CVE-2026-6556

CRITICALPre-NVD 9.19.1—
EchelonGraph scoreMEDIUM confidence

This critical-severity CVE scores 9.1 under a secondary CVSS source (NVD's own analysis pending). EPSS exploit probability: 0.3%, top 79% of all CVEs by exploit prediction. GitHub Security Advisory data not yet ingested — confidence will rise once GHSA publishes (typical lag: hours to days for open-source ecosystem CVEs; never for infrastructure-only CVEs).

Triggered by: NVD CVSS baseline
Sources: epss, secondary
9.1
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS: 0%CVSS: 9.1Exploit: NoneExposed: 0

No vendor fix yet — apply a workaround or compensating control (WAF / firewall / segmentation) and watch for a patch.

@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 v3
9.1
EG Score
9.1(medium)
EPSS
21.4%
KEV
Not listed

Published

June 30, 2026

Last Modified

July 1, 2026

Advisory Details (2)

Auto-updated Jun 30, 2026
Patch available. Sources: github.
github Patch Available

@fastify/express vulnerable to middleware bypass via non-string mount paths in prefixed plugins · Advisory · fastify/fastify-express · GitHub

https://github.com/fastify/fastify-express/security/advisories/GHSA-3wf5-7852-vcfq
generic

Security Advisories | OpenJS Foundation CVE Numbering Authority

https://cna.openjsf.org/security-advisories.html

Weakness Classification(1)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

Data Freshness Timeline

(refreshed 56× in last 7d / 97× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

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  72. 2026-06-30 14:32 UTCNVD updatefirst tracked

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Frequently asked(5)

What is CVE-2026-6556?
CVE-2026-6556 is a critical vulnerability published on June 30, 2026. @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…
When was CVE-2026-6556 disclosed?
CVE-2026-6556 was first published in the National Vulnerability Database on June 30, 2026, with the most recent update on July 1, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2026-6556 actively exploited?
CVE-2026-6556 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 21.4% percentile likelihood of exploitation in the next 30 days — higher percentiles indicate greater predicted risk.
What is the CVSS score of CVE-2026-6556?
CVE-2026-6556 has a CVSS v3 base score of 9.1 (NVD).
How do I remediate CVE-2026-6556?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2026-6556, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

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