# CVE-2026-120215: @fastify/cors vulnerable to Denial of Service on a CORS preflight with the onSend hook


- **Severity:** Low 3.7 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L`)
- **Exploited in the wild:** Not known
- **Fix available:** No
- **Published:** 2 Nov 2026
- **Updated:** 2 Nov 2026
- **Also known as:** GHSA-8c3j-mm64-257c

## What is CVE-2026-120215?

CVE-2026-120215 is a low-severity vulnerability in @fastify/cors, published on 2 Nov 2026. 

### Official description

@fastify/cors is the CORS plugin for the Fastify web framework. In versions 8.2.0 through 11.3.x, an application that registers the plugin with the non-default onSend hook option crashes on a CORS preflight. On an OPTIONS request that carries the Origin and Access-Control-Request-Method headers, the plugin calls reply.send from inside the onSend hook and re-enters the send pipeline, so a second write of the response headers throws an uncaught ERR_HTTP_HEADERS_SENT that can terminate the Node.js process when no process-level handler is installed. An unauthenticated remote client can trigger this with a single preflight request, so the impact is availability only, and exploitation requires the non-default onSend configuration. The issue is fixed in @fastify/cors 11.4.0, where onSend is no longer an accepted hook value and registration fails with a clear error instead of crashing at request time, and users should upgrade to 11.4.0 or later. As a workaround, use any other hook such as the default onRequest, or set preflight to false.

Weakness: CWE-248 (Uncaught Exception)

## Which versions are affected by CVE-2026-120215?


## How do I fix CVE-2026-120215?

No fix has been published for CVE-2026-120215 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-120215 being exploited?

There is no known exploitation of CVE-2026-120215 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-120215 and fix it if it is affected.

What it is: CVE-2026-120215 is a low-severity vulnerability in @fastify/cors, published on 2 Nov 2026. 
Exploitation: There is no known exploitation of CVE-2026-120215 in the wild and no public exploit code.

Affected product: @fastify/cors. 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-2026-120215/ (Markdown: https://insights.sbomify.com/CVE-2026-120215/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-120215
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-120215
- OSV: https://osv.dev/vulnerability/CVE-2026-120215

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-120215/
Summaries are generated and may contain mistakes; the official description above is the record.