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

> Attackers can cause denial of service and property injection via prototype pollution in the unserialize function of the Locutus JavaScript library.

- **Severity:** Medium 6.9 (`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`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 27 Mar 2026
- **Updated:** 30 Mar 2026
- **Also known as:** GHSA-4mph-v827-f877

## What is CVE-2026-33993?

CVE-2026-33993 is a medium-severity unsafe deserialization vulnerability in locutusjs locutus, published on 27 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-1321 (Improperly Controlled Modification of Object Prototype Attributes)

## Which versions are affected by CVE-2026-33993?

2 open source packages ship an affected version, including locutus. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `locutus` | npm | All versions | 3.0.25 |
| `locutus` | npm | All versions | 3.0.25 |

Products: Locutus Locutus.

## How do I fix CVE-2026-33993?

Upgrade locutus to 3.0.25 or later.

## Is CVE-2026-33993 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33993 exists.

## Other vulnerabilities in locutusjs locutus

- [CVE-2026-32304](https://insights.sbomify.com/CVE-2026-32304/): Locutus: RCE via unsanitized input in create_function() (critical 9.8)
- [CVE-2020-7719](https://insights.sbomify.com/CVE-2020-7719/): Prototype Pollution (critical 9.8)
- [CVE-2026-25521](https://insights.sbomify.com/CVE-2026-25521/): Locutus is vulnerable to Prototype Pollution (critical 9.4)
- [CVE-2026-29091](https://insights.sbomify.com/CVE-2026-29091/): Locutus: Remote Code Execution (RCE) in locutus call_user_func_array due to Code Injection (high 8.1)
- [CVE-2026-33994](https://insights.sbomify.com/CVE-2026-33994/): Locutus Prototype Pollution due to incomplete fix for CVE-2026-25521 (medium 6.3)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-33993 and fix it if it is affected.

What it is: CVE-2026-33993 is a medium-severity unsafe deserialization vulnerability in locutusjs locutus, published on 27 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33993 exists.

Affected packages and fixed versions:
- locutus (npm): affected all versions; fixed in 3.0.25
- locutus (npm): affected all versions; fixed in 3.0.25

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-33993/ (Markdown: https://insights.sbomify.com/CVE-2026-33993/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-33993
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-33993
- OSV: https://osv.dev/vulnerability/CVE-2026-33993

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-33993/
Summaries are generated and may contain mistakes; the official description above is the record.