diff --git a/common/changes/@rushstack/node-core-library/mojazayeri-prevent-merge-prototype-pollution_2026-08-28-23-45.json b/common/changes/@rushstack/node-core-library/mojazayeri-prevent-merge-prototype-pollution_2026-08-28-23-45.json
new file mode 100644
index 0000000000..ee2855218a
--- /dev/null
+++ b/common/changes/@rushstack/node-core-library/mojazayeri-prevent-merge-prototype-pollution_2026-08-28-23-45.json
@@ -0,0 +1,11 @@
+{
+  "changes": [
+    {
+      "packageName": "@rushstack/node-core-library",
+      "comment": "Prevent prototype pollution when Objects.mergeWith() processes attacker-controlled properties.",
+      "type": "patch"
+    }
+  ],
+  "packageName": "@rushstack/node-core-library",
+  "email": "mojazayeri@users.noreply.github.com"
+}
diff --git a/libraries/node-core-library/src/objects/mergeWith.ts b/libraries/node-core-library/src/objects/mergeWith.ts
index c31d8fcdde..cfb611847c 100644
--- a/libraries/node-core-library/src/objects/mergeWith.ts
+++ b/libraries/node-core-library/src/objects/mergeWith.ts
@@ -30,16 +30,31 @@ export function mergeWith<TTarget extends object, TSource extends object>(
   const targetRecord: Record<string, unknown> = target as unknown as Record<string, unknown>;
   const sourceRecord: Record<string, unknown> = source as unknown as Record<string, unknown>;
   for (const [key, srcValue] of Object.entries(sourceRecord)) {
-    const objValue: unknown = targetRecord[key];
+    const objValue: unknown = Object.hasOwnProperty.call(targetRecord, key)
+      ? targetRecord[key]
+      : undefined;
     const customized: unknown = customizer?.(objValue, srcValue, key);
     if (customized !== undefined) {
-      targetRecord[key] = customized;
+      _setProperty(targetRecord, key, customized);
     } else if (isRecord(srcValue) && isRecord(objValue)) {
       mergeWith(objValue, srcValue, customizer);
     } else {
-      targetRecord[key] = srcValue;
+      _setProperty(targetRecord, key, srcValue);
     }
   }
 
   return target;
 }
+
+function _setProperty(target: Record<string, unknown>, key: string, value: unknown): void {
+  if (key === '__proto__') {
+    Object.defineProperty(target, key, {
+      configurable: true,
+      enumerable: true,
+      value,
+      writable: true
+    });
+  } else {
+    target[key] = value;
+  }
+}
diff --git a/libraries/node-core-library/src/objects/test/mergeWith.test.ts b/libraries/node-core-library/src/objects/test/mergeWith.test.ts
index 333e2a4615..dd1a43e854 100644
--- a/libraries/node-core-library/src/objects/test/mergeWith.test.ts
+++ b/libraries/node-core-library/src/objects/test/mergeWith.test.ts
@@ -72,6 +72,28 @@ describe(mergeWith.name, () => {
       mergeWith(target, { a: 1 });
       expect(target).toEqual({ a: 1 });
     });
+
+    it('does not merge into inherited target properties', () => {
+      const prototype: { settings: Record<string, unknown> } = { settings: { inherited: true } };
+      const target: Record<string, unknown> = Object.create(prototype);
+
+      mergeWith(target, { settings: { own: true } });
+
+      expect(prototype.settings).toEqual({ inherited: true });
+      expect(target.settings).toEqual({ own: true });
+      expect(Object.hasOwnProperty.call(target, 'settings')).toBe(true);
+    });
+
+    it('does not pollute Object.prototype through __proto__', () => {
+      const source: Record<string, unknown> = JSON.parse('{"__proto__":{"polluted":true}}');
+      const target: Record<string, unknown> = {};
+
+      mergeWith(target, source);
+
+      expect((Object.prototype as Record<string, unknown>).polluted).toBeUndefined();
+      expect(Object.hasOwnProperty.call(target, '__proto__')).toBe(true);
+      expect(target.__proto__).toEqual({ polluted: true });
+    });
   });
 
   describe('customizer behavior', () => {