CVE-2026-21637 is a vulnerability in Node.js's TLS implementation related to error handling in the Pre-Shared Key (PSK) and Application-Layer Protocol Negotiation (ALPN) callbacks. These callbacks are invoked during the TLS handshake to process client-supplied data. The flaw occurs when synchronous exceptions thrown inside these callbacks are not properly caught or wrapped, causing them to bypass the standard TLS error handling mechanisms such as 'tlsClientError' and 'error' events. This results in two primary failure modes: immediate termination of the Node.js process or silent leaks of file descriptors. The latter leads to gradual resource exhaustion, culminating in denial of service. Because the callbacks handle attacker-controlled input during the handshake, a remote adversary can repeatedly trigger these exceptions, effectively crashing or exhausting the TLS server without authentication or user interaction. The vulnerability affects a broad range of Node.js versions, from early versions (4.0) through recent ones (25.2.1), indicating a long-standing issue. The CVSS 3.0 base score of 5.9 reflects a medium severity, with network attack vector, high attack complexity, no privileges required, no user interaction, and impact limited to availability. No patches are linked yet, and no known exploits have been observed in the wild. The underlying CWE is CWE-400 (Uncontrolled Resource Consumption), emphasizing the denial of service nature of this flaw.

For European organizations, this vulnerability poses a significant risk to the availability of services relying on Node.js TLS servers that utilize PSK or ALPN callbacks. Such services could experience unexpected crashes or resource exhaustion, leading to denial of service conditions. This could disrupt critical web applications, APIs, or microservices that depend on secure TLS connections. Industries with high reliance on Node.js for backend services, including finance, healthcare, and e-commerce, may face operational interruptions and potential reputational damage. Additionally, denial of service attacks exploiting this flaw could be used as a smokescreen for other malicious activities or to degrade service during geopolitical tensions. The broad range of affected Node.js versions means many legacy and current deployments are vulnerable, increasing the attack surface. The lack of required authentication or user interaction lowers the barrier for attackers to exploit this remotely. However, the high attack complexity somewhat limits widespread exploitation. Still, organizations must consider the risk of service outages and plan accordingly.

European organizations should immediately audit their Node.js deployments to identify TLS servers using PSK or ALPN callbacks. Until official patches are released, developers should implement defensive coding practices by wrapping these callbacks in try-catch blocks to safely handle synchronous exceptions and prevent process crashes or resource leaks. Monitoring for abnormal process terminations or file descriptor usage can help detect exploitation attempts. Employing rate limiting and connection throttling on TLS handshake requests can reduce the risk of resource exhaustion. Organizations should also consider upgrading Node.js to versions where this issue is fixed once patches become available. Network-level protections such as Web Application Firewalls (WAFs) or intrusion prevention systems (IPS) can be tuned to detect and block suspicious TLS handshake patterns. Regularly reviewing Node.js release notes and security advisories will ensure timely application of fixes. Finally, implementing robust incident response plans for denial of service events will minimize operational impact.