# CVE-2026-77063: multer vulnerable to file size limit bypass via async fileFilter race condition


- **Severity:** Low 3.7 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Fix available:** Yes
- **Published:** 28 Aug 2026
- **Updated:** 31 Aug 2026
- **Also known as:** CLEANSTART-2026-VJ23809, ECHO-bfbe-94c6-77e4, GHSA-qvfw-j98x-7q72, MINI-5vr7-rrcj-32vh, MINI-ffvp-cj38-6g5j, MINI-q8vv-78mf-f2pf, ROOT-APP-NPM-CVE-2026-77063

## What is CVE-2026-77063?

CVE-2026-77063 is a low-severity vulnerability in multer, published on 28 Aug 2026. 

### Official description

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.

Weakness: CWE-362 (Concurrent Execution using Shared Resource with Improper Synchronization)

## Which versions are affected by CVE-2026-77063?

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


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `multer` | npm | All versions | 2.3.0 |
| `multer` | npm | All versions | 2.3.0 |

Products: Expressjs Multer.

## How do I fix CVE-2026-77063?

Upgrade multer to 2.3.0 or later.

## Is CVE-2026-77063 being exploited?

There is no known exploitation of CVE-2026-77063 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-2026-77063 and fix it if it is affected.

What it is: CVE-2026-77063 is a low-severity vulnerability in multer, published on 28 Aug 2026. 
Exploitation: There is no known exploitation of CVE-2026-77063 in the wild and no public exploit code.

Affected packages and fixed versions:
- multer (npm): affected all versions; fixed in 2.3.0
- multer (npm): affected all versions; fixed in 2.3.0

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-2026-77063/ (Markdown: https://insights.sbomify.com/CVE-2026-77063/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-77063
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-77063
- OSV: https://osv.dev/vulnerability/CVE-2026-77063

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-77063/
Summaries are generated and may contain mistakes; the official description above is the record.