CVE-2026-19481: @fastify/busboy Denial of Service Vulnerability

HIGH
Advisory Identifier: GHSA-x8mw-p69m-v3mx
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CVSS 7.5
CWE-754
Published
Advisory Summary

CVE-2026-19481 is a HIGH (CVSS 7.5) denial of service vulnerability (CWE-754: Improper Check for Unusual or Exceptional Conditions) affecting @fastify/busboy libraries in the npm ecosystem. The issue involves @fastify/busboy vulnerable to denial of service via prototype-named multipart part header. Remediation is available by upgrading affected packages to version 3.2.1.

CVE-2026-19481 at a Glance

CVE Identifier CVE-2026-19481
GHSA Identifier GHSA-x8mw-p69m-v3mx
Severity HIGH
CVSS v3.1 Base Score 7.5
CWE Classification CWE-754: Improper Check for Unusual or Exceptional Conditions
Affected Product @fastify/busboy
Package Ecosystem npm
Published Date
Last Updated

Security Impact

CVSS: 7.5

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality None (C:N)
Integrity None (I:N)
Availability High (A:H)

How to Fix CVE-2026-19481

To remediate this vulnerability, upgrade all affected @fastify/busboy dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

@fastify/busboy fixed: 3.2.1 
npm install @fastify/busboy@3.2.1
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

npm ls @fastify/busboy

How to Check if You Are Affected

To determine whether your Node.js application is affected by CVE-2026-19481, inspect your project dependencies using npm:

npm ls @fastify/busboy

Analyze the terminal output to locate any version of @fastify/busboy in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 3.2.1), your application is affected.

Note: The npm ls @fastify/busboy command inspects dependencies declared in your package.json and lockfile; it does not perform automated runtime dynamic scanning.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-754: Improper Check for Unusual or Exceptional Conditions, where the product does not check or incorrectly checks for unusual or exceptional conditions that are not expected to occur frequently during day to day operation of the product. In @fastify/busboy, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

### 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.

CVE-2026-19481 Affected @fastify/busboy Versions

1 component(s)
Fixed Versions:
3.2.1
Product / Package Ecosystem Affected Versions Fixed Version
@fastify/busboy
npm/@fastify/busboy
npm
≥ 1.0.0
< 3.2.1
3.2.1

References

GitHub Advisory / Wiki ADVISORY
https://github.com/fastify/busboy/security/advisories/GHSA-x8mw-p69m-v3mx
NVD — CVE-2026-19481 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-19481
Security Fix Commit FIX COMMIT
https://github.com/fastify/busboy/commit/957a24b66d5915e6d0a6ac988c9dbcee86373e9b
@fastify/busboy Security Information SECURITY
https://cna.openjsf.org/security-advisories.html
@fastify/busboy Source Repository REPOSITORY
https://github.com/fastify/busboy
@fastify/busboy Release RELEASE
https://github.com/fastify/busboy/releases/tag/v3.2.1

Frequently Asked Questions

What is CVE-2026-19481?
CVE-2026-19481 is a security vulnerability in @fastify/busboy involving @fastify/busboy vulnerable to Denial of Service via prototype-named multipart part header under CWE-754: Improper Check for Unusual or Exceptional Conditions.
Which @fastify/busboy versions are affected?
Affected versions include releases prior to 3.2.1 across affected packages: @fastify/busboy.
What version fixes CVE-2026-19481?
Upgrading to 3.2.1 (or the latest patched release for your respective branch) resolves the vulnerability.
Is @fastify/busboy affected?
Yes, @fastify/busboy is affected in releases prior to 3.2.1. Upgrade your dependency to 3.2.1 or later.
What is the CVSS score of CVE-2026-19481?
CVE-2026-19481 has a CVSS v3.1 base score of 7.5 (HIGH) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.
What CWE is associated with CVE-2026-19481?
CVE-2026-19481 is classified under CWE-754: Improper Check for Unusual or Exceptional Conditions (The product does not check or incorrectly checks for unusual or exceptional conditions that are not expected to occur frequently during day to day operation of the product.).
How can I check npm dependencies for CVE-2026-19481?
Run the dependency tree command: 'npm ls @fastify/busboy'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-x8mw-p69m-v3mx",
  "modified" : "2026-10-02T23:16:36Z",
  "published" : "2026-10-02T23:16:36Z",
  "aliases" : [ "CVE-2026-19481" ],
  "summary" : "@fastify/busboy vulnerable to Denial of Service via prototype-named multipart part header",
  "details" : "### Impact\n\nVersions 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.\n\n### Patches\n\nFixed in version 3.2.1.\n\n### Workarounds\n\nAttach 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.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "@fastify/busboy"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.0.0"
      }, {
        "fixed" : "3.2.1"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/fastify/busboy/security/advisories/GHSA-x8mw-p69m-v3mx"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-19481"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/fastify/busboy/commit/957a24b66d5915e6d0a6ac988c9dbcee86373e9b"
  }, {
    "type" : "WEB",
    "url" : "https://cna.openjsf.org/security-advisories.html"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/fastify/busboy"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/fastify/busboy/releases/tag/v3.2.1"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-754" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-10-02T23:16:36Z",
    "nvd_published_at" : "2026-08-13T09:17:12Z"
  }
}