CVE-2026-14198 json
@fastify/middie vulnerable to authorization bypass via encoded slash in path parameter values
published 4mo ago modified 4mo ago also EUVD-2026-40946
- cvss
- 9.1
- version 3.1, CNA
- cisa kev
- Not listed
- not in CISA's catalog
- epss
- 0.5%
- EPSS 0.5% chance of exploitation in 30 days, percentile 44%
- public exploits
- 0
- none known
Summary
@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.
Patch / fix
- @fastify/middie @fastify/middie 9.3.3
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Check with FencerCISA SSVC
- Exploitation
- none
- Automatable
- yes
- Technical impact
- total
- Assessed
- 4mo ago
Exploit prediction
0.52% chance of exploitation in the next 30 days, higher than 44% of all scored CVEs (FIRST EPSS, 2026-11-09).
CVSS scores
| assigned by | version | score | vector |
|---|---|---|---|
| CNA | 3.1 | 9.1 | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N |
Affected products
- @fastify/middie @fastify/middie
Classification
- Weakness
-
- CWE-436 Interpretation Conflict
References
-
github.com
/fastify/middie/security/advisories/GHSA-2v46-jxjm-7q3v
mitigation, vendor-advisory
-
cna.openjsf.org
/security-advisories.html
vendor-advisory
Sources
- CVE List CVE-2026-14198
- NVD CVE-2026-14198
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