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

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

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

EPSS

No dated EPSS signal available.

As published by the source; long descriptions are shortened.

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…

Published by the source

3 Nov 2026, 17:53 UTC

Last modified by the source

3 Nov 2026, 18:02 UTC

Source last checked

4 Nov 2026, 06:10 UTC

1. Revision 2 · amended · 4 Nov 2026

   Amended from osv record GHSA-33r5-xmm8-v6x5

2. Revision 1 · created · 3 Nov 2026

   Published from ghsa record GHSA-33r5-xmm8-v6x5

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# Patch Next advisory brief: CVE-2026-44267

Advisory: Elo JavaScript emitter has sandbox escape to arbitrary Node.js / browser code execution via unfiltered `.constructor` member access
IDs: CVE-2026-44267, GHSA-33r5-xmm8-v6x5
Record: https://patchnext.com/advisories/cve-2026-44267 (revision 2, source checked 4 Nov 2026, 06:10 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Critical (CVSS 9.8)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: @enspirit/elo 1.0.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm @enspirit/elo: affected < 1.0.1; fixed in 1.0.1

## Suggested inspection (read-only)
- `npm ls @enspirit/elo --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on @enspirit/elo.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-33r5-xmm8-v6x5 (GitHub-reviewed advisory GHSA-33r5-xmm8-v6x5 lists affected ranges: npm @enspirit/elo >=0 <=1.0.0)
- Verified: https://osv.dev/vulnerability/GHSA-33r5-xmm8-v6x5 (OSV record GHSA-33r5-xmm8-v6x5 lists affected ranges: npm @enspirit/elo >=0 <1.0.1 (fixed))

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