# CVE-2026-3419: Fastify's Missing End Anchor in "subtypeNameReg" Allows Malformed Content-Types to Pass Validation

> Attackers can bypass Content-Type validation in Fastify by sending malformed headers, potentially routing requests to unintended parsers.

- **Severity:** Medium 5.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/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:** 6 Mar 2026
- **Updated:** 9 Mar 2026
- **Also known as:** GHSA-573f-x89g-hqp9

## What is CVE-2026-3419?

CVE-2026-3419 is a medium-severity vulnerability in fastify, published on 6 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Fastify incorrectly accepts malformed `Content-Type` headers containing trailing characters after the subtype token, in violation of RFC 9110 §8.3.1(https://httpwg.org/specs/rfc9110.html#field.content-type). For example, a request sent with Content-Type: application/json garbage passes validation and is processed normally, rather than being rejected with 415 Unsupported Media Type.

When regex-based content-type parsers are in use (a documented Fastify feature), the malformed value is matched against registered parsers using the full string including the trailing garbage. This means a request with an invalid content-type may be routed to and processed by a parser it should never have reached.

Impact:
An attacker can send requests with RFC-invalid Content-Type headers that bypass validity checks, reach content-type parser matching, and be processed by the server. Requests that should be rejected at the validation stage are instead handled as if the content-type were valid.

Workarounds:
Deploy a WAF rule to protect against this

Fix:

The fix is available starting with v5.8.1.

Weakness: CWE-185 (Incorrect Regular Expression)

## Which versions are affected by CVE-2026-3419?

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


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `fastify` | npm | From 5.7.2 | 5.8.1 |
| `fastify` | npm | From 5.7.2 | 5.8.1 |

Products: Fastify Fastify.

## How do I fix CVE-2026-3419?

Upgrade fastify to 5.8.1 or later.

## Is CVE-2026-3419 being exploited?

There is no known exploitation of CVE-2026-3419 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-2026-33806](https://insights.sbomify.com/CVE-2026-33806/): fastify vulnerable to Body Schema Validation Bypass via Leading Space in Content-Type Header (high 7.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-3419 and fix it if it is affected.

What it is: CVE-2026-3419 is a medium-severity vulnerability in fastify, published on 6 Mar 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-2026-3419 in the wild and no public exploit code.

Affected packages and fixed versions:
- fastify (npm): affected from 5.7.2; fixed in 5.8.1
- fastify (npm): affected from 5.7.2; fixed in 5.8.1

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-2026-3419/ (Markdown: https://insights.sbomify.com/CVE-2026-3419/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-3419
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-3419
- OSV: https://osv.dev/vulnerability/CVE-2026-3419

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-3419/
Summaries are generated and may contain mistakes; the official description above is the record.