# 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

Status: published. Published: 2026-11-04. Updated: 2026-11-04.

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.

## Remediation

- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: CNA: VulnCheck) — https://github.com/hapijs/joi/commit/76f5771e1afe527cfda24be0e1a920ffa54888d6
- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: CNA: VulnCheck) — https://github.com/hapijs/joi/commit/dae9467cec27e787bfdea8b276595177d52d28ec
- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: CNA: VulnCheck) — https://github.com/hapijs/joi/pull/3164

## Affected products and versions

- hapijs / joi (joi): ≥ 17.13.6; < 17.13.9 (affected, semver); ≥ 18.2.5; < 18.3.1 (affected, semver). Source: CNA: VulnCheck

## Severity and prioritization

- CVSS 4.0: 6.3 (medium); 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; source: CNA: VulnCheck

- CVSS 3.1: 3.7 (low); CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N; source: CNA: VulnCheck

- CISA KEV: No listing in collected sources

- EPSS: Not available

- SSVC: Not available

- Weaknesses: CWE-1321

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://github.com/hapijs/joi/commit/76f5771e1afe527cfda24be0e1a920ffa54888d6 (CNA: VulnCheck; patch)

- https://github.com/hapijs/joi (CNA: VulnCheck; product)

- https://github.com/hapijs/joi/commit/dae9467cec27e787bfdea8b276595177d52d28ec (CNA: VulnCheck; patch)

- https://github.com/hapijs/joi/pull/3164 (CNA: VulnCheck; patch, issue-tracking)

- https://github.com/hapijs/joi/security/advisories/GHSA-6w3j-5fw6-r9vr (CNA: VulnCheck; related, vendor-advisory)

- https://github.com/hapijs/joi/blob/v18.3.0/lib/messages.js#L64-L67 (CNA: VulnCheck; technical-description)

- 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 (CNA: VulnCheck; third-party-advisory)

## Provenance

- CNA: VulnCheck: https://www.cve.org/CVERecord?id=CVE-2026-122131; retrieved 2026-11-04; source updated 2026-11-04

Revision: a8e1f05a7fa8878c9f7280f9999c48c093a17cff2a14b1d94f2c3fc603387c74

Canonical: https://y2.dev/cve/CVE-2026-122131

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