# CVE-2026-5079: multer vulnerable to Denial of Service via deeply nested field names

> An attacker can cause denial of service in applications using multer by sending multipart data with deeply nested field names, exhausting server resources.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 15 Jun 2026
- **Updated:** 15 Jun 2026
- **Also known as:** CLSA-2026-1784077507, ECHO-f70f-5c59-82fe, GHSA-72gw-mp4g-v24j, MINI-7xr7-35g3-9mpw, MINI-p5qv-w9q8-hhqf, ROOT-APP-NPM-CVE-2026-5079

## What is CVE-2026-5079?

CVE-2026-5079 is a high-severity denial of service vulnerability in multer, published on 15 Jun 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Impact: multer versions 1.0.0 through 2.1.1 and 3.0.0-alpha.1 are vulnerable to a Denial of Service via deeply nested field names in multipart form data. The append-field dependency parses bracket notation in field names with no limit on nesting depth, allowing an attacker to force allocation of deeply nested object structures that consume CPU and memory. A single HTTP request with a crafted multipart body is sufficient to exploit this.

Patches: Users should upgrade to multer 2.2.0 (2.x line) or 3.0.0-alpha.2 (3.x prerelease) and configure the new limits.fieldNestingDepth option to the minimum depth their application requires.

Workarounds: Set limits.fields to a reasonable value to reduce the number of fields an attacker can send per request. This does not fully mitigate the issue but limits the impact.

Weakness: CWE-400 (Uncontrolled Resource Consumption)

## Which versions are affected by CVE-2026-5079?

4 open source packages ship an affected version, including multer. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `multer` | npm | From 1.0.0 | 2.2.0 |
| `multer` | npm | From 1.0.0 | 2.2.0 |
| `multer` | npm | From 3.0.0-alpha.1 | 3.0.0-alpha.2 |
| `multer` | npm | From 3.0.0-alpha.1 | 3.0.0-alpha.2 |

Products: Expressjs Multer.

## How do I fix CVE-2026-5079?

Upgrade multer to a fixed version for your release line: 2.2.0 or 3.0.0-alpha.2.

## Is CVE-2026-5079 being exploited?

There is no known exploitation of CVE-2026-5079 in the wild and no public exploit code.

## Other vulnerabilities in multer

- [CVE-2026-3520](https://insights.sbomify.com/CVE-2026-3520/): Multer vulnerable to Denial of Service via uncontrolled recursion (high 8.7)
- [CVE-2026-2359](https://insights.sbomify.com/CVE-2026-2359/): Multer vulnerable to Denial of Service via resource exhaustion (high 8.7)
- [CVE-2026-3304](https://insights.sbomify.com/CVE-2026-3304/): Multer vulnerable to Denial of Service via incomplete cleanup (high 8.7)
- [CVE-2026-77037](https://insights.sbomify.com/CVE-2026-77037/): multer vulnerable to Denial of Service via file descriptor leak on aborted uploads (high 7.5)
- [CVE-2026-77078](https://insights.sbomify.com/CVE-2026-77078/): multer vulnerable to Denial of Service via crafted multipart field names (high 7.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-5079 and fix it if it is affected.

What it is: CVE-2026-5079 is a high-severity denial of service vulnerability in multer, published on 15 Jun 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-5079 in the wild and no public exploit code.

Affected packages and fixed versions:
- multer (npm): affected from 1.0.0; fixed in 2.2.0
- multer (npm): affected from 1.0.0; fixed in 2.2.0
- multer (npm): affected from 3.0.0-alpha.1; fixed in 3.0.0-alpha.2
- multer (npm): affected from 3.0.0-alpha.1; fixed in 3.0.0-alpha.2

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-5079/ (Markdown: https://insights.sbomify.com/CVE-2026-5079/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-5079
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-5079
- OSV: https://osv.dev/vulnerability/CVE-2026-5079

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-5079/
Summaries are generated and may contain mistakes; the official description above is the record.