CVE-2026-44267: Elo JavaScript emitter has sandbox escape to arbitrary Node.js / browser code execution via unfiltered `.constructor` member access

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• No confirmed exploitation. It is not in CISA’s KEV catalog. That can change quickly after a public write-up; re-check if you are affected.
• EPSS 0.8%, percentile 58: FIRST’s model estimates a 0.8% chance of exploitation activity in the next 30 days, higher than 58% of all scored CVEs (moderate). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Only applies if your dependency tree includes @enspirit/elo (npm) <= 1.0.0, fixed in 1.0.1. Check your lockfile, including transitive dependencies.

What it is

Elo is a purely functional expression language for no-code tools. Prior to 1.0.1, plain Elo strings supplied to defaultFormats.elo.parse(), bin/elo, or elo-lang.org/try can use .constructor.constructor to reach JavaScript Function because the memberaccess handler in src/transform.ts does not reject meta-properties, src/compilers/javascript.ts emits the surviving property directly, and src/parser.ts accepts constructor as an ordinary identifier. The resulting generated JavaScript executes attacker-selected code in the host runtime. This permits browser script execution and session compromise on the auto-running playground and arbitrary Node.js code execution on library, CI, or DevOps hosts. This plain-string path is distinct from the programmatic AST-object injection issue. This issue is fixed in version 1.0.1.

From the GitHub advisory

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 memberaccess 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.

Weakness type: CWE-79, CWE-94, CWE-693, CWE-1336.

Product: @enspirit/elo (npm) · Assigned by: GitHub_M · Published: 2026-11-03

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Privileges required: none (no login)
• User interaction: none
• Scope: unchanged

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• FIRST EPSS (model of 2026-11-04): 0.83% probability of exploitation activity in the next 30 days; percentile 58.

Timeline

• 2026-11-03 — CVE record last updated
• 2026-11-03 — CVE record published by GitHub_M
• 2026-11-03 — GitHub-reviewed advisory GHSA-33r5-xmm8-v6x5 published (critical)

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-33r5-xmm8-v6x5 github.com
2. CVE.org record CVE-2026-44267 cve.org
3. NVD entry CVE-2026-44267 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com
3. github.com github.com

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