CVE-2026-18549: @fastify/multipart is a multipart form-data parser for Fastify

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HIGH 7.5REMOTEUPGRADE

CVE-2026-18549 is a high-severity flaw (CVSS 7.5) affecting fastify-multipart. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

@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 han

Why this priority

High severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.6% (top 51%)
CISA exploited list	No
Weakness	CWE-400, CWE-664
Published	2026-08-15
Last modified	2026-09-02

Publisher (CNA): openjs · Vendor: fastify

Last updated 2026-09-02 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-08-15 14:17 UTCPublished CVE record published
2026-09-02 15:13 UTCUpdated Record last modified

fastify fastify-multipart UPGRADE

Affected: >= 5.3.0 and < 10.1.1

Fixed in: 10.1.1

Safe to downgrade to: a release before 5.3.0

1. Check installed version

apt-cache policy fastify-multipart || rpm -q fastify-multipart || fastify-multipart --version

2. UPGRADE to 10.1.1 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade fastify-multipart

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade fastify-multipart

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 5.3.0 (Debian / Ubuntu)

apt-cache madison fastify-multipart sudo apt install fastify-multipart=<VERSION_BEFORE_5.3.0> sudo apt-mark hold fastify-multipart

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list fastify-multipart sudo dnf downgrade fastify-multipart-<VERSION_BEFORE_5.3.0> sudo dnf versionlock add fastify-multipart

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-18549?

fastify-multipart: upgrade to 10.1.1. Exact commands for each OS are on this page.

Is CVE-2026-18549 being exploited?

CVE-2026-18549 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-18549 affect?

CVE-2026-18549 affects fastify-multipart. Severity is high (CVSS 7.5). @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 und

How urgent is CVE-2026-18549?

Priority 43/100. High severity; Remotely exploitable, no login, no user action

Related CVEs

CVE-2026-19474 HIGH 7.5 UPGRADE @fastify/multipart is a multipart form-data parser for Fastify

References

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