CVE-2026-33804
@fastify/middie vulnerable to middleware bypass via deprecated ignoreDuplicateSlashes option
CVE-2026-33804 is a high vulnerability (CVSS 7.4) in @fastify/middie, published April 16, 2026. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 0.28%, the 20th percentile. Fixed in @fastify/middie 9.3.2.
@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.
Attack profile
How easy it is to exploit
- Attack vector
- Network
- Complexity
- High
- Privileges needed
- None
- User interaction
- None
What an attacker can affect
- Confidentiality
- High
- Integrity
- High
- Availability
- None
Exploit likelihood
What each source reports
CVE List Score used
@fastify/middie vulnerable to middleware bypass via deprecated ignoreDuplicateSlashes option
@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.
- Record
CVE-2026-33804- CVSS 3.1
- 7.4 High
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N - Weakness
- CWE-436
- Updated
- 2026-11-02
EUVDEUVD-2026-23235
@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.
- Record
EUVD-2026-23235- CVSS 3.1
- 7.4 High
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N - Updated
- 2026-11-02
GitHub AdvisoriesGHSA-v9ww-2j6r-98q6
@fastify/middie vulnerable to middleware bypass via deprecated ignoreDuplicateSlashes option
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 (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 }.
- Record
GHSA-v9ww-2j6r-98q6- CVSS 3.1
- 7.4 High
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N - Weakness
- CWE-436
- Updated
- 2026-11-02
NVD
@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.
- Record
CVE-2026-33804- CVSS 3.1
- 7.4 High
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N - CVSS 3.1
- 9.1 Critical
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N - Weakness
- CWE-436
- Updated
- 2026-11-02
OSV
@fastify/middie vulnerable to middleware bypass via deprecated ignoreDuplicateSlashes option
@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.
- Record
CVE-2026-33804- CVSS 3.1
- 7.4 High
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N - Weakness
- CWE-436
- Updated
- 2026-11-02
OSVGHSA-v9ww-2j6r-98q6
@fastify/middie vulnerable to middleware bypass via deprecated ignoreDuplicateSlashes option
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 (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 }.
- Record
GHSA-v9ww-2j6r-98q6- CVSS 3.1
- 7.4 High
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N - Weakness
- CWE-436
- Updated
- 2026-11-02
OSVCLSA-2026-1784856385 No score
TuxCare security update for @fastify/middie (3 CVEs)
TuxCare rebuilt @fastify/middie to address 3 CVEs. Fixed in 8.3.3-tuxcare.2.
- Record
CLSA-2026-1784856385- Updated
- 2026-11-02
Affected products
Affected packages
| Package | Affected | Fixed |
|---|---|---|
@fastify/middienpm |
through 9.3.1 | 9.3.2 |
@fastify/middieTuxCare:npm |
8.3.3-tuxcare.0 up to 8.3.3-tuxcare.2, plus listed versions 8.3.38.3.3-tuxcare.1 |
8.3.3-tuxcare.2 |
@fastify/middienpm |
before 9.3.2 | 9.3.2 |
History
| When | Changed | Sources |
|---|---|---|
| 2026-11-02 | created | CVE List, EUVD, GitHub Advisories, NVD, OSV |