# CVE-2026-33806: fastify vulnerable to Body Schema Validation Bypass via Leading Space in Content-Type Header

> Attackers can bypass body schema validation in Fastify applications by adding a leading space to the Content-Type header, potentially allowing invalid data processing.

- **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:** 15 Apr 2026
- **Updated:** 15 Jul 2026
- **Also known as:** MINI-j3pc-8mch-536r

## What is CVE-2026-33806?

CVE-2026-33806 is a high-severity vulnerability in fastify, published on 15 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Impact:

Fastify applications using schema.body.content for per-content-type body validation can have validation bypassed entirely by prepending a space to the Content-Type header. The body is still parsed correctly but schema validation is skipped.

This is a regression introduced in fastify >= 5.3.2 by the fix for CVE-2025-32442

Patches:

Upgrade to fastify v5.8.5 or later.

Workarounds:

None. Upgrade to the patched version.

Weakness: CWE-1289 (Improper Validation of Unsafe Equivalence in Input), CWE-1287 (Improper Validation of Specified Type of Input)

## Which versions are affected by CVE-2026-33806?

It affects 1 product.


Products: Fastify Fastify.

## How do I fix CVE-2026-33806?

Upgrade to a fixed version.

## Is CVE-2026-33806 being exploited?

There is no known exploitation of CVE-2026-33806 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-2026-84428](https://insights.sbomify.com/CVE-2026-84428/): fastify vulnerable to header validation bypass via incomplete schema case normalization (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-2026-25223](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-2026-33806 and fix it if it is affected.

What it is: CVE-2026-33806 is a high-severity vulnerability in fastify, published on 15 Apr 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-2026-33806 in the wild and no public exploit code.

Affected product: fastify. 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-2026-33806/ (Markdown: https://insights.sbomify.com/CVE-2026-33806/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-33806
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-33806
- OSV: https://osv.dev/vulnerability/CVE-2026-33806

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-33806/
Summaries are generated and may contain mistakes; the official description above is the record.