CVE-2026-117531 @fastify/multipart CVE debrief

Who should care

Defenders and developers using the @fastify/multipart plugin in their Fastify applications should assess their exposure and upgrade to version 10.1.7 or later to prevent potential DoS attacks.

Why it matters

CVE-2026-117531 is a vulnerability in the @fastify/multipart plugin for the Fastify web framework that can lead to a denial-of-service (DoS) attack. Defenders and developers using this plugin should assess their exposure and upgrade to version 10.1.7 or later to prevent potential DoS attacks. The impact is availability only, and exploitation depends on the application using a delayed file consumer.

• Denial-of-service (DoS) attack possible
• Server resource exhaustion possible
• Application availability impacted

Technical summary

The @fastify/multipart plugin for the Fastify web framework does not propagate parser errors to the file stream when a request body ends inside a file part with no closing boundary. This can lead to a denial-of-service (DoS) attack, where an unauthenticated remote client can accumulate pending connections and exhaust server resources. The issue arises because the plugin destroys the in-progress file stream without an error when a request body ends inside a file part with no closing boundary, even though the Content-Length is complete and no file size limit is exceeded. An unauthenticated remote client can repeat these malformed uploads against a route that uses a delayed file consumer to accumulate pending

Defensive priority

Medium

Recommended defensive actions

• Upgrade to @fastify/multipart version 10.1.7 or later
• Review and update affected applications using the vulnerable plugin
• Monitor server resources for potential DoS attacks
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up
• Review the supplied official advisory or CVE record to validate affected scope, severity, and vendor guidance
• Plan vendor-supported updates or mitigations through normal change control where exposure is confirmed
• Check relevant monitoring, detection, and logs for exposed assets that need extra review

Evidence notes

The CVE record and NVD entry provide details about the vulnerability, including its description, CVSS score, and affected versions. However, the corpus does not establish versions, exploitation, impact, or remediation beyond upgrading to 10.1.7 or later.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.