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.