Locutus brings stdlibs of other programming languages to JavaScript for educational purposes. Prior to version 3.0.25, the "unserialize()" function in "locutus/php/var/unserialize" assigns deserialized keys to plain objects via bracket notation without filtering the "__proto__" key. When a PHP serialized payload contains "__proto__" as an array or object key, JavaScript's "__proto__" setter is invoked, replacing the deserialized object's prototype with attacker-controlled content. This enables property injection, for...in propagation of injected properties, and denial of service via built-in method override. This is distinct from the previously reported prototype pollution in "parse_str" (GHSA-f98m-q3hr-p5wq, GHSA-rxrv-835q-v5mh) — "unserialize" is a different function with no mitigation applied. Version 3.0.25 patches the issue.