CVE-2026-33804Network attack vector

Authentication bypass via duplicate-slash normalization mismatch

Published Apr 16, 2026 · Updated Apr 16, 2026

Authentication bypass in Fastify @fastify/middie 9.3.1 and earlier allows remote attackers to skip path-scoped middleware. The plugin reads ignoreDuplicateSlashes only from routerOptions, so top-level configuration leaves middie matching raw paths while Fastify normalizes duplicate slashes. The flaw is reachable only with the deprecated top-level option enabled, and crafted duplicate-slash requests can evade authentication or authorization middleware.

CVSS severity7.4
High
EPSS probability0.28%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Authentication bypass in Fastify @fastify/middie 9.3.1 and earlier allows remote attackers to skip path-scoped middleware. The plugin reads ignoreDuplicateSlashes only from routerOptions, so top-level configuration leaves middie matching raw paths while Fastify normalizes duplicate slashes. The flaw is reachable only with the deprecated top-level option enabled, and crafted duplicate-slash requests can evade authentication or authorization middleware.

The record

CVE
CVE-2026-33804
Published
Apr 16, 2026
Updated
Apr 16, 2026
Vendor
Fastify
Product
@fastify/middie
Classifications
CWE-436, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 16, 2026CVE publishedPublication date reported by the CVE source.
  2. Apr 16, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=0 <9.3.2

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Fastify · @fastify/middieversion=0 <9.3.2

Published CVSS scores

9.1NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
7.4OpenJS FoundationCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-436Interpretation Conflict
T1190Exploit Public-Facing Application

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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