---
id: CVE-2026-77063
aliases:
  - GHSA-qvfw-j98x-7q72
title: >-
  multer vulnerable to file size limit bypass via async fileFilter race
  condition
summary: >-
  multer vulnerable to file size limit bypass via async fileFilter race
  condition
severity: low
cvss: 3.7
cwe:
  - CWE-362
vendor: multer
product: multer
ecosystem: npm
affected:
  - multer < 2.3.0
patched:
  - multer 2.3.0
published: '2026-09-08'
updated: '2026-09-08'
sourceUpdated: '2026-09-08T21:29:43Z'
source: GHSA
sourceUrl: 'https://github.com/advisories/GHSA-qvfw-j98x-7q72'
references:
  - url: >-
      https://github.com/expressjs/multer/security/advisories/GHSA-qvfw-j98x-7q72
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2026-77063'
  - url: >-
      https://github.com/expressjs/multer/commit/ab6aeae650328cf31799dcfa4c3e116ba8faaacc
  - url: 'https://cna.openjsf.org/security-advisories.html'
  - url: 'https://github.com/expressjs/multer/releases/tag/v2.3.0'
  - url: 'https://github.com/advisories/GHSA-qvfw-j98x-7q72'
tags:
  - ghsa
  - npm
epss: 0.0016
epssPercentile: 0.05604
ingestedAt: '2026-09-08T22:12:30.967Z'
---

## Overview

### Impact

When `multer` is configured with an asynchronous `fileFilter`, the `limits.fileSize` limit can be bypassed. The `'limit'` event is registered inside the async `fileFilter` callback, so if a file exceeds `limits.fileSize` before that callback runs, the event is missed and the oversized upload is accepted instead of being rejected with a `LIMIT_FILE_SIZE` error. Applications that rely on `limits.fileSize` to reject oversized uploads are affected on all upload methods (`.single()`, `.array()`, `.fields()`, `.any()`). Uploads using a synchronous `fileFilter` are not affected.

### Patches

Users should upgrade to `2.3.0`.

### Workarounds

Use a synchronous `fileFilter`, or validate the uploaded file size after the upload completes.

## Affected packages

- `multer < 2.3.0`

## Remediation

Upgrade to a patched release:

- `multer 2.3.0`