CVE-2026-33804
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 9.3.1 and earlier are vulnerable to middleware bypass when the deprecated Fastify ignoreDuplicateSlashes option is enabled. The middleware path matching logic does not account for duplicate slash normalization performed by Fastify's router, allowing requests with duplicate slashes to bypass middleware authentication and authorization checks. This only affects applications using the deprecated ignoreDuplicateSlashes option. Upgrade to @fastify/middie 9.3.2 to fix this issue. There are no workarounds other than disabling the ignoreDuplicateSlashes option.
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 versions 9.3.1 and earlier have a middleware bypass vulnerability when the deprecated ignoreDuplicateSlashes option is enabled. The middleware path matching logic fails to account for duplicate slash normalization performed by Fastify's router, allowing requests with duplicate slashes (e.g., //admin) to bypass authentication and authorization checks that middleware is supposed to enforce.
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< 9.3.2CVSS 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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Identify @fastify/middie versionCheck your package.json or run 'npm list @fastify/middie' to determine the installed versionAffected if The installed version is 9.3.1 or earlier (< 9.3.2)
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Locate Fastify configuration fileExamine your Fastify initialization code (typically the main server file) for the fastify.register call for @fastify/middie and any Fastify configuration optionsAffected if The ignoreDuplicateSlashes option is set to true in the Fastify configuration
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Verify middleware is in useCheck if @fastify/middie is registered in your application and if authentication/authorization middleware is mounted on path-based routesAffected if Middie middleware handles request routing and authentication is applied to specific paths
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Test for slash normalization behaviorSend a test request with duplicate slashes (e.g., GET //admin) to an endpoint that should be protected by middlewareAffected if The request with duplicate slashes successfully reaches a protected endpoint that would normally require authentication
You are affected if @fastify/middie version is below 9.3.2 AND ignoreDuplicateSlashes is enabled in your Fastify configuration, allowing requests with duplicate slashes to bypass middleware protection.
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 · scoped9.3.2
Upgrade to @fastify/middie version 9.3.2 or later. There are no workarounds except disabling the deprecated ignoreDuplicateSlashes option in the Fastify configuration.
@fastify/middie 9.3.2
- Update @fastify/middie to version 9.3.2 or later using your package manager (e.g., npm install @fastify/middie@^9.3.2, yarn add @fastify/middie@^9.3.2, or pnpm add @fastify/middie@^9.3.2)
- Verify the installed version matches 9.3.2 or higher
- Run your application's test suite to ensure the middleware bypass vulnerability is resolved
- After upgrading, consider removing the deprecated ignoreDuplicateSlashes option from your Fastify configuration as it is no longer needed and poses security risks
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation2.0 h
- Implementation2.0 h
- Testing2.0 h
- Review / QA1.0 h
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Free · runs locallyCheck whether your project pulls in CVE-2026-33804 — 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-2026-33804 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
No notes yet
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- Version or environment caveats, and links to real fixes
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