CVE-2026-33993CWE-1321

Locutus has Prototype Pollution via __proto__ Key Injection in unserialize()

Medium · published March 27, 2026

CVSS v4.0
6.9
EPSS
1%
Percentile
54.6
In the wild
Unconfirmed
What it is

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.

The record
Technical detail
CVSS v4.0
6.9 · MEDIUM
Vector
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N
EPSS
0.00754 · 54.6th percentile
Weakness
CWE-1321 · Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')
Published
2026-03-27T22:14Z
EPSS history
Timeline
  • 27 MAR 22:14Z
    Locutus has Prototype Pollution via __proto__ Key Injection in unserialize()
    cvelistv5