CVE-2026-3520 in multer
Summary
by MITRE • 03/04/2026
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.
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Analysis
by VulDB Data Team • 03/31/2026
The vulnerability identified as CVE-2026-3520 affects Multer, a widely-used node.js middleware component designed to handle multipart/form-data requests commonly encountered in file upload scenarios. This middleware serves as a critical component in web applications that process file uploads, making it a potential attack surface for malicious actors seeking to disrupt service availability. The vulnerability specifically manifests in versions prior to 2.1.1, where the middleware fails to properly validate and process malformed request data, creating opportunities for attackers to exploit the underlying implementation flaws. The issue resides within the request parsing logic that handles the complex structure of multipart data streams, which can be manipulated to cause unexpected behavior in the application's execution flow.
The technical flaw within Multer stems from inadequate input validation and error handling mechanisms when processing malformed multipart requests. When an attacker crafts specifically designed requests containing malformed boundary delimiters or corrupted data structures, the middleware's parsing algorithm becomes susceptible to stack overflow conditions. This occurs because the recursive or iterative parsing functions fail to implement proper bounds checking and depth limitations, allowing malicious input to cause excessive stack consumption. The vulnerability operates at the application layer and can be classified under CWE-674, which addresses "Uncontrolled Recursion," and potentially CWE-400, addressing "Uncontrolled Resource Consumption." The attack vector requires only the ability to send HTTP requests to the vulnerable endpoint, making it particularly dangerous as it can be exploited through automated tools and does not require authentication or elevated privileges.
The operational impact of this vulnerability extends beyond simple service disruption, as it can lead to complete application unavailability and potential system instability. When a stack overflow occurs, the node.js process may crash or become unresponsive, requiring manual intervention to restore service. In high-traffic environments, this vulnerability can be exploited to create sustained denial of service conditions that can impact multiple users simultaneously. The vulnerability affects all applications that rely on Multer for file upload functionality, including web applications, content management systems, and file sharing platforms. Organizations using vulnerable versions may experience downtime, loss of user trust, and potential business disruption. The attack can be executed remotely without requiring any special privileges, making it particularly concerning for publicly accessible web services. According to ATT&CK framework, this vulnerability maps to T1499.004, which covers "Endpoint Denial of Service," and T1211, addressing "Social Engineering," as attackers may use this vulnerability as part of broader attack campaigns.
Mitigation strategies for CVE-2026-3520 center exclusively on upgrading to Multer version 2.1.1 or later, as this release contains the necessary patches to address the stack overflow conditions. Organizations should conduct immediate vulnerability assessments to identify all systems using vulnerable versions of the middleware and implement coordinated upgrade schedules to minimize service disruption. While no known workarounds exist for this specific vulnerability, administrators can consider implementing additional layers of protection such as rate limiting, input validation at the reverse proxy level, and monitoring for unusual request patterns that might indicate exploitation attempts. Security teams should also review their incident response procedures to ensure rapid detection and remediation capabilities. The patch addresses the core issue by implementing proper recursion limits, enhanced input validation, and improved error handling within the multipart parsing routines, preventing the conditions that lead to stack overflow scenarios. Organizations should also consider implementing web application firewalls or intrusion detection systems to monitor for suspicious request patterns that could indicate attempts to exploit this vulnerability. Regular security audits and dependency management practices should be strengthened to prevent similar issues in other components of the application stack.