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CVE-2026-22031

Fastify Fastify/middie vulnerability

8.4 HIGHPublic exploitEPSS 0.53%
  1. Published19 Jan 2026

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.

CVSS 3.1 breakdown

Attack vectorNetwork
Attack complexityHigh
Privileges requiredLow
User interactionNone
ScopeChanged
ConfidentialityHigh
IntegrityHigh
AvailabilityLow
CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:L

Affected products

Fastify Fastify/middie< 9.1.0

Remediation

Fixed in
Fastify Fastify/middie≥ 9.1.0

References

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Frequently asked

Is CVE-2026-22031 being actively exploited?

CVE-2026-22031 is not in the CISA KEV catalog. A public exploit reference exists, so treat it as exploitable. Its EPSS exploit probability is 0.53%.

How severe is CVE-2026-22031?

CVE-2026-22031 has a CVSS 3.1 base score of 8.4, rated high. EPSS estimates a 0.53% probability of exploitation.

Which products are affected by CVE-2026-22031?

Affected products include Fastify Fastify/middie.

How do I fix CVE-2026-22031?

Upgrade affected software to a fixed version, for example Fastify Fastify/middie 9.1.0 or later, then confirm no affected versions remain in your inventory.

How Secuno handles this

Secuno correlates vulnerabilities like CVE-2026-22031 against the real software inventory on every managed device, every day. Rather than a static severity, each match is scored in context: exploit availability, EPSS, the asset's exposure, and business criticality decide where it lands in the queue. If an affected version is present anywhere across your portfolio, it surfaces automatically with a prioritised remediation path.

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Data from the NVD, CISA KEV catalog and FIRST EPSS. CVSS 3.1 base score shown where available.