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

Source: https://junglewise.ai/threats/cve-2026-77078-multer-denial-of-service-via-crafted-multipart-field-names  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-08-28. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2026-77078 · Severity: high · CVSS 7.5 · Published 2026-08-28

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.

Technologies: [multer (npm)](https://junglewise.ai/threats/technologies/npm-multer). Vendors: [npm](https://junglewise.ai/threats/vendors/npm).

## Quick answers

**Is CVE-2026-77078 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-77078 being exploited in the wild.

**How severe is CVE-2026-77078?** High severity, with a CVSS score of 7.5, published 28 August 2026.

**Which products and versions are affected?** multer (npm) SEMVER:introduced=0,fixed=2.3.0.

**Is there a fix for CVE-2026-77078?** Yes. multer (npm): upgrade to 2.3.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-77078 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-77078?** Check whether you run multer (npm) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- multer (npm): upgrade to 2.3.0 or later

  ```sh
  npm install multer@2.3.0
  ```


## 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

## Identifiers

- CVE-2026-77078
- GHSA-wc9g-mqfw-jrwm

## Timeline

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

## References

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

## Related threats

- [multer denial of service in asynchronous fileFilter after request teardown](https://junglewise.ai/threats/cve-2026-113872-multer-is-a-middleware-for-node-js-that-handles-multipart-form)
- [Express multer Denial of Service via orphaned disk writes](https://junglewise.ai/threats/cve-2026-88932-express-multer-denial-of-service-via-orphaned-disk-writes)
- [multer denial of service via oversized array index](https://junglewise.ai/threats/cve-2026-82333-multer-denial-of-service-via-oversized-array-index-in-field-names)
- [multer file size limit bypass via async fileFilter race condition](https://junglewise.ai/threats/cve-2026-77063-multer-file-size-limit-bypass-via-async-filefilter-race-condition)
- [Express multer file descriptor leak on aborted uploads](https://junglewise.ai/threats/cve-2026-77037-express-multer-file-descriptor-leak-on-aborted-uploads)