CVE-2026-84368

joi: Prototype pollution via a proto language key in custom messages

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.

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.

CVE-2026-84368 has a CVSS score of 3.7 (Low). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (17.13.6, 18.2.5); upgrading removes the vulnerable code path.

Kodem intelligence

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Upgrade to version 18.2.5 or 17.13.6.

1. What is CVE-2026-84368? CVE-2026-84368 is a low-severity security vulnerability in joi (npm), affecting versions >= 17.2.0, < 17.13.6. It is fixed in 17.13.6, 18.2.5.
2. How severe is CVE-2026-84368? CVE-2026-84368 has a CVSS score of 3.7 (Low). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which packages are affected by CVE-2026-84368?
   • joi (npm) (versions >= 17.2.0, < 17.13.6)
   • @hapi/joi (npm) (versions >= 16.0.0, <= 17.1.1)
4. Is there a fix for CVE-2026-84368? Yes. CVE-2026-84368 is fixed in 17.13.6, 18.2.5. Upgrade to this version or later.
5. Is CVE-2026-84368 exploitable, and should I be worried? Whether CVE-2026-84368 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-84368 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-84368?
   • Upgrade joi to 17.13.6 or later
   • Upgrade joi to 18.2.5 or later