CVE-2026-77078: multer Denial of Service via crafted multipart field names

Executive brief

Multer is a widely-used Node.js library for handling file uploads in web applications. This vulnerability allows an unauthenticated attacker to crash any application using multer by sending a specially crafted multipart form request with malicious field names. An attack requires only a single HTTP request and causes immediate service outage, disrupting availability for all users until the application is restarted.

Technical details

An uncaught exception (CWE-248) in multer's multipart field parsing logic allows two specially crafted text field names to trigger a RangeError: Invalid array length. The vulnerability exists in the field parsing component which is reached during normal multipart/form-data request processing. No authentication or user interaction is required; the attack vector is network-based and exploitable from any remote client. When triggered, the exception is not caught by multer's error handler and propagates uncaught, terminating the Node.js process. The patch is available in version 2.3.0 and later.

Affected products

• Express.js Foundation multer < 2.3.0

Timeline

• 2026-08-28: disclosed
• 2026-09-08: patched: Version 2.3.0 released

References

• https://api.github.com/users/O4FDev
• https://github.com/O4FDev
• https://api.github.com/users/O4FDev/gists%7B/gist_id%7D
• https://api.github.com/users/O4FDev/repos
• https://avatars.githubusercontent.com/u/52675277?v=4
• https://api.github.com/users/O4FDev/events%7B/privacy%7D