Prototype Pollution in Locutus
CVE-2026-33994
Locutus brings stdlibs of other programming languages to JavaScript for educational purposes. Starting in version 2.0.39 and prior to version 3.0.25, a prototype pollution vulnerability exists in the `parse_str` function of the npm package locutus. An attacker can pollute `Object.prototype` by overriding `RegExp.prototype.test` and then passing a crafted query string to `parse_str`, bypassing the prototype pollution guard. This vulnerability stems from an incomplete fix for CVE-2026-25521. The CVE-2026-25521 patch replaced the `String.prototype.includes()`-based guard with a `RegExp.prototype.test()`-based guard. However, `RegExp.prototype.test` is itself a writable prototype method that can be overridden, making the new guard bypassable in the same way as the original — trading one hijackable built-in for another. Version 3.0.25 contains an updated fix.
Vulnerability class: Prototype Pollution
Published · last modified .
CVSS v3 metric
CVSS v3 base score 9.8 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H.
EPSS exploit prediction
EPSS: 0.008 (53.4th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 53rd percentile — 53.4% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.006 · newest: 0.008 · change: +0.002
Affected products
- Locutus
- Locutusjs Locutus — versions >= 2.0.39, < 3.0.25
Weakness classification (CWE)
References
- github.com/locutusjs/locutus/security/advisories/GHSA-vc8f-x9pp-wf5p (Exploit, Vendor Advisory)
- github.com/locutusjs/locutus/pull/597 (Issue Tracking)
- github.com/locutusjs/locutus/commit/345a6211e1e6f939f96a7090bfeff642c9fcf9e4 (Patch)
- github.com/locutusjs/locutus/releases/tag/v3.0.25 (Release Notes)
Frequently asked questions
- What is CVE-2026-33994?
- CVE-2026-33994 is a critical-severity vulnerability in Locutus, classified under Improperly Controlled Modification of Object Prototype Attributes (Prototype Pollution). CVSS score: 9.8/10. Published 2026-03-27.
- How severe is CVE-2026-33994?
- Critical severity. CVSS v3 base score is 9.8 out of 10.