LiquidJS is a Shopify / GitHub Pages compatible template engine in pure JavaScript. Prior to 10.27.2, enabling ownPropertyOnly does not consistently restrict inherited array indices because negative indexing, .first, .last, the first filter, the last filter, join, reverse, slice, compact, and for-loop iteration can read prototype-provided elements outside readJSProperty(). An attacker who can influence prototype state or inherited array-index data and cause templates to render affected operations can disclose values that ownPropertyOnly is expected to hide. Direct positive indexing and ordinary object prototype reads are blocked, but the alternate array paths remain affected. This issue is fixed in 10.27.2.

Affected: npm: liquidjs

## Summary

LiquidJS’s `ownPropertyOnly` option is intended to prevent templates from reading inherited or prototype properties from scope data. With `ownPropertyOnly: true`, direct access such as:

```liquid {{ a[0] }} ```

correctly blocks an inherited array index. However, the same inherited value is still disclosed through:

```text .first .last negative indexing for-loop iteration first last join reverse slice compact ```

## Impact

This is an information disclosure issue when an application relies on `ownPropertyOnly: true` to safely render templates over untrusted or prototype-polluted scope data.

Object prototype property reads are blocked as expected, but inherited `Array.prototype` index reads are not consistently blocked. An attacker who can influence prototype state or inherited array-index data may cause templates to disclose values that `ownPropertyOnly` is expected to hide.

## Proof of concept

```js const { Liquid } = require("liquidjs");

const engine = new Liquid({ ownPropertyOnly: true });

Array.prototype[0] = "ARRAY_PROTO_POLLUTED"; Object.prototype.secret = "OBJECT_PROTO_POLLUTED";

const a = []; a.length = 1;

const o = {};

for (const [src, scope] of [ ["{{ a[0] }}", { a }], ["{{ a[-1] }}", { a }], ["{{ o.secret }}", { o }], ["{{ a.first }}", { a }], ["{{ a.last }}", { a }], ["{{ a | first }}", { a }], ["{{ a | last }}", { a }], ["{{ a | join: ',' }}", { a }], ["{{ a | reverse | first }}", { a }], ["{{ a | slice: 0, 1 | join:…