@fastify/busboy vulnerable to Denial of Service via prototype-named multipart part header

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

7.5 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS

• CVE-2026-19481: 0.49% estimated exploitation activity in the next 30 days; percentile 42.42 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Impact Versions of @fastify/busboy from 1.0.0 and prior to 3.2.1 are vulnerable to a Denial of Service. The multipart header parser stores part-header names on a plain JavaScript object, so a part header named proto or constructor resolves to an inherited value that is not an array, and the parser throws TypeError: this.header[h].push is not a function. Through the documented req.pipe(busboy) integration this surfaces as an error event, while direct write()/end() usage throws synchronously and can terminate the Node.js process if uncaught. The parser runs before application middleware, so any unauthenticated client that can submit multipart/form-data is affected. Patches Fixed in version 3.2.1. Workarounds Attach an error listener to the Busboy stream so the parser failure is handled rather than crashing the process, and wrap direct write()/end() calls in a try/catch. Upgrading to 3.2.1 removes the failure entirely.

Published by the source

2 Oct 2026, 23:16 UTC

Last modified by the source

2 Oct 2026, 23:30 UTC

Source last checked

26 Oct 2026, 13:06 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-x8mw-p69m-v3mx

2. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GHSA-x8mw-p69m-v3mx

3. Revision 1 · created · 23 Oct 2026

   Published from ghsa record GHSA-x8mw-p69m-v3mx

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# Patch Next advisory brief: CVE-2026-19481

Advisory: @fastify/busboy vulnerable to Denial of Service via prototype-named multipart part header
IDs: CVE-2026-19481, GHSA-x8mw-p69m-v3mx
Record: https://patchnext.com/advisories/cve-2026-19481 (revision 3, source checked 26 Oct 2026, 13:06 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: @fastify/busboy 3.2.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm @fastify/busboy: affected >= 1.0.0, < 3.2.1; fixed in 3.2.1

## Suggested inspection (read-only)
- `npm ls @fastify/busboy --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on @fastify/busboy.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-x8mw-p69m-v3mx (GitHub-reviewed advisory GHSA-x8mw-p69m-v3mx lists affected ranges: npm @fastify/busboy >=1.0.0 <3.2.1)
- Verified: https://osv.dev/vulnerability/GHSA-x8mw-p69m-v3mx (OSV record GHSA-x8mw-p69m-v3mx lists affected ranges: npm @fastify/busboy >=1.0.0 <3.2.1 (fixed))

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