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EUVD-2026-278435

fkie_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.

GHSA-vc8f-x9pp-wf5p — Locutus Prototype Pollution due to incomplete fix for CVE-2026-25521

Affected: npm: locutus

## Summary

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](https://github.com/locutusjs/locutus/security/advisories/GHSA-rxrv-835q-v5mh). 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.

## Package

locutus (npm)

## Affected versions

>= 2.0.39, <= 3.0.24

Tested and confirmed vulnerable on **2.0.39** and **3.0.24** (latest). Version 2.0.38 (pre-fix) uses a different guard (`String.prototype.includes`) and is not affected by this specific bypass.

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## Description

### Details

The vulnerability resides in `parse_str.js` where the `RegExp.prototype.test()` function is used to check whether user-provided input contains forbidden keys:

```javascript if (/__proto__|constructor|prototype/.test(key)) { break } ```

The previous guard (fixed in CVE-2026-25521) used `String.prototype.includes()`:

```javascript if (key.includes('__proto__')) { break } ```

The CVE-2026-25521 fix correctly identified…