CVE-2026-33807
CRITICALDescription
@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 to fix
TridentStack Control can deploy fixes like this automatically across your Windows, macOS, and Linux fleet. See how it works
Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.
CVSS v3 Vector
Exploitability
Impact
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
Exploit Intelligence
Low risk: more likely to be exploited than 36% of all known CVEs.
References
Related Vulnerabilities
Other CWE-436 vulnerabilities, ordered by exploit likelihood. View all
| CVE | Severity | CVSS | EPSS | Exploited | Fix |
|---|---|---|---|---|---|
| CVE-2026-63030 | Critical | 9.8 | 97% | KEV | Fix |
| CVE-2025-25292 | Critical | 9.8 | 65% | - | Fix |
| CVE-2022-37436 | Medium | 5.3 | 58% | - | Fix |
| CVE-2021-28474 | High | 8.8 | 51% | - | Fix |
| CVE-2025-25291 | Critical | 9.8 | 20% | - | Fix |
| CVE-2019-17596 | High | 7.5 | 4.7% | - | Fix |
Common questions
How do I fix CVE-2026-33807?
Upgrade fastify\/express to 4.0.5 or later.
Is CVE-2026-33807 being actively exploited?
Not that we know of. CVE-2026-33807 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.43% is the estimated probability that it will be exploited in the next 30 days. That is higher than 36% of all scored CVEs.
How severe is CVE-2026-33807?
CVE-2026-33807 has a CVSS v3 base score of 9.1, rated critical. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.
What does CVE-2026-33807 affect?
Published advisories record a fix for fastify\/express (NVD). Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.
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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-06-01.