critical CVSS 9.8

CVE-2026-33993·PHP vulnerability

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.

Severity
critical
Software
PHP
Fixed in
3.0.25
Published
2026-03-27

Affected versions

Until: 3.0.25

Fixed in: 3.0.25

How to fix this CVE

Update PHP and the Locutus library to version 3.0.25 or later to patch the prototype pollution vulnerability in the unserialize() function. This vulnerability allows attackers to inject malicious properties into deserialized objects by exploiting the __proto__ key, potentially leading to denial of service or property injection attacks. Ensure all systems running affected versions are patched immediately.

sudo dnf update php

Defensia detects this vulnerability

How to check if you are affected

  1. Step 1: Check the installed PHP version by running `php -v` and verify it is 3.0.25 or later if using Locutus library
  2. Step 2: Identify if Locutus is installed in your PHP environment by checking `composer show | grep locutus` or searching for locutus in your vendor directory
  3. Step 3: Review application logs for unusual object deserialization patterns or unexpected prototype chain modifications that could indicate exploitation attempts
  4. Step 4: Verify the fix by confirming the Locutus version in composer.lock shows `"version": "v3.0.25"` or higher

FAQ

What is CVE-2026-33993?

CVE-2026-33993 is a critical prototype pollution vulnerability in the Locutus library's unserialize() function that fails to filter the __proto__ key during object deserialization. This allows attackers to inject arbitrary properties into JavaScript object prototypes, potentially causing denial of service or enabling further exploitation.

Is CVE-2026-33993 being actively exploited?

No, CVE-2026-33993 is not currently listed on the CISA Known Exploited Vulnerabilities (KEV) catalog and no public exploits have been disclosed, though organizations should still prioritize patching given the critical CVSS 9.8 severity score.

What versions of PHP are affected by CVE-2026-33993?

All versions of Locutus prior to 3.0.25 are affected by this vulnerability. If you are using Locutus integrated with PHP, ensure both are updated to the patched versions.

How do I check if my server is vulnerable to CVE-2026-33993?

Run `composer show locutus/locutus` to check the installed version, or examine your composer.lock file for the Locutus version. If it is below 3.0.25, your system is vulnerable.

Does Defensia detect CVE-2026-33993?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Locutus or PHP is installed on a monitored server, CVE-2026-33993 will appear in your dashboard with remediation steps.

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References

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