What’s Vulnerable › Vulnerabilities

CVE-2026-33993: Locutus has Prototype Pollution via __proto__ Key Injection in unserialize()

• Severity: critical (CVSS 9.8 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.6% probability of exploitation in the next 30 days
• Public exploit or detection template: yes
• Published: 2026-03-27 · updated 2026-10-26
• Weakness: CWE-1321

Description

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.

Affected products

• locutusjs locutus: before 3.0.25 — fixed in 3.0.25

How to fix

Upgrade to a fixed version: locutusjs locutus 3.0.25. Check the vendor advisory for mitigations.

References

• https://github.com/locutusjs/locutus/commit/345a6211e1e6f939f96a7090bfeff642c9fcf9e4
• https://github.com/locutusjs/locutus/pull/597
• https://github.com/locutusjs/locutus/releases/tag/v3.0.25
• https://github.com/locutusjs/locutus/security/advisories/GHSA-4mph-v827-f877

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