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.
// cvss v3.1 vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High
Checked 2026-10-04 at 01:14 UTC. The most recent advisory here was published 2026-10-02. Updated continuously from NVD, GHSA, OSV and CNA feeds.