---
title: "GHSA-6w3j-5fw6-r9vr: joi: Prototype pollution via a `__proto__` language key in custom messages"
description: "Low severity. joi: Prototype pollution via a `__proto__` language key in custom messages. Affects joi; fixed in 18.2.5."
url: https://inlinr.com/advisories/GHSA-6w3j-5fw6-r9vr
lang: en
---

[Packages](https://inlinr.com/packages) / [joi](https://inlinr.com/packages/joi) / [Security](https://inlinr.com/packages/joi/security)

Low Vulnerability GHSA-6w3j-5fw6-r9vr CVE-2026-84368 Published 8 Sep 2026

# joi: Prototype pollution via a \`\_\_proto\_\_\` language key in custom messages

Your version 

Affected releases · [joi](https://inlinr.com/packages/joi)

- 18.x 18.0.0 – 18.2.4 Fixed in 18.2.5
- 17.x 17.2.0 – 17.13.5 Fixed in 17.13.6

On 18.x, the newest line:

```
npm install joi@18.2.5
```

Every release on a timeline

18.x 

1. Affected

18.0.0 – 18.2.4  
11 releases
2. Fixed

18.2.5 – 18.3.1  
7 releases, latest

17.x 

1. Not affected

17.1.1  
1 release
2. Affected

17.2.0 – 17.13.5  
38 releases
3. Fixed

17.13.6 – 17.13.9  
4 releases

256 stable releases of joi, oldest to newest, one row per major.

## What happens

Impact

An application that passes attacker-controlled data into joi's custom message configuration (`messages()`, `message()`, `prefs({ messages })`, `Joi.extend({ messages })` or `rule({ message })`) lets the attacker write properties onto `Object.prototype`, where every object in the process then inherits them. A key named `__proto__` was treated as a language name, and the code reused the object it found at that key, which resolves to the prototype rather than to a new own property; a key named `constructor` did the same to the `Object` function's statics. A consuming application that gates on the mere presence of a property (`if (user.isAdmin)`) can be made to take the wrong branch for every object it inspects.

This is not reachable from data that joi validates. Custom messages are schema-construction configuration, normally written by the application developer. Exploitation therefore requires an application that feeds untrusted input straight into schema construction.

Patches

Upgrade to version 18.2.5 or 17.13.6.

Workarounds

Do not pass untrusted input into `messages()`, `message()`, `prefs({ messages })`, `Joi.extend({ messages })` or `rule({ message })`. Or validate that they don't contain any `__proto__` or `constructor` property.

## How it is exploited

Exploitable over the network, with no account and no action from a user.

- **Confidentiality**: none
- **Integrity**: low
- **Availability**: none

CVSS:3.1/​AV:N/​AC:H/​PR:N/​UI:N/​S:U/​C:N/​I:L/​A:N

## References

- [Reference](https://github.com/hapijs/joi/security/advisories/GHSA-6w3j-5fw6-r9vr "https://github.com/hapijs/joi/security/advisories/GHSA-6w3j-5fw6-r9vr")  
github.com/hapijs/joi/security/advisories/GHSA-…
- [NVD](https://nvd.nist.gov/vuln/detail/CVE-2026-84368 "https://nvd.nist.gov/vuln/detail/CVE-2026-84368")  
nvd.nist.gov/vuln/detail/CVE-2026-84368
- [Pull request](https://github.com/hapijs/joi/pull/3138 "https://github.com/hapijs/joi/pull/3138")  
#3138
- [Pull request](https://github.com/hapijs/joi/pull/3139 "https://github.com/hapijs/joi/pull/3139")  
#3139
- [Fix commit](https://github.com/hapijs/joi/commit/8d0b808f3e874d28f9078f61b7742290989afb36 "https://github.com/hapijs/joi/commit/8d0b808f3e874d28f9078f61b7742290989afb36")  
8d0b808
- [Fix commit](https://github.com/hapijs/joi/commit/90d07571923c90e3432a328fc041d1cda03d30fa "https://github.com/hapijs/joi/commit/90d07571923c90e3432a328fc041d1cda03d30fa")  
90d0757
- [Package](https://github.com/hapijs/joi "https://github.com/hapijs/joi")  
github.com/hapijs/joi