CVE-2026-6556
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · unedited@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.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceThe @fastify/express plugin fails to rewrite the plugin prefix for middleware mount paths when they are arrays or regular expressions. Only string paths get the prefix applied. This causes middleware using non-string paths in prefixed plugin scopes to be skipped entirely, allowing attackers to bypass authentication, authorization, rate limiting, or auditing controls.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data< 4.0.7CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- High
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Check installed @fastify/express versionRun 'npm list @fastify/express' or check package.json dependenciesAffected if Version is below 4.0.7 (e.g., 4.0.6, 4.0.5, etc.)
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Identify use of prefixed plugin scopesSearch codebase for fastify.register() or fastify.plugin() calls that include a 'prefix' option (e.g., fastify.register(plugin, { prefix: '/api' }))Affected if Any plugins are registered with a prefix option
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Identify middleware using array or regex paths in prefixed scopesSearch for app.use() or equivalent middleware calls inside prefixed plugin scopes that use arrays (e.g., ['/path1', '/path2']) or regular expressions (e.g., /\/api\/.*/) as mount pathsAffected if Non-string middleware paths exist within a prefixed plugin scope
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Verify middleware execution in prefixed routesTest or review logs to confirm that middleware mounted with array/regex paths in prefixed scopes is NOT being executed for requests to prefixed routesAffected if Middleware with array/regex paths is being skipped in prefixed plugin contexts
You are affected if @fastify/express version is below 4.0.7 AND your application uses middleware with array or regex mount paths inside plugin scopes that have a prefix option configured.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scoped4.0.7
Upgrade @fastify/express to version 4.0.7 or later. If immediate upgrading is not possible, refactor middleware to use string mount paths instead of arrays or regular expressions within prefixed plugin scopes.
@fastify/express 4.0.7
- Check current @fastify/express version in project dependencies (package.json or package-lock.json)
- Update @fastify/express dependency to version 4.0.7 using package manager (npm install @fastify/[email protected] or yarn add @fastify/[email protected])
- Reinstall dependencies to ensure the new version is applied
- Verify the application starts without errors
- Test all routes within prefixed plugin scopes to confirm middleware (authentication, authorization, rate limiting, auditing) is now properly applied to non-string mount paths (arrays and regular expressions)
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation2.0 h
- Implementation2.0 h
- Testing3.0 h
- Review / QA1.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,224.
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Free · runs locallyCheck whether your project pulls in CVE-2026-6556 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-6556 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
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- Version or environment caveats, and links to real fixes
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