fastify busboy up to 3.2.0 Header Parser prototype pollution

Summaryinfo

A vulnerability has been found in fastify busboy up to 3.2.0 and classified as critical. Affected by this issue is some unknown functionality of the component Header Parser. The manipulation leads to prototype pollution. This vulnerability is uniquely identified as CVE-2026-19481. The attack is possible to be carried out remotely. No exploit exists.

Detailsinfo

A vulnerability was found in fastify busboy up to 3.2.0. It has been rated as critical. This issue affects an unknown part of the component Header Parser. The manipulation with an unknown input leads to a prototype pollution vulnerability. Using CWE to declare the problem leads to CWE-1321. The product receives input from an upstream component that specifies attributes that are to be initialized or updated in an object, but it does not properly control modifications of attributes of the object prototype. Impacted is confidentiality, integrity, and availability. The summary by CVE is:

@fastify/busboy is a multipart form-data parser. In versions 1.0.0 through 3.2.0, an attacker who can submit multipart form-data can crash the parser by sending a part header whose name is a prototype-inherited property such as __proto__ or constructor. The internal header parser stores headers in a plain JavaScript object and assumes each value is an array, so an inherited property name resolves to a truthy non-array value and triggers a TypeError. In the common pipe integration the failure surfaces as an error event, but in direct write or end usage the exception is thrown synchronously and can terminate the Node.js process, causing an unauthenticated denial of service. The issue is fixed in @fastify/busboy 3.2.1, which creates the header object with a null prototype. Users should upgrade to 3.2.1.

The advisory is shared at github.com. The identification of this vulnerability is CVE-2026-19481 since 08/10/2026. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. Neither technical details nor an exploit are publicly available. MITRE ATT&CK project uses the attack technique T1059 for this issue.

Upgrading to version 3.2.1 eliminates this vulnerability.

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Productinfo

Vendor

• fastify

Name

• busboy

Version

• 3.0
• 3.1
• 3.2.0

Website

• Product: https://github.com/fastify/busboy/

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