CVE-2026-3520 Multer vulnerable to Denial of Service via uncontrolled recursion

High 8.7 Published 4 Mar 2026 Updated 15 Jul 2026

An attacker can cause a denial of service via stack overflow by sending malformed multipart requests to applications using this Node.js middleware.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: Available

What is CVE-2026-3520?

CVE-2026-3520 is a high-severity denial of service vulnerability in expressjs multer, published on 4 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

Multer is a node.js middleware for handling `multipart/form-data`. A vulnerability in Multer prior to version 2.1.1 allows an attacker to trigger a Denial of Service (DoS) by sending malformed requests, potentially causing stack overflow. Users should upgrade to version 2.1.1 to receive a patch. No known workarounds are available.

Weakness: CWE-770, CWE-674

Which versions are affected by CVE-2026-3520?

2 open source packages ship an affected version, including multer. It affects 1 product.

Package	Affected	Fixed in
`multer` npm	All versions	`2.1.1`
`multer` npm	All versions	`2.1.1`

Products include Expressjs Multer.

How do I fix CVE-2026-3520?

Upgrade multer to 2.1.1 or later.

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-2026-3520 being exploited?

There is no known exploitation of CVE-2026-3520 in the wild and no public exploit code.

Sources

Also known as ECHO-8994-222a-8499, GHSA-5528-5vmv-3xc2, MINI-2266-hqgj-fvp4, ROOT-APP-NPM-CVE-2026-3520.