CVE-2026-33804 | BDVDB-2026-327407 Detail

@fastify/middie@fastify/middie

Description#

@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.

Impact#

Per CVSS v3.1: confidentiality impact high, integrity impact high, availability impact none. Attack vector: network.

Weaknesses#

CWE-436 Interpretation Conflict

References#

Exploit#

No public exploit known

History#

Version 12026-04-16Record published in the CVE List.

Published

Published: 2026-04-16
Last modified: 2026-04-16

Vendor information

Vendor: @fastify/middie
Product: @fastify/middie

All @fastify/middie vulnerabilities

Score

7.4 High

EPSS

less than 1%, Very Low

Estimated chance of exploitation in the next 30 days. EPSS percentile: 20.4.

KEV

Not listed in the CISA Known Exploited Vulnerabilities catalog.

Impact

Not classified

Action

No specific action is recorded in the source data. Check the references for vendor guidance.

Affected Systems

Vulnerable
< 9.3.2
Fixed
9.3.2
Vulnerable
Not stated
Fixed
9.3.2

Scoring System

CVSS v4.0

Not available

CVSS v3.1

7.4 High

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N

Metric breakdown
Attack Vector	Network
Attack Complexity	High
Privileges Required	None
User Interaction	None
Scope	Unchanged
Confidentiality	High
Integrity	High
Availability	None

CVSS v3.0

Not available

CVSS v2

Not available

Source: CVE List v5 (record); CISA KEV (kev, action); FIRST EPSS (epss).