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CVE-2026-33807

Fastify Fastify/express vulnerability

9.1 CRITICALPublic exploitEPSS 0.53%
  1. Published15 Apr 2026

Description

@fastify/express v4.0.4 and earlier contains a path handling bug in the onRegister function that causes middleware paths to be doubled when inherited by child plugins. When a child plugin is registered with a prefix that matches a middleware path, the middleware path is prefixed a second time, causing it to never match incoming requests. This results in complete bypass of Express middleware security controls, including authentication, authorization, and rate limiting, for all routes defined within affected child plugin scopes. No special configuration or request crafting is required. Upgrade to @fastify/express v4.0.5 or later.

CVSS 3.1 breakdown

Attack vectorNetwork
Attack complexityLow
Privileges requiredNone
User interactionNone
ScopeUnchanged
ConfidentialityHigh
IntegrityHigh
AvailabilityNone
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

Affected products

Fastify Fastify/express< 4.0.5

Remediation

Fixed in
Fastify Fastify/express≥ 4.0.5

References

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Frequently asked

Is CVE-2026-33807 being actively exploited?

CVE-2026-33807 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-2026-33807?

CVE-2026-33807 has a CVSS 3.1 base score of 9.1, rated critical. EPSS estimates a 0.53% probability of exploitation.

Which products are affected by CVE-2026-33807?

Affected products include Fastify Fastify/express.

How do I fix CVE-2026-33807?

Upgrade affected software to a fixed version, for example Fastify Fastify/express 4.0.5 or later, then confirm no affected versions remain in your inventory.

How Secuno handles this

Secuno correlates vulnerabilities like CVE-2026-33807 against the real software inventory on every managed device, as each hourly inventory arrives. 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.