What’s Vulnerable › Vulnerabilities

CVE-2026-84368: joi: Prototype pollution via a `__proto__` language key in custom messages

• Severity: low (CVSS 3.7 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.4% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-09-01 · updated 2026-09-09
• Weakness: CWE-1321

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.

Affected products

• hapijs joi: from 16.0.0, <= 17.1.1; from 17.2.0, < 17.13.6; from 18.0.0, < 18.2.5

References

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

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