CVE-2026-18427: @fastify/static before version 10.1.3 contains an incomplete fix for a previous route guard bypass

Q: How do I fix CVE-2026-18427?

fastify-static: upgrade to 10.1.3. Exact commands for each OS are on this page.

Q: Is CVE-2026-18427 being exploited?

CVE-2026-18427 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2026-18427 affect?

CVE-2026-18427 affects fastify-static. Severity is high (CVSS 7.5). @fastify/static before version 10.1.3 contains an incomplete fix for a previous route guard bypass. The static file handler rejected only parent directory segments, but it did not canonicalize dot segments, duplicate slashes, encoded dots,

Q: How urgent is CVE-2026-18427?

Priority 44/100. High severity; Remotely exploitable, no login, no user action

This month

HIGH 7.5REMOTEUPGRADE

CVE-2026-18427 is a high-severity flaw (CVSS 7.5) affecting fastify-static. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

@fastify/static before version 10.1.3 contains an incomplete fix for a previous route guard bypass. The static file handler rejected only parent directory segments, but it did not canonicalize dot segments, duplicate slashes, encoded dots, or backslashes before route matching and before delegating to the send layer. As a result, an unauthenticated attacker could request a file protected by a route based guard using a non canonical path form that

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	0.7% (top 49%)
CISA exploited list	No
Weakness	CWE-22, CWE-436
Published	2026-08-06
Last modified	2026-09-04

Publisher (CNA): openjs · Vendor: fastify

Last updated 2026-09-04 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-08-06 16:16 UTCPublished CVE record published
2026-09-04 18:16 UTCUpdated Record last modified

fastify fastify-static UPGRADE

Affected: < 10.1.3

Fixed in: 10.1.3

Safe to downgrade to: no unaffected older release

1. Check installed version

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

2. UPGRADE to 10.1.3 (Debian / Ubuntu)

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

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade fastify-static

Note

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

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-2026-18427?

fastify-static: upgrade to 10.1.3. Exact commands for each OS are on this page.

Is CVE-2026-18427 being exploited?

CVE-2026-18427 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-2026-18427 affect?

CVE-2026-18427 affects fastify-static. Severity is high (CVSS 7.5). @fastify/static before version 10.1.3 contains an incomplete fix for a previous route guard bypass. The static file handler rejected only parent directory segments, but it did not canonicalize dot segments, duplicate slashes, encoded dots,

How urgent is CVE-2026-18427?

Priority 44/100. High severity; Remotely exploitable, no login, no user action

Related CVEs

CVE-2026-7120 MEDIUM 5.3 UPGRADE @fastify/static evaluates the allowedPath callback before normalizing dot segments and duplicate path separato
CVE-2026-15074 HIGH 7.5 UPGRADE @fastify/static up to and including version 10.1.0 fails to reject dot-dot path segments in request pathnames

References

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