CVE-2026-14181: @fastify/middie standalone engine vulnerable to Denial of Service via malformed percent-encoded paths
@fastify/middie versions 9.1.0 through 9.3.2 fail to guard the URL normalization step used by the standalone engine when incoming request paths contain malformed percent-encoded sequences. Inputs such as an incomplete percent escape or a truncated multibyte sequence cause the underlying decoder to throw synchronously, and the exception escapes the middie normalize step and terminates the Node.js process. The bypass affects applications that call middie.run directly on the standalone engine API, causing an immediate denial of service for all connected clients until restart. Applications using the Fastify plugin path are not affected because Fastifys error handler catches the exception. Patches: upgrade to @fastify/middie 9.3.3. Workarounds: migrate from the standalone engine API to the Fastify plugin path, where the framework error handler catches the exception.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
@fastify/middieto a version that resolves this vulnerability.Fixed in 9.3.3 - Configuration
If applicable, migrate from the standalone engine API (middie.run) to the Fastify plugin path so Fastify’s error handler catches the synchronous exception thrown during URL normalization.
Fastify application routing/error handling Request handling path = migrate to Fastify plugin path (avoid standalone middie.run) - Operational
Because the denial of service can terminate the Node.js process, restart the affected service after remediation.
Event History
Frequently Asked Questions
What is the severity of CVE-2026-14181?
The severity of CVE-2026-14181 is rated as high with a CVSS score of 7.5.
How do I fix CVE-2026-14181?
To fix CVE-2026-14181, upgrade @fastify/middie to version 9.3.3 or later.
What types of attacks does CVE-2026-14181 expose applications to?
CVE-2026-14181 exposes applications to Denial of Service attacks using malformed percent-encoded paths.
Which versions of @fastify/middie are affected by CVE-2026-14181?
Versions 9.1.0 through 9.3.2 of @fastify/middie are affected by CVE-2026-14181.
What is the impact of CVE-2026-14181 on system availability?
CVE-2026-14181 can lead to system unavailability due to Denial of Service conditions caused by malformed input.