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TI-2026-17731 Aliases CVE-2026-22037GHSA-g6q3-96cp-5r5m

@fastify/express vulnerable to Improper Handling of URL Encoding (Hex Encoding)

Priority HIGH 72CWE-177Published · Updated

Key facts

Attack vector
Network
Privileges required
Low
User interaction
None
Attack complexity
High
Impact type
Security bypass
Confidentiality · integrity · availability
High · High · Low
Fix status
Fixed

Attack profile from CVSS 3.1 · CNA. Fix status from CNA and GitHub. Impact type from CWE-288.

Summary

The @fastify/express plugin adds full Express compatibility to Fastify. A security vulnerability exists in @fastify/express prior to version 4.0.3 where middleware registered with a specific path prefix can be bypassed using URL-encoded characters (e.g., `/%61dmin` instead of `/admin`). While the middleware engine fails to match the encoded path and skips execution, the underlying Fastify router correctly decodes the path and matches the route handler, allowing attackers to access protected endpoints without the middleware constraints. The vulnerability is caused by how @fastify/express matches requests against registered middleware paths. This vulnerability is similar to, but differs from, CVE-2026-22031 because this is a different npm module with its own code. Version 4.0.3 of @fastify/express contains a patch fort the issue.

Why it matters now

Exploitation Proof of concept CISA SSVC · not in CISA KEV
EPSS 0.00349 28th percentileScored 2026-11-07
Fix Fixed fastify-express (fastify): 4.0.3@fastify/express (npm): 4.0.3

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation
Poc
Automatable
No
Technical impact
Total

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

VendorProductAffected versionsFixed inStatusSource
fastifyfastify-expressbefore 4.0.34.0.3Patch availableCNA
—@fastify/express (npm)before 4.0.34.0.3Patch availableGHSA-g6q3-96cp-5r5m
  • fastify-express Vendorfastify Affected before 4.0.3 Fixed in 4.0.3 Patch available CNA
  • @fastify/express (npm) Vendor— Affected before 4.0.3 Fixed in 4.0.3 Patch available GHSA-g6q3-96cp-5r5m

Remediation

  • Fix · CNA fastify-express (fastify) Fixed in 4.0.3
  • Fix · GHSA-g6q3-96cp-5r5m @fastify/express (npm) Fixed in 4.0.3 Advisory →

Timeline

  1. 2026-10-02 Threatist record created from CVE-2026-22037
  2. 2026-10-03 Alias added: GHSA-g6q3-96cp-5r5m

References

Advisory 1

Other 4

  • TI-2026-26180 @fastify/express vulnerable to middleware authentication bypass via URL normalization… CVSS 4.0CRITICAL 9.1
  • TI-2026-26179 @fastify/express vulnerable to middleware path doubling causing authentication bypass in child plugin scopes CVSS 3.1CRITICAL 9.1
  • TI-2026-26177 @fastify/reply-from vulnerable to connection header abuse enabling stripping of proxy-added headers CVSS 4.0CRITICAL 9.0
  • TI-2026-05498 @fastify/middie vulnerable to middleware authentication bypass in child plugin scopes CVSS 3.1CRITICAL 9.1
  • TI-2023-15307 Cross-site request forgery in npm @fastify/oauth2 CVSS 3.1HIGH 8.8
  • TI-2022-18664 Denial of service in Fastify via Content-Type header CVSS 3.1HIGH 7.5
See all 76 →

Sources

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