CVE-2026-77063: multer vulnerable to file size limit bypass via async fileFilter race condition
multer is a middleware for handling multipart/form-data in Node.js. When an application uses an asynchronous fileFilter together with the fileSize limit, a race condition in multer's file stream handling can allow a file that exceeds the configured size limit to bypass the size-limit rejection. All versions before 2.3.0 are affected. The impact is limited because the underlying multipart parser still truncates the stream at the size limit, so this is a bypass of the limit rejection rather than uncontrolled resource consumption. The issue is fixed in multer 2.3.0. Upgrade to multer 2.3.0 to remediate.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
multerto a version that resolves this vulnerability.Fixed in 2.3.0
Event History
Frequently Asked Questions
Which applications are affected by this issue?
Applications using multer versions before 2.3.0 are affected only when they combine an asynchronous fileFilter with multer's fileSize limit.
What does an attacker need to exploit the bypass?
An attacker needs network access to an upload endpoint that uses the affected asynchronous fileFilter and fileSize configuration. No privileges or user interaction are required, but exploitation has high attack complexity.
Does this allow unrestricted file uploads or resource exhaustion?
No. The underlying multipart parser still truncates the upload stream at the configured size limit; the flaw bypasses the configured size-limit rejection rather than allowing uncontrolled resource consumption.
How can the issue be remediated?
Upgrade multer to version 2.3.0. This version fixes the asynchronous fileFilter race condition.