CVE-2026-19481 @fastify/busboy vulnerable to Denial of Service via prototype-named multipart part header
An attacker can crash the Node.js process by sending multipart headers with prototype names, causing denial of service in applications using this parser.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-19481?
CVE-2026-19481 is a high-severity denial of service vulnerability in @fastify/busboy, published on 13 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
@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.
Weakness: CWE-754
Which versions are affected by CVE-2026-19481?
2 open source packages ship an affected version, including @fastify/busboy. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
@fastify/busboy npm | From 1.0.0 | 3.2.1 |
@fastify/busboy npm | From 1.0.0 | 3.2.1 |
Products include Fastify Fastify\/Busyboy.
How do I fix CVE-2026-19481?
Upgrade @fastify/busboy to 3.2.1 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-19481 being exploited?
There is no known exploitation of CVE-2026-19481 in the wild and no public exploit code.
Other vulnerabilities in @fastify/busboy
Sources
Also known as GHSA-x8mw-p69m-v3mx, MINI-vrm5-m8v6-vxxr, ROOT-APP-NPM-CVE-2026-19481.