CVE-2026-18549 @fastify/multipart vulnerable to Denial of Service via aborted upload after fileSize limit
An unauthenticated attacker can cause disk and event-loop exhaustion by aborting uploads after the file size limit is reached, hanging request handlers.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-18549?
CVE-2026-18549 is a high-severity denial of service vulnerability in @fastify/multipart, published on 15 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Which versions are affected by CVE-2026-18549?
It affects 1 product.
Products include Fastify Fastify-Multipart.
How do I fix CVE-2026-18549?
No fix has been published for CVE-2026-18549 yet. Limit exposure of the affected software until one is available.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-18549 being exploited?
There is no known exploitation of CVE-2026-18549 in the wild and no public exploit code.
Other vulnerabilities in @fastify/multipart
- CVE-2026-19474 @fastify/multipart vulnerable to Denial of Service via temporary file leak on aborted upload
- CVE-2023-25576 @fastify/multipart vulnerable to DoS due to unlimited number of parts
- CVE-2021-23597 Denial of Service (DoS)
- CVE-2020-8136 Uncontrolled Resource Consumption in fastify-multipart
Sources
Also known as GHSA-vmph-573x-85f6.