CVE-2026-6556: Fastify Fastify/express broken access control (CVSS 9.1)

Description

@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 plugin scopes, so middleware registered with those forms does not match the actual prefixed request path. Applications that use path-scoped middleware for authentication, authorization, rate limiting, or auditing on routes inside a prefixed scope can be bypassed by sending a request to the prefixed route, because Fastify still matches the route but the middleware is skipped. Patches: upgrade to @fastify/express 4.0.7. Workarounds: use string mount paths instead of arrays or regular expressions in prefixed plugins, or register one use call per path.

CVSS 3.1 breakdown

Attack vectorNetwork

Attack complexityLow

Privileges requiredNone

User interactionNone

ScopeUnchanged

ConfidentialityHigh

IntegrityHigh

AvailabilityNone

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

Affected products

Fastify Fastify/express< 4.0.7

Remediation

Fixed in

Fastify Fastify/express≥ 4.0.7

Patch & vendor advisories

References

https://cna.openjsf.org/security-advisories.htmlVendor Advisory
https://github.com/fastify/fastify-express/security/advisories/GHSA-3wf5-7852-vcfqMitigationVendor Advisory

Related CVEs

Recent high-severity vulnerabilities sharing a weakness type.

Frequently asked

Is CVE-2026-6556 being actively exploited?

CVE-2026-6556 is not in the CISA KEV catalog. Its EPSS exploit probability is 0.52%.

How severe is CVE-2026-6556?

CVE-2026-6556 has a CVSS 3.1 base score of 9.1, rated critical. EPSS estimates a 0.52% probability of exploitation.

Which products are affected by CVE-2026-6556?

Affected products include Fastify Fastify/express.

How do I fix CVE-2026-6556?

Upgrade affected software to a fixed version, for example Fastify Fastify/express 4.0.7 or later, then confirm no affected versions remain in your inventory.

How Secuno handles this

Secuno correlates vulnerabilities like CVE-2026-6556 against the real software inventory on every managed device, every day. Rather than a static severity, each match is scored in context: exploit availability, EPSS, the asset's exposure, and business criticality decide where it lands in the queue. If an affected version is present anywhere across your portfolio, it surfaces automatically with a prioritised remediation path.

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