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Denial of Service in @fastify/middie standalone engine URL normalization

IdentifiersCVE-2026-14181CWE-248

CVE-2026-14181 affects @fastify/middie versions 9.1.0 through 9.3.2. The vulnerability is in the URL normalization step used by the standalone engine API when processing incoming request paths containing malformed percent-encoded sequences. Inputs such as incomplete percent escapes or truncated multibyte sequences cause the underlying decoder to throw a synchronous exception. In the standalone engine path, this exception is not guarded or caught within middie’s normalize step, allowing it to escape and terminate the Node.js process. The issue affects applications that invoke middie.run directly on the standalone engine API. Applications using the Fastify plugin integration are not affected because Fastify’s error handling catches the exception.

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ANALYST BRIEF

Impact, mitigation & remediation

What it means. What to do now. Patch path, mitigations, and the assume-compromise checklist.

Impact

What an attacker gets, and what they’ve been doing with it.

A remote attacker can trigger an immediate denial of service by sending a crafted HTTP request path containing malformed percent-encoded data to a vulnerable application using the standalone engine API. Successful exploitation crashes the Node.js process, interrupting service for all connected and subsequent clients until the process is restarted or otherwise recovered by supervision tooling.

Mitigation

If you can’t patch tonight, do this now.

If immediate patching is not possible, avoid use of the standalone engine API and migrate to the Fastify plugin path, where Fastify’s error handler catches the exception. As an additional defensive measure, place strict request validation or filtering in front of the application to reject malformed percent-encoded request paths before they reach the vulnerable normalization logic, though this is only a workaround and not a substitute for upgrading.

Remediation

Patch, then assume compromise.

Upgrade @fastify/middie to version 9.3.3 or later, which patches the unhandled exception condition in the standalone engine URL normalization path.
PUBLIC EXPLOITS

Exploits

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VALID 0 / 0 TOTALView more in app

No public exploit code observed for this vulnerability.

ACTIVITY FEED

Recent activity

9 sources tracked across advisories and community write-ups. News coverage will land here when it surfaces.

9 SOURCESView more in app

No news coverage yet. Advisories and community discussion only.

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Exposure mapping

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Threat actor evidence

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Associated malware

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Detection signatures

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Vendor-by-vendor mapping

Cross-references every affected SKU, including bundled OEM variants.

Social activity9

Community discussion across Reddit, Mastodon, and other social sources.

EXTERNAL REFERENCES
MITRE CVE
cve.org · CVE-2026-14181
NVD
nvd.nist.gov/vuln/detail/CVE-2026-14181
MITRE CWE · CWE-248
cwe.mitre.org