
+			};
+
+			const src = {
+				b: {
+					d: {
+						f: 4,
+					},
+				},
+				g: 5,
+			};
+
+			const result = merge(dst, src);
+
+			expect(result).to.deep.equal({
+				a: 1,
+				b: {
+					c: 2,
+					d: {
+						e: 3,
+						f: 4,
+					},
+				},
+				g: 5,
+			});
+		});
+
+		it('should handle arrays correctly', () => {
+			const dst = { arr: [1, 2, 3] };
+			const src = { arr: [4, 5, 6] };
+
+			const result = merge(dst, src);
+			expect(result.arr).to.deep.equal([4, 5, 6]);
+		});
+
+		it('should handle undefined values', () => {
+			const dst = { a: 1, b: 2 };
+			const src = { b: undefined, c: 3 };
+
+			const result = merge(dst, src);
+			expect(result).to.deep.equal({ a: 1, b: 2, c: 3 });
+		});
+
+		it('should protect against prototype pollution', () => {
+			const originalProto = Object.prototype.toString;
+			const maliciousPayload = JSON.parse(
+				'{"__proto__": {"polluted": true}}'
+			) as Record<string, unknown>;
+
+			const dst = { legitimate: 'data' };
+			merge(dst, maliciousPayload);
+
+			// Check if prototype was not polluted
+			expect(({} as any).polluted).to.be.undefined;
+			expect((Object.prototype as any).polluted).to.be.undefined;
+			expect(Object.prototype.toString).to.equal(originalProto);
+		});
+
+		it('should handle multiple sources', () => {
+			const dst = { a: 1 };
+			const src1 = { b: 2 };
+			const src2 = { c: 3 };
+
+			const result = merge(dst, src1, src2);
+			expect(result).to.deep.equal({ a: 1, b: 2, c: 3 });
+		});
+
+		it('should protect against constructor pollution', () => {
+			const dst = {};
+			const malicious = JSON.parse('{ "constructor": { "prototype": { "polluted": true } } }') as Record<string, unknown>;
+
+			merge(dst, malicious);
+
+			expect(({} as any).polluted).to.be.undefined;
+			expect((Object.prototype as any).polluted).to.be.undefined;
+		});
+
+		it('should protect against nested prototype pollution', () => {
+			const dst = { nested: {} };
+			const malicious = JSON.parse('{"nested":{"__proto__": { "polluted": true }}}') as Record<string, unknown>;
+
+			merge(dst, malicious);
+
+			expect(({} as any).polluted).to.be.undefined;
+			expect((Object.prototype as any).polluted).to.be.undefined;
+		});
+
+		it('should handle circular references safely', () => {
+			const dst: Record<string, unknown> = { a: 1 };
+			dst.circular = dst;
+
+			const src = { b: 2 };
+
+			const result = merge(dst, src);
+			expect(result.b).to.equal(2);
+			expect(result.circular).to.equal(result);
+		});
+	});
+
+	describe('clone', () => {
+		it('should deep clone objects', () => {
+			const original = {
+				a: 1,
+				b: {
+					c: 2,
+					d: [1, 2, { e: 3 }],
+				},
+			};
+
+			const cloned = clone(original);
+
+			expect(cloned).to.deep.equal(original);
+			expect(cloned).to.not.equal(original);
+			expect(cloned.b).to.not.equal(original.b);
+			expect(cloned.b.d).to.not.equal(original.b.d);
+		});
+
+		it('should handle primitive values', () => {
+			expect(clone(42)).to.equal(42);
+			expect(clone('string')).to.equal('string');
+			expect(clone(null)).to.equal(null);
+			expect(clone(undefined)).to.equal(undefined);
+		});
+
+		it('should protect against __proto__ chain climbing', () => {
+			const malicious = JSON.parse('{"__proto__": {"__proto__": {"polluted": true}}}');
+
+			clone(malicious);
+
+			expect(({} as any).polluted).to.be.undefined;
+			expect((Object.prototype as any).polluted).to.be.undefined;
+		});
+
+		it('should protect against prototype pollution', () => {
+			const originalProto = Object.prototype.toString;
+			const maliciousObject = JSON.parse('{ "__proto__": { "polluted": true }, "value": 1234 }');
+
+			const cloned = clone(maliciousObject);
+
+			expect(cloned.value).to.equal(1234);
+			expect(({} as any).polluted).to.be.undefined;
+			expect((Object.prototype as any).polluted).to.be.undefined;
+			expect(Object.prototype.toString).to.equal(originalProto);
+		});
+	});
+	/* eslint-enable
+		@typescript-eslint/unbound-method,
+		@typescript-eslint/no-explicit-any,
+		@typescript-eslint/no-unsafe-member-access,
+		@typescript-eslint/no-unused-expressions,
+	*/
+});