Resource exhaustion in Fastify Fastify-Multipart
CVE-2026-18549
@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.
Vulnerability class: DoS (Denial of Service)
Published · last modified .
CVSS v3 metric
CVSS v3 base score 7.5 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.
EPSS exploit prediction
EPSS: 0.006 (46.8th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 47th percentile — 46.8% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.003 · newest: 0.006 · change: +0.003
Affected products
- Fastify Fastify-Multipart
- @Fastify/multipart — versions 5.3.0, 10.1.1
Weakness classification (CWE)
References
- ce714d77-add3-4f53-aff5-83d477b104bb (Mitigation, Vendor Advisory)
- ce714d77-add3-4f53-aff5-83d477b104bb (Third Party Advisory)
Frequently asked questions
- What is CVE-2026-18549?
- CVE-2026-18549 is a high-severity vulnerability in Fastify Fastify-Multipart, classified under Uncontrolled Resource Consumption. CVSS score: 7.5/10. Published 2026-08-15.
- How severe is CVE-2026-18549?
- High severity. CVSS v3 base score is 7.5 out of 10.