Vulnerability · Published Mar 27, 2026 · Assigned by GitHub_M
CVE-2026-33994Locutus Prototype Pollution due to incomplete fix for CVE-2026-25521
Critical 9.8
In plain English
A critical-severity prototype pollution in locutus from locutusjs. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.6% (higher than 47% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official description
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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| npm | locutus | 3.0.25 | GHSA-vc8f-x9pp-wf5p |
Affected products (as reported)
Weakness: CWE-1321 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
References
- https://github.com/locutusjs/locutus/security/advisories/GHSA-vc8f-x9pp-wf5p x_refsource_CONFIRM
- https://github.com/locutusjs/locutus/pull/597 x_refsource_MISC
- https://github.com/locutusjs/locutus/commit/345a6211e1e6f939f96a7090bfeff642c9fcf9e4 x_refsource_MISC
- https://github.com/locutusjs/locutus/releases/tag/v3.0.25 x_refsource_MISC