CVE-2026-21637
Published: 20 January 2026
Summary
CVE-2026-21637 is a high-severity Uncontrolled Resource Consumption (CWE-400) vulnerability in Nodejs Node.Js. Its CVSS base score is 7.5 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked at the 13.6th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.
Threat & Defense at a Glance
Threat & Defense Details
Likely Mitigating ControlsAI
Per-CVE control mapping for this CVE has not run yet; the list below is derived from the weakness types (CWEs) cited in the NVD entry.
Limiting concurrent sessions directly prevents uncontrolled resource consumption by capping the number of active sessions per user or account.
Analysis identifies uncontrolled resource consumption indicative of denial-of-service or abuse attempts.
Contingency plan testing includes resource exhaustion scenarios to verify recovery, making it harder for attackers to sustain exploits that cause uncontrolled consumption.
Updated contingency plans include current procedures to detect, contain, and recover from resource exhaustion, limiting an attacker's ability to sustain impact from uncontrolled consumption.
Alternate site allows resumption of operations if resource exhaustion at the primary site is exploited to cause unavailability.
Alternate telecommunications services enable resumption of essential functions when primary services become unavailable due to uncontrolled resource consumption.
The team can analyze and respond to resource exhaustion incidents, reducing the impact of attacks that exploit uncontrolled consumption weaknesses.
Timely maintenance support and spare parts enable rapid recovery from failures induced by uncontrolled resource consumption, shortening the impact window of denial-of-service attacks.
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
Vulnerability enables remote exploitation of public-facing Node.js TLS server (T1190) via attacker-controlled handshake input, directly resulting in application/system DoS through crashes or resource exhaustion (T1499.004).
NVD Description
A flaw in Node.js TLS error handling allows remote attackers to crash or exhaust resources of a TLS server when `pskCallback` or `ALPNCallback` are in use. Synchronous exceptions thrown during these callbacks bypass standard TLS error handling paths (tlsClientError and…
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error), causing either immediate process termination or silent file descriptor leaks that eventually lead to denial of service. Because these callbacks process attacker-controlled input during the TLS handshake, a remote client can repeatedly trigger the issue. This vulnerability affects TLS servers using PSK or ALPN callbacks across Node.js versions where these callbacks throw without being safely wrapped.
Deeper analysisAI
CVE-2026-21637 is a flaw in Node.js TLS error handling that affects TLS servers using `pskCallback` or `ALPNCallback`. Synchronous exceptions thrown during these callbacks bypass standard TLS error handling paths, such as tlsClientError and error events, resulting in immediate process termination or silent file descriptor leaks that lead to denial of service. These callbacks process attacker-controlled input during the TLS handshake, and the vulnerability impacts Node.js versions where such callbacks throw exceptions without safe wrapping.
Remote unauthenticated attackers can exploit this vulnerability over the network with low complexity during TLS handshakes. By repeatedly triggering synchronous exceptions in the callbacks, attackers can crash the server process or cause gradual resource exhaustion through file descriptor leaks, achieving denial of service. The issue is rated with a CVSS v3.1 base score of 7.5 (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H) and maps to CWE-400 (Uncontrolled Resource Consumption).
The Node.js security advisory provides details on patches and mitigation in its December 2025 security releases blog post at https://nodejs.org/en/blog/vulnerability/december-2025-security-releases.
Details
- CWE(s)