Multer is a widely used middleware in the Node.js ecosystem for handling multipart/form-data, commonly used in file upload scenarios. CVE-2026-3304 is a vulnerability identified in Multer versions prior to 2.1.0, categorized under CWE-459, which relates to incomplete cleanup leading to resource exhaustion. Specifically, an attacker can craft malformed multipart/form-data requests that cause Multer to improperly manage allocated resources, resulting in a Denial of Service (DoS) condition. This DoS arises from resource exhaustion, such as memory or file descriptors, which can degrade or crash the server application. The vulnerability requires no privileges or user interaction, making it remotely exploitable over the network. The CVSS 4.0 vector (AV:N/AC:L/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N) indicates network attack vector, low complexity, no privileges or user interaction required, and a high impact on availability. Multer 2.1.0 includes a patch that properly handles malformed requests to prevent resource leaks and exhaustion. No alternative mitigations or workarounds are currently available, emphasizing the importance of upgrading. Although no known exploits have been observed in the wild, the vulnerability's characteristics make it a significant risk for any Node.js application using vulnerable Multer versions for file uploads.

The primary impact of CVE-2026-3304 is Denial of Service, which can severely disrupt web applications relying on Multer for handling file uploads. An attacker can cause server resource exhaustion, leading to application crashes, degraded performance, or unavailability. This can affect user experience, business continuity, and potentially lead to financial losses or reputational damage. Since Multer is commonly used in web services worldwide, the vulnerability poses a broad risk to organizations running Node.js applications with file upload capabilities. The lack of required authentication or user interaction increases the threat level, as attackers can exploit the vulnerability remotely and anonymously. Additionally, the DoS could be leveraged as part of a larger attack chain or to distract defenders while other malicious activities occur. The absence of known exploits in the wild currently limits immediate risk, but the vulnerability's high CVSS score and ease of exploitation suggest it could be targeted in the future.

The definitive mitigation for CVE-2026-3304 is to upgrade Multer to version 2.1.0 or later, where the vulnerability has been patched. Organizations should audit their codebases and dependencies to identify usage of vulnerable Multer versions and prioritize patching in development, staging, and production environments. In the absence of patching, implementing strict input validation and limiting the size and rate of multipart/form-data requests at the application or web server level may reduce exposure, though these are not full mitigations. Employing Web Application Firewalls (WAFs) with rules to detect and block malformed multipart requests could provide temporary protection. Monitoring server resource usage and setting appropriate resource limits (e.g., memory, file descriptors) can help detect and mitigate DoS attempts. Additionally, logging and alerting on unusual request patterns or errors related to multipart handling can aid early detection. However, these measures should complement, not replace, upgrading Multer. Finally, developers should follow secure coding practices for handling file uploads and keep dependencies up to date to minimize similar risks.