CVE-2026-33804 json
@fastify/middie vulnerable to middleware bypass via deprecated ignoreDuplicateSlashes option
published 2026-04-16 6 mo ago modified 2026-06-17 4 mo ago also GHSA-v9ww-2j6r-98q6 , EUVD-2026-23235
- cvss
- 7.4
- version 3.1, CNA
- cisa kev
- Not listed
- not in CISA's catalog
- epss
- 0.3%
- EPSS 0.3% chance of exploitation in 30 days, percentile 20%
- public exploits
- 0
- none known
Summary
@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.
Patch / fix
- npm @fastify/middie 9.3.2
- @fastify/middie @fastify/middie 9.3.2
CISA SSVC
- Exploitation
- none
- Automatable
- no
- Technical impact
- total
- Assessed
- 6 mo ago
Exploit prediction
0.28% chance of exploitation in the next 30 days, higher than 20% of all scored CVEs (FIRST EPSS, 2026-11-08).
CVSS scores
| assigned by | version | score | vector |
|---|---|---|---|
| CNA | 3.1 | 7.4 | CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N |
| NVD | 3.1 | 9.1 | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N |
| GitHub | 3.1 | 7.4 | CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N |
Affected packages
| ecosystem | package | affected | fixed in |
|---|---|---|---|
| npm | @fastify/middie | <9.3.2 |
9.3.2 |
Affected products
- @fastify/middie @fastify/middie
Classification
- Weakness
-
- CWE-436 Interpretation Conflict
References
-
github.com
/fastify/middie/security/advisories/GHSA-v9ww-2j6r-98q6
web, mitigation, vendor-advisory
-
cna.openjsf.org
/security-advisories.html
web, vendor-advisory
-
nvd.nist.gov
/vuln/detail/CVE-2026-33804
advisory
-
github.com
/fastify/middie
package
Sources
- CVE List CVE-2026-33804
- GitHub Advisory GHSA-v9ww-2j6r-98q6
- NVD CVE-2026-33804
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