dbcve.org
Fastify\/middieApplication · Fastify

CVE-2026-14198

CRITICAL · 9.1 CVSS v3.1 Published 2026-07-01
Fix available
A fix is available. Upgrade to 9.3.3 or later.
See remediation →
100/100
Remediation priority · Urgent
Remotely reachable No privileges Zero-click 8 weeks old

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/middie versions 9.1.0 through 9.3.2 decode the encoded slash %2F inside path parameter values before matching middleware paths, while Fastify's underlying router preserves the encoding during route lookup. The two layers disagree on the canonical request path, so the middleware fails to match a URL that the route handler does match. When middleware is used for authentication, authorization, rate limiting, or auditing on parameterized paths, an attacker can reach the protected handler by sending a single crafted URL with an encoded slash in the parameter position. The bypass is HTTP method agnostic and requires no authentication or special preconditions. Patches: upgrade to @fastify/middie 9.3.3. Workarounds: avoid parameterized middleware paths for security decisions, or enforce authentication at the route handler or via a Fastify hook that runs after the router has resolved the request.

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 confidence

@fastify/middie decodes %2F in path parameter values before middleware path matching, while Fastify's router preserves the encoding. This inconsistency causes middleware to fail matching URLs that the route handler successfully matches, allowing attackers to bypass security controls like authentication, authorization, rate limiting, or auditing on parameterized routes.

MitigationUpgrade @fastify/middie to version 9.3.3 or later. As a workaround, avoid using parameterized middleware paths for security decisions and enforce authentication at the route handler level or via Fastify hooks that run after route resolution.

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
Fastify\/middieApplication
Affected:>= 9.1.0, < 9.3.3

CVSS 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 checks

Work through these to decide whether this CVE applies to you.

  1. Identify @fastify/middie installation
    Run npm list @fastify/middie or check package.json dependencies to see if @fastify/middie is installed
    Affected if @fastify/middie is present in the dependencies
  2. Check installed version
    Run npm list @fastify/middie to get the exact version number, then compare to the affected range >= 9.1.0 and < 9.3.3
    Affected if The installed version falls within >= 9.1.0 and < 9.3.3
  3. Inspect middleware path configurations
    Search codebase for middie.register or app.use calls with parameterized paths such as /:param or /:id patterns that handle security-sensitive operations
    Affected if Parameterized middleware paths are used for authentication, authorization, rate limiting, or auditing
  4. Identify security controls in middleware
    Review middleware code to determine if security decisions (auth checks, rate limits, access control) are enforced at the middleware level on parameterized routes
    Affected if Security controls are enforced in middleware that matches against parameterized paths

You are affected if @fastify/middie version 9.1.0 through 9.2.x is installed AND you have parameterized middleware paths handling security-sensitive operations.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

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 · scoped
Upgrade available Upgrade to 9.3.3 or later
Fixed in 9.3.3
Interim mitigation

Upgrade @fastify/middie to version 9.3.3 or later. As a workaround, avoid using parameterized middleware paths for security decisions and enforce authentication at the route handler level or via Fastify hooks that run after route resolution.

Recommended fix High confidence

@fastify/middie version 9.3.3

  1. Check the current version of @fastify/middie in your project (npm list @fastify/middie or look in package.json)
  2. Update @fastify/middie to version 9.3.3 using your package manager (npm install @fastify/[email protected], yarn upgrade @fastify/[email protected], or pnpm update @fastify/[email protected])
  3. Run npm install or your lockfile update command to ensure the new version is installed
  4. Verify the installed version is 9.3.3 (npm list @fastify/middie)
  5. Test your application to confirm middleware path matching works correctly, including routes with encoded slashes in parameters

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Fastify\/middie Scoped from the published advisory
  • Consultation2.0 h
  • Implementation2.0 h
  • Testing2.0 h
  • Review / QA1.0 h
7.0 hours of engineering $1,240
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-14198 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data