What is CVE-2026-6556?

CVE-2026-6556 is a critical-severity vulnerability in @Fastify/express, classified under Improper Authorization. CVSS score: 9.1/10. Published 2026-06-30.

How severe is CVE-2026-6556?

Critical severity. CVSS v3 base score is 9.1 out of 10.

Auth bypass in @Fastify/express

CVE-2026-6556

@fastify/express versions 4.0.6 and earlier only rewrite the plugin prefix for middleware mount paths when the path argument is a string. Non-string mount paths (arrays of paths and regular expressions) are left unprefixed inside prefixed…

CVSS v3 metric

CVSS v3 base score 9.1 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N.

Affected products

@Fastify/express — versions 0, 4.0.7

Weakness classification (CWE)

CWE-285 — Improper Authorization

References

ce714d77-add3-4f53-aff5-83d477b104bb
ce714d77-add3-4f53-aff5-83d477b104bb

Frequently asked questions

What is CVE-2026-6556?: CVE-2026-6556 is a critical-severity vulnerability in @Fastify/express, classified under Improper Authorization. CVSS score: 9.1/10. Published 2026-06-30.
How severe is CVE-2026-6556?: Critical severity. CVSS v3 base score is 9.1 out of 10.