CVE-2026-6556 json
@fastify/express vulnerable to middleware bypass via non-string mount paths in prefixed plugins
published 4mo ago modified 4mo ago also EUVD-2026-40309
- cvss
- 9.1
- version 3.1, CNA
- cisa kev
- Not listed
- not in CISA's catalog
- epss
- 0.5%
- EPSS 0.5% chance of exploitation in 30 days, percentile 44%
- public exploits
- 0
- none known
Summary
@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.
Patch / fix
- @fastify/express @fastify/express 4.0.7
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Check with FencerCISA SSVC
- Exploitation
- none
- Automatable
- yes
- Technical impact
- total
- Assessed
- 4mo ago
Exploit prediction
0.52% chance of exploitation in the next 30 days, higher than 44% of all scored CVEs (FIRST EPSS, 2026-11-11).
CVSS scores
| assigned by | version | score | vector |
|---|---|---|---|
| CNA | 3.1 | 9.1 | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N |
Affected products
- @fastify/express @fastify/express
Classification
- Weakness
-
- CWE-285 Improper Authorization
References
-
github.com
/fastify/fastify-express/security/advisories/GHSA-3wf5-7852-vcfq
mitigation, vendor-advisory
-
cna.openjsf.org
/security-advisories.html
vendor-advisory
Sources
- CVE List CVE-2026-6556
- NVD CVE-2026-6556
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