# CVE-2026-18549: @fastify/multipart vulnerable to Denial of Service via aborted upload after fileSize limit

> An unauthenticated attacker can cause disk and event-loop exhaustion by aborting uploads after the file size limit is reached, hanging request handlers.

- **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:** No
- **Published:** 15 Aug 2026
- **Updated:** 17 Aug 2026
- **Also known as:** GHSA-vmph-573x-85f6

## What is CVE-2026-18549?

CVE-2026-18549 is a high-severity denial of service vulnerability in @fastify/multipart, published on 15 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

@fastify/multipart is a multipart form-data parser for Fastify. In versions from 5.3.0 up to but not including 10.1.1, when the busboy fileSize limit truncates a file part, the plugin clears its internal current-file reference while the underlying stream is still open. If the client then aborts the connection before sending the terminating boundary, the abort cleanup finds no stream to destroy, so saveRequestFiles() never settles, the request handler hangs, and the temporary file already written to disk is never cleaned up. An unauthenticated client can repeat this to permanently leak temporary files and suspended handler executions, leading to disk and event-loop exhaustion. The issue is fixed in @fastify/multipart 10.1.1. Users should upgrade to 10.1.1.

Weakness: CWE-664 (Improper Control of a Resource Through its Lifetime), CWE-400 (Uncontrolled Resource Consumption)

## Which versions are affected by CVE-2026-18549?

It affects 1 product.


Products: Fastify Fastify-Multipart.

## How do I fix CVE-2026-18549?

No fix has been published for CVE-2026-18549 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-18549 being exploited?

There is no known exploitation of CVE-2026-18549 in the wild and no public exploit code.

## Other vulnerabilities in @fastify/multipart

- [CVE-2026-19474](https://insights.sbomify.com/CVE-2026-19474/): @fastify/multipart vulnerable to Denial of Service via temporary file leak on aborted upload (high 7.5)
- [CVE-2023-25576](https://insights.sbomify.com/CVE-2023-25576/): @fastify/multipart vulnerable to DoS due to unlimited number of parts (high 7.5)
- [CVE-2021-23597](https://insights.sbomify.com/CVE-2021-23597/): Denial of Service (DoS) (high 7.5)
- [CVE-2020-8136](https://insights.sbomify.com/CVE-2020-8136/): Uncontrolled Resource Consumption in fastify-multipart (high 7.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-18549 and fix it if it is affected.

What it is: CVE-2026-18549 is a high-severity denial of service vulnerability in @fastify/multipart, published on 15 Aug 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-2026-18549 in the wild and no public exploit code.

Affected product: @fastify/multipart. No package-level version data is available, so check the vendor advisory linked from the reference page.

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-2026-18549/ (Markdown: https://insights.sbomify.com/CVE-2026-18549/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-18549
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-18549
- OSV: https://osv.dev/vulnerability/CVE-2026-18549

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-18549/
Summaries are generated and may contain mistakes; the official description above is the record.