How severe is CVE-2026-33807?

CVE-2026-33807 has a CVSS 3.x base score of 9.1, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability none.

Is CVE-2026-33807 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (35th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-33807?

CVE-2026-33807 affects Fastify Fastify/express. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-33807?

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

Does CVE-2026-33807 have an EU (EUVD) identifier?

Yes. CVE-2026-33807 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-22880.

When was CVE-2026-33807 published?

CVE-2026-33807 was published on 2026-04-15 and last updated on 2026-06-17.

CVE-2026-33807

CVE-2026-33807 is a critical-severity vulnerability in Fastify Fastify/express with a CVSS 3.x base score of 9.1. 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-436.

Key facts

Severity: Critical (CVSS 3.x base score 9.1)
EPSS exploit prediction: 0% (35th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-22880
Weakness: CWE-436
Affected product: Fastify Fastify/express
Published: 2026-04-15
Last modified: 2026-06-17

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.

Frequently asked questions

What is CVE-2026-33807?: @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.
How severe is CVE-2026-33807?: CVE-2026-33807 has a CVSS 3.x base score of 9.1, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability none.
Is CVE-2026-33807 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (35th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-33807?: CVE-2026-33807 affects Fastify Fastify/express. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-33807?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.
Does CVE-2026-33807 have an EU (EUVD) identifier?: Yes. CVE-2026-33807 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-22880.
When was CVE-2026-33807 published?: CVE-2026-33807 was published on 2026-04-15 and last updated on 2026-06-17.

References

Affected products (1)

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

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