CVE-2026-33808: @fastify/express has a middleware authentication bypass via URL normalization gaps (duplicate slashes and semicolons)

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• Not in CISA KEV, but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
• EPSS 0.6%, percentile 45: FIRST’s model estimates a 0.6% chance of exploitation activity in the next 30 days, higher than 45% of all scored CVEs (low). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Only applies if your dependency tree includes @fastify/express (npm) <= 4.0.4, fixed in 4.0.5. Check your lockfile, including transitive dependencies.

What it is

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.

From the GitHub advisory

Summary

@fastify/express v4.0.4 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 two vectors:

1. Duplicate slashes (//admin/dashboard) when ignoreDuplicateSlashes: true is configured 2. Semicolon delimiters (/admin;bypass) when useSemicolonDelimiter: true is configured

In both cases, Fastify's 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.

Note: This is distinct from GHSA-g6q3-96cp-5r5m (CVE-2026-22037), which addressed URL percent-encoding bypass and was patched in v4.0.3. These normalization gaps remain in v4.0.4. A similar class of normalization issue was addressed in @fastify/middie via GHSA-8p85-9qpw-fwgw (CVE-2026-2880), but @fastify/express does not include the equivalent fixes.

Weakness type: CWE-436.

Product: @fastify/express (npm) · Assigned by: openjs · Published: 2026-04-15

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Attack requirements: present (depends on deployment conditions)
• Privileges required: none (no login)
• User interaction: none

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation poc, automatable yes, technical impact total.
• FIRST EPSS (model of 2026-10-21): 0.55% probability of exploitation activity in the next 30 days; percentile 45.

Timeline

• 2026-04-15 — CVE record last updated
• 2026-04-15 — CVE record published by openjs
• 2026-04-16 — GitHub-reviewed advisory GHSA-6hw5-45gm-fj88 published (critical)

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-6hw5-45gm-fj88 github.com
2. CVE.org record CVE-2026-33808 cve.org
3. NVD entry CVE-2026-33808 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. cna.openjsf.org cna.openjsf.org

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