CVE-2026-113872: multer vulnerable to Denial of Service via a pending asynchronous fileFilter after request teardown
Impact
When multer is configured with an asynchronous fileFilter, a single request can crash the Node.js process. If a request is aborted (for example when one file is rejected) while another file's async fileFilter is still pending, the pending callback later resumes a parsing stream that multer has already torn down. busboy then emits an error on a stream that no longer has an error listener, which Node raises as an unhandled 'error' event, and the process exits. A single unauthenticated request is enough, so any application using multer with an asynchronous fileFilter is affected, regardless of storage engine. Uploads using a synchronous fileFilter are not affected.
Patches
Users should upgrade to multer 2.5.0 or later.
Workarounds
Use a synchronous fileFilter.
Other sources
multer is a middleware for Node.js that handles multipart/form-data file uploads. In versions from 2.0.0 before 2.5.0, when multer is configured with an asynchronous fileFilter, a single request can crash the Node.js process. If the request is aborted while one file's asynchronous fileFilter is still pending, the pending callback later resumes a parsing stream that multer has already torn down, and the underlying parser emits an error on a stream that no longer has an error listener, which Node raises as an unhandled error event so the process exits. Any application using multer with an asynchronous fileFilter is affected regardless of storage engine, while uploads using a synchronous fileFilter are not affected. The issue is fixed in multer 2.5.0, and users should upgrade to 2.5.0 or later. As a workaround, use a synchronous fileFilter.
— MITRE
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
npm/multerto a version that resolves this vulnerability.Fixed in 2.5.0 - Upgrade
Upgrade
debian/node-multerto a version that resolves this vulnerability.Fixed in 2.5.0+~cs3.3.0-1 - Upgrade
Upgrade
multerto a version that resolves this vulnerability.Fixed in 2.5.0 - Configuration
Configure multer to use a synchronous fileFilter.
multer fileFilter = synchronous
Event History
Frequently Asked Questions
Which deployments are exposed to this denial-of-service condition?
Applications using multer versions 2.0.0 through before 2.5.0 are affected only when configured with an asynchronous fileFilter. The issue applies regardless of the selected storage engine; uploads using a synchronous fileFilter are not affected.
What must an attacker do to trigger the crash?
An attacker needs to abort an upload request while an asynchronous fileFilter callback for a file is still pending. When that callback subsequently completes, it can cause an unhandled error event that exits the Node.js process.
What can be done if an immediate upgrade is not possible?
Replace the asynchronous fileFilter with a synchronous fileFilter as a workaround. The permanent fix is to upgrade multer to version 2.5.0 or later.