# CVE-2026-122131: Joi 17.13.6 before 17.13.9 and 18.2.5 before 18.3.1 Prototype Replacement via __proto__ Language Key


- **Severity:** Medium 6.3 (`CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Fix available:** No
- **Published:** 4 Nov 2026
- **Updated:** 5 Nov 2026
- **Also known as:** GHSA-gw37-7cc7-xmm6

## What is CVE-2026-122131?

CVE-2026-122131 is a medium-severity vulnerability in hapijs joi, published on 4 Nov 2026. 

### Official description

Joi 17.13.6 before 17.13.9 and 18.2.5 before 18.3.1 contains a prototype pollution vulnerability in lib/messages.js compile() and merge() that fails to reject a __proto__ language key. Attackers supplying untrusted message configuration via any().messages(), prefs, validate options or Joi.extend can replace the compiled messages prototype, altering or breaking validation error rendering and dependent logic.

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

## Which versions are affected by CVE-2026-122131?


## How do I fix CVE-2026-122131?

No fix has been published for CVE-2026-122131 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-122131 being exploited?

There is no known exploitation of CVE-2026-122131 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-122131 and fix it if it is affected.

What it is: CVE-2026-122131 is a medium-severity vulnerability in hapijs joi, published on 4 Nov 2026. 
Exploitation: There is no known exploitation of CVE-2026-122131 in the wild and no public exploit code.

Affected product: hapijs joi. No package-level version data is available, so check the vendor advisory linked from the reference page.

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-2026-122131/ (Markdown: https://insights.sbomify.com/CVE-2026-122131/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-122131
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-122131
- OSV: https://osv.dev/vulnerability/CVE-2026-122131

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-122131/
Summaries are generated and may contain mistakes; the official description above is the record.