CVE-2026-18549: fastify fastify-multipart Uncontrolled Resource Consumption

Vulnerability summary

CVE-2026-18549 is a high (CVSS 7.5) Uncontrolled Resource Consumption vulnerability in fastify fastify-multipart. EPSS estimates a 0.60% chance of exploitation in the next 30 days, higher than 47.8% of scored vulnerabilities.

Timeline

1. 15-08-2026
   PublishedVulnerability first disclosed.
2. 17-08-2026
   Last modifiedVulnerability information updated.

Description

@fastify/multipart is a multipart form-data parser for Fastify. In versions from 5.3.0 up to but not including 10.1.1, when the busboy fileSize limit truncates a file part, the plugin clears its internal current-file reference while the underlying stream is still open. If the client then aborts the connection before sending the terminating boundary, the abort cleanup finds no stream to destroy, so saveRequestFiles() never settles, the request handler hangs, and the temporary file already written to disk is never cleaned up. An unauthenticated client can repeat this to permanently leak temporary files and suspended handler executions, leading to disk and event-loop exhaustion. The issue is fixed in @fastify/multipart 10.1.1. Users should upgrade to 10.1.1.

• CWE-400Uncontrolled Resource ConsumptionThe product does not properly control the allocation and maintenance of a limited resource.
• CWE-664Improper Control of a Resource Through its LifetimeThe product does not maintain or incorrectly maintains control over a resource throughout its lifetime of creation, use, and release.