VulnerabilityAnalyzed

CVE-2026-33808

This allows complete bypass of path-scoped authentication middleware via duplicate slashes when ignoreDuplicateSlashes is enabled, or via semicolon delimiters when useSemicolonDelimiter is enabled.

CRITICAL 9.1EPSS 0.48%

Does this matter?

High impact if exploited, but EPSS currently rates exploitation as unlikely (0.48%). Schedule it in the normal patch cycle and watch for a rise in EPSS or a public exploit.

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.

CVSS 4.0: 9.1 CRITICALCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/M
EPSS: 0.48% probability · 41th percentile
Public exploits: None in Exploit-DB
CISA KEV: Not listed
Weakness: CWE-436
Affected: fastify/fastify\/express
Source: ce714d77-add3-4f53-aff5-83d477b104bb

EPSS trend

EPSS 0.5% since 26 October 2026; no change recorded yet. Points are recorded when the score first appears and whenever it moves by a percentage point or more.

References

https://cna.openjsf.org/security-advisories.htmlVendor Advisory
https://github.com/fastify/fastify-express/security/advisories/GHSA-6hw5-45gm-fj88Exploit, Mitigation, Vendor Advisory
https://github.com/fastify/fastify-express/security/advisories/GHSA-6hw5-45gm-fj88Exploit, Mitigation, Vendor Advisory

Source: NVD record, EPSS from FIRST.org, KEV from CISA, exploits from Exploit-DB. Refreshed daily. Download this record as JSON.