CVE-2026-3520 is a vulnerability classified under CWE-674 (Uncontrolled Recursion) found in the Multer middleware component of the expressjs project. Multer is widely used in Node.js applications to process multipart/form-data, commonly for file uploads. The vulnerability arises from improper handling of malformed multipart requests, which can cause the middleware to enter uncontrolled recursive calls. This recursion leads to a stack overflow condition, resulting in a Denial of Service (DoS) where the affected application crashes or becomes unresponsive. The flaw affects all versions of Multer prior to 2.1.1. The vulnerability can be exploited remotely without any authentication or user interaction, making it highly accessible to attackers. The CVSS v4.0 score is 8.7 (high severity), reflecting the ease of exploitation and the significant impact on availability. No known workarounds exist, and the only remediation is upgrading to Multer version 2.1.1, which contains the patch. Although no active exploits have been reported, the widespread use of Multer in Node.js web applications makes this a critical issue for developers and organizations relying on this middleware for handling file uploads.

The primary impact of CVE-2026-3520 is a Denial of Service condition caused by stack overflow due to uncontrolled recursion. This can disrupt web services that rely on Multer for handling multipart/form-data, potentially causing application crashes or server unavailability. Organizations running vulnerable versions of Multer risk service interruptions, which can lead to loss of business continuity, degraded user experience, and reputational damage. Since the vulnerability requires no authentication and can be triggered remotely, it increases the attack surface significantly. Attackers could leverage this flaw to launch DoS attacks against critical web applications, including APIs and file upload endpoints, potentially affecting cloud services, SaaS platforms, and enterprise web portals. The lack of known exploits in the wild currently reduces immediate risk, but the high severity and ease of exploitation mean that attackers may develop exploits rapidly once the vulnerability becomes widely known.

To mitigate CVE-2026-3520, organizations should immediately upgrade all instances of Multer to version 2.1.1 or later, where the vulnerability is patched. Developers should audit their dependencies to ensure no indirect usage of vulnerable Multer versions exists. Implementing input validation and limiting the size and complexity of multipart requests at the application or web server level can reduce the risk of malformed request exploitation. Employing Web Application Firewalls (WAFs) with rules to detect and block suspicious multipart/form-data patterns may provide additional protection. Monitoring application logs for unusual request patterns or crashes related to multipart handling can help detect attempted exploitation. Finally, organizations should incorporate Multer version checks into their CI/CD pipelines to prevent deployment of vulnerable versions in the future.