How severe is CVE-2026-33804?

CVE-2026-33804 has a CVSS 3.x base score of 7.4, rated high severity. It is exploitable over network with high attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability none.

Is CVE-2026-33804 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (20th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-33804?

CVE-2026-33804 affects Fastify Fastify/middie. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-33804?

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

Does CVE-2026-33804 have an EU (EUVD) identifier?

Yes. CVE-2026-33804 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-23235.

When was CVE-2026-33804 published?

CVE-2026-33804 was published on 2026-04-16 and last updated on 2026-06-17.

CVE-2026-33804

CVE-2026-33804 is a high-severity vulnerability in Fastify Fastify/middie with a CVSS 3.x base score of 7.4. 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: High (CVSS 3.x base score 7.4)
EPSS exploit prediction: 0% (20th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-23235
Weakness: CWE-436
Affected product: Fastify Fastify/middie
Published: 2026-04-16
Last modified: 2026-06-17

Description

@fastify/middie versions 9.3.1 and earlier are vulnerable to middleware bypass when the deprecated Fastify ignoreDuplicateSlashes option is enabled. The middleware path matching logic does not account for duplicate slash normalization performed by Fastify's router, allowing requests with duplicate slashes to bypass middleware authentication and authorization checks. This only affects applications using the deprecated ignoreDuplicateSlashes option. Upgrade to @fastify/middie 9.3.2 to fix this issue. There are no workarounds other than disabling the ignoreDuplicateSlashes option.

Frequently asked questions

What is CVE-2026-33804?: @fastify/middie versions 9.3.1 and earlier are vulnerable to middleware bypass when the deprecated Fastify ignoreDuplicateSlashes option is enabled. The middleware path matching logic does not account for duplicate slash normalization performed by Fastify's router, allowing requests with duplicate slashes to bypass middleware authentication and authorization checks. This only affects applications using the deprecated ignoreDuplicateSlashes option. Upgrade to @fastify/middie 9.3.2 to fix this issue. There are no workarounds other than disabling the ignoreDuplicateSlashes option.
How severe is CVE-2026-33804?: CVE-2026-33804 has a CVSS 3.x base score of 7.4, rated high severity. It is exploitable over network with high attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability none.
Is CVE-2026-33804 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (20th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-33804?: CVE-2026-33804 affects Fastify Fastify/middie. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-33804?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
Does CVE-2026-33804 have an EU (EUVD) identifier?: Yes. CVE-2026-33804 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-23235.
When was CVE-2026-33804 published?: CVE-2026-33804 was published on 2026-04-16 and last updated on 2026-06-17.

References

Affected products (1)

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

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