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CVE-2026-33807

CRITICALCVSS 9.1/10EPSS 0.43%

Last modified

CVE-2026-33807 is a critical-severity vulnerability rated 9.1/10 on the CVSS scale. @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. EPSS estimates a 0.43% chance of exploitation in the next 30 days.

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.

Metrics

CVSS 3.1
9.1/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

EPSS Probability
0.43%

34.4th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

Affected Software

VendorProductVersions
FastifyFastify\/Express< 4.0.5

References

Timeline

Published
Last Modified
Status
Analyzed

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 score of 9.1/10 (CRITICAL severity). The EPSS model estimates a 0.43% probability of exploitation in the next 30 days.
How do I fix CVE-2026-33807?
Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2026-33807?

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Source: NVD / NIST