CVE-2026-33804

criticalpriority · track · 36EPSS 0.003 · 20 %ileEUUS
grimoire priorityP2 · Scheduled
Exploitable?
—
no known exploitation
Likely / severe?
0.28% (20th) EPSS
CVSS 9.1 · critical
Is it mine?
3 products
Check your exposure
Who says so?
4 sources
cnnvd · enisa_euvd · nvd · osv

EPSS · scored 2026-10-27

@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.

01

cvss

Technical severity, 0–10. One row per scoring source.

9.1
010

base score 0–10 · higher = more severe · hover a vector to decode⚠ sources disagree · high–critical · 7.4–9.1

v3.19.1criticalCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:Nprimary · NVD / NIST USexploitability 3.9 · impact 5.2
v3.17.4highCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:Nsecondary · ENISA EU
v3.17.4highCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:Nsecondaryexploitability 2.2 · impact 5.2
v3.17.4highCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:Nsecondary · OSV
v3.18.3highCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N/E:Usecondary · CVSS-BT (t0sche)
02

ssvc

A stakeholder decision — act, attend, or track — from exploitation and impact.

Exploitation: none · Automatable: no · Technical Impact: totalcoordinatorcisa_ssvcUS
03

weaknesses

The CWE weakness classes this flaw belongs to.

CWE-436USCWE-436US
04

affected products (3)

Products and version ranges known to be affected.

05

advisories (1)

Vendor and ecosystem advisories describing this.

  • GHSA-v9ww-2j6r-98q6@fastify/middie vulnerable to middleware bypass via deprecated ignoreDuplicateSlashes option2026-04-16ghsa · osvUS

    ### Impact `@fastify/middie` v9.3.1 and earlier does not read the deprecated (but still functional) top-level `ignoreDuplicateSlashes` option, only reading from `routerOptions`. This creates a normalization gap: Fastify's router normalizes duplicate slashes but middie does not, allowing middleware bypass via URLs with duplicate leading slashes (e.g., `//admin/secret`). This only affects applications using the deprecated top-level configuration style (`fastify({ ignoreDuplicateSlashes: true })`). Applications using `routerOptions: { ignoreDuplicateSlashes: true }` are not affected. This is distinct from [GHSA-8p85-9qpw-fwgw](https://github.com/fastify/middie/security/advisories/GHSA-8p85-9qpw-fwgw) (CVE-2026-2880), which was patched in v9.2.0. ### Patches Upgrade to `@fastify/middie` >= 9.3.2. ### Workarounds Migrate from deprecated top-level `ignoreDuplicateSlashes: true` to `routerOptions: { ignoreDuplicateSlashes: true }`.

06

also known as (2)

Other names and identifiers for this entity.

07

sources

Which sources assert this record, and when.

cnnvdenisa_euvdnvdosvNVDCVE.orgMITRE
published 2026-04-16modified 2026-06-17
08

references (6)

External citations — patches, advisories, analysis.

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