CVE-2026-2359 identifies a vulnerability in the Multer middleware component of the Express.js framework, which is widely used in Node.js applications to handle multipart/form-data, such as file uploads. The vulnerability is classified under CWE-772 (Missing Release of Resource after Effective Lifetime), indicating improper resource management. Specifically, an attacker can initiate a file upload and then drop the connection prematurely, causing Multer to allocate resources that are not properly released. This leads to resource exhaustion on the server, resulting in a Denial of Service (DoS) condition. The vulnerability affects all Multer versions prior to 2.1.0. Exploitation is straightforward as it requires no authentication, no user interaction, and can be triggered remotely over the network. The CVSS 4.0 vector (AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N) reflects a network attack vector with low complexity and no privileges or user interaction needed, causing high impact on availability. No known workarounds exist, making upgrading to version 2.1.0 the only effective mitigation. Although no exploits have been observed in the wild yet, the ease of exploitation and the critical nature of the vulnerability make it a significant threat to any web application using vulnerable Multer versions.

The primary impact of CVE-2026-2359 is Denial of Service through resource exhaustion, which can disrupt web services relying on Multer for file uploads. Organizations running Node.js applications with vulnerable Multer versions may experience degraded performance or complete service outages, affecting availability and potentially leading to loss of business continuity. This can impact customer trust and operational efficiency, especially for services that rely heavily on file upload functionality such as content management systems, e-commerce platforms, and cloud services. The vulnerability does not directly compromise confidentiality or integrity but can be leveraged as a vector for broader attacks by causing service disruptions. Given the widespread use of Express.js and Multer in modern web development, the scope of affected systems is large, spanning startups to large enterprises globally.

The definitive mitigation is to upgrade Multer to version 2.1.0 or later, where the vulnerability has been patched. Organizations should audit their dependencies to identify vulnerable Multer versions and prioritize patching in their deployment pipelines. Additionally, implementing robust resource management and connection timeout policies at the application and infrastructure level can help limit the impact of dropped connections. Web servers and load balancers should be configured to detect and terminate stalled or incomplete uploads promptly. Monitoring for unusual spikes in resource usage related to file uploads can provide early warning signs of exploitation attempts. Employing rate limiting and connection throttling on upload endpoints can further reduce the risk of resource exhaustion. Finally, developers should review their error handling and cleanup logic around file uploads to ensure resources are released correctly even in abnormal connection scenarios.