How severe is CVE-2026-33808?

CVE-2026-33808 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-2026-33808 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (38th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-33808?

CVE-2026-33808 affects Fastify Fastify/express. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-33808?

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-2026-33808 have an EU (EUVD) identifier?

Yes. CVE-2026-33808 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-22881.

When was CVE-2026-33808 published?

CVE-2026-33808 was published on 2026-04-15 and last updated on 2026-06-17.

CVE-2026-33808

CVE-2026-33808 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)
CVSS v4: 9.1
EPSS exploit prediction: 0% (38th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-22881
Weakness: CWE-436
Affected product: Fastify Fastify/express
Published: 2026-04-15
Last modified: 2026-06-17

Description

Impact@fastify/express v4.0.4 and earlier fails to normalize URLs before passing them to Express middleware when Fastify router normalization options are enabled. This allows complete bypass of path-scoped authentication middleware via duplicate slashes when ignoreDuplicateSlashes is enabled, or via semicolon delimiters when useSemicolonDelimiter is enabled. In both cases, Fastify router normalizes the URL and matches the route, but @fastify/express passes the original un-normalized URL to Express middleware, which fails to match and is skipped. An unauthenticated attacker can access protected routes by manipulating the URL path. PatchesUpgrade to @fastify/express v4.0.5 or later.

Frequently asked questions

What is CVE-2026-33808?: Impact@fastify/express v4.0.4 and earlier fails to normalize URLs before passing them to Express middleware when Fastify router normalization options are enabled. This allows complete bypass of path-scoped authentication middleware via duplicate slashes when ignoreDuplicateSlashes is enabled, or via semicolon delimiters when useSemicolonDelimiter is enabled. In both cases, Fastify router normalizes the URL and matches the route, but @fastify/express passes the original un-normalized URL to Express middleware, which fails to match and is skipped. An unauthenticated attacker can access protected routes by manipulating the URL path. PatchesUpgrade to @fastify/express v4.0.5 or later.
How severe is CVE-2026-33808?: CVE-2026-33808 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-2026-33808 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (38th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-33808?: CVE-2026-33808 affects Fastify Fastify/express. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-33808?: 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-2026-33808 have an EU (EUVD) identifier?: Yes. CVE-2026-33808 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-22881.
When was CVE-2026-33808 published?: CVE-2026-33808 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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