How severe is CVE-2026-22031?

CVE-2026-22031 has a CVSS 3.x base score of 8.4, rated high severity. It is exploitable over network with high attack complexity, requires low privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability low.

Is CVE-2026-22031 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (37th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-22031?

CVE-2026-22031 affects Fastify Fastify/middie. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-22031?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.

Does CVE-2026-22031 have an EU (EUVD) identifier?

Yes. CVE-2026-22031 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-3321.

When was CVE-2026-22031 published?

CVE-2026-22031 was published on 2026-01-19 and last updated on 2026-06-17.

CVE-2026-22031

CVE-2026-22031 is a high-severity vulnerability in Fastify Fastify/middie with a CVSS 3.x base score of 8.4. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-177.

Key facts

Severity: High (CVSS 3.x base score 8.4)
EPSS exploit prediction: 0% (37th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-3321
Weakness: CWE-177
Affected product: Fastify Fastify/middie
Published: 2026-01-19
Last modified: 2026-06-17

Description

@fastify/middie is the plugin that adds middleware support on steroids to Fastify. A security vulnerability exists in @fastify/middie prior to version 9.1.0 where middleware registered with a specific path prefix can be bypassed using URL-encoded characters (e.g., `/%61dmin` instead of `/admin`). While the middleware engine fails to match the encoded path and skips execution, the underlying Fastify router correctly decodes the path and matches the route handler, allowing attackers to access protected endpoints without the middleware constraints. Version 9.1.0 fixes the issue.

Frequently asked questions

What is CVE-2026-22031?: @fastify/middie is the plugin that adds middleware support on steroids to Fastify. A security vulnerability exists in @fastify/middie prior to version 9.1.0 where middleware registered with a specific path prefix can be bypassed using URL-encoded characters (e.g., `/%61dmin` instead of `/admin`). While the middleware engine fails to match the encoded path and skips execution, the underlying Fastify router correctly decodes the path and matches the route handler, allowing attackers to access protected endpoints without the middleware constraints. Version 9.1.0 fixes the issue.
How severe is CVE-2026-22031?: CVE-2026-22031 has a CVSS 3.x base score of 8.4, rated high severity. It is exploitable over network with high attack complexity, requires low privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability low.
Is CVE-2026-22031 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (37th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-22031?: CVE-2026-22031 affects Fastify Fastify/middie. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-22031?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
Does CVE-2026-22031 have an EU (EUVD) identifier?: Yes. CVE-2026-22031 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-3321.
When was CVE-2026-22031 published?: CVE-2026-22031 was published on 2026-01-19 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:fastify:fastify\/middie:*:*:*:*:*:node.js:*:*

More vulnerabilities in Fastify Fastify/middie

CVE-2026-14198 — Critical (CVSS 9.1): @fastify/middie versions 9.1.0 through 9.3.2 decode the encoded slash %2F inside path parameter values before matching…
CVE-2026-6270 — Critical (CVSS 9.1): @fastify/middie versions 9.3.1 and earlier do not register inherited middleware directly on child plugin engine…
CVE-2026-2880 — Critical (CVSS 9.1): A vulnerability in @fastify/middie versions < 9.2.0 can result in authentication/authorization bypass when using…
CVE-2026-14181 — High (CVSS 7.5): @fastify/middie versions 9.1.0 through 9.3.2 fail to guard the URL normalization step used by the standalone engine…
CVE-2026-33804 — High (CVSS 7.4): @fastify/middie versions 9.3.1 and earlier are vulnerable to middleware bypass when the deprecated Fastify…

All CVEs affecting Fastify Fastify/middie →

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CVE-2026-6414 — Medium (CVSS 5.9): @fastify/static versions 8.0.0 through 9.1.0 decode percent-encoded path separators (%2F) before filesystem resolution,…
CVE-2024-48866 — Medium (CVSS 5.3): An improper handling of URL encoding (Hex Encoding) vulnerability has been reported to affect several QNAP operating…
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