# CVE-2026-84428: fastify vulnerable to header validation bypass via incomplete schema case normalization

> An unauthenticated attacker can bypass conditional header validation checks in Fastify by exploiting incomplete case normalization in JSON Schema dependencies.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 4 Sep 2026
- **Updated:** 4 Sep 2026
- **Also known as:** CLSA-2026-1791634874, CLSA-2026-1792242589, GHSA-9q9j-q6p8-xq58, MINI-chf9-9mrc-3rhf

## What is CVE-2026-84428?

CVE-2026-84428 is a high-severity authentication bypass vulnerability in fastify, published on 4 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

fastify versions before 5.12.2 implement the case-insensitive nature of HTTP header names by lowercasing names in a route's header schema before compiling it, but the transformation is incomplete: it lowercases the properties keys and the root-level required array, and does not lowercase the trigger and dependent names inside the JSON Schema Draft 7 dependencies keyword. Because Node stores request header names in lowercase, a canonical-case dependency such as requiring an authentication header whenever a privileged-mode header is present never matches, and the presence assertion is silently skipped. An unauthenticated remote client can therefore send the header that activates a privileged branch while omitting the header the dependency was meant to require, bypassing the conditional check. Users should upgrade to fastify 5.12.2 or later.

Weakness: CWE-178 (Improper Handling of Case Sensitivity)

## Which versions are affected by CVE-2026-84428?

2 open source packages ship an affected version, including fastify. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `fastify` | npm | All versions | 5.12.2 |
| `fastify` | npm | All versions | 5.12.2 |

Products: Fastify Fastify.

## How do I fix CVE-2026-84428?

Upgrade fastify to 5.12.2 or later.

## Is CVE-2026-84428 being exploited?

There is no known exploitation of CVE-2026-84428 in the wild and no public exploit code.

## Other vulnerabilities in fastify

- [CVE-2026-84504](https://insights.sbomify.com/CVE-2026-84504/): fastify vulnerable to request body replacement via an async validation result collision (high 8.1)
- [CVE-2026-76169](https://insights.sbomify.com/CVE-2026-76169/): fastify vulnerable to authentication bypass via malformed URLs reaching encapsulated not-found handlers (high 7.5)
- [CVE-2026-84469](https://insights.sbomify.com/CVE-2026-84469/): fastify vulnerable to request validation bypass via skipped boolean false schemas (high 7.5)
- [CVE-2026-33806](https://insights.sbomify.com/CVE-2026-33806/): fastify vulnerable to Body Schema Validation Bypass via Leading Space in Content-Type Header (high 7.5)
- [CVE-2026-25223](https://insights.sbomify.com/CVE-2026-25223/): Fastify's Content-Type header tab character allows body validation bypass (high 7.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-84428 and fix it if it is affected.

What it is: CVE-2026-84428 is a high-severity authentication bypass vulnerability in fastify, published on 4 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-84428 in the wild and no public exploit code.

Affected packages and fixed versions:
- fastify (npm): affected all versions; fixed in 5.12.2
- fastify (npm): affected all versions; fixed in 5.12.2

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-2026-84428/ (Markdown: https://insights.sbomify.com/CVE-2026-84428/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-84428
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-84428
- OSV: https://osv.dev/vulnerability/CVE-2026-84428

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-84428/
Summaries are generated and may contain mistakes; the official description above is the record.