high CVSS 7.5

CVE-2026-21637·Node.js vulnerability

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 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.

Severity
high
Software
Node.js
Fixed in
25.3.0
Published
2026-01-20

Affected versions

From: 25.0.0

Until: 25.3.0

Fixed in: 25.3.0

How to fix this CVE

Update Node.js to version 25.3.0 or later to patch the TLS callback exception handling flaw. This vulnerability allows remote attackers to trigger denial of service through crafted TLS handshakes when PSK or ALPN callbacks are enabled. Apply the update immediately across all affected servers running versions 25.0.0 through 25.2.x.

sudo dnf update nodejs

Defensia detects this vulnerability

What an exploitation attempt looks like

Sample log line indicative of exploitation attempts:

Multiple rapid TLS handshake attempts with malformed or exception-triggering ALPN/PSK extension data followed by server process termination or connection resets without graceful error responses.

How to check if you are affected

  1. Step 1: Run `node --version` to check the currently installed Node.js version. Versions 25.0.0 to 25.2.x are vulnerable.
  2. Step 2: Identify TLS servers in your environment by searching for `pskCallback` or `ALPNCallback` configuration in application source code or Node.js process arguments using `ps aux | grep node`.
  3. Step 3: Monitor system logs and Node.js error logs for sudden process crashes or repeated error patterns during TLS handshakes; look for EBADF (bad file descriptor) errors or abrupt process terminations without stack traces.
  4. Step 4: After updating to version 25.3.0+, verify with `node --version` and restart all Node.js services to ensure the patched version is active.

FAQ

What is CVE-2026-21637?

CVE-2026-21637 is a TLS error handling vulnerability in Node.js that allows remote attackers to crash or exhaust server resources by triggering unhandled exceptions in PSK or ALPN callbacks during the TLS handshake phase.

Is CVE-2026-21637 being actively exploited?

No, this vulnerability is not currently listed on the CISA KEV catalog and no public exploits are available, though the attack requires minimal complexity and could be weaponized by unauthenticated remote clients.

What versions of Node.js are affected by CVE-2026-21637?

Node.js versions 25.0.0 through 25.2.x are vulnerable. Version 25.3.0 and later contain the fix.

How do I check if my server is vulnerable to CVE-2026-21637?

Run `node --version` and compare against version 25.3.0. If your version is between 25.0.0 and 25.2.x inclusive, and your application uses TLS with PSK or ALPN callbacks, your server is vulnerable.

Does Defensia detect CVE-2026-21637?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Node.js is installed on a monitored server, CVE-2026-21637 will appear in your dashboard with remediation steps.

Related Node.js CVEs

CVE-2026-21636CVSS 10A flaw in Node.js's permission model allows Unix Domain Socket (UDS) connections to bypass network restrictions when `--permission` is enabled. Even without `--allow-net`, attacker-controlled inputs (such as URLs or socketPath options) can connect to arbitrary local sockets via net, tls, or undici/fetch. This breaks the intended security boundary of the permission model and enables access to privileged local services, potentially leading to privilege escalation, data exposure, or local code execution. * The issue affects users of the Node.js permission model on version v25. In the moment of this vulnerability, network permissions (`--allow-net`) are still in the experimental phase.
CVE-2025-55130CVSS 9.1A flaw in Node.js’s Permissions model allows attackers to bypass `--allow-fs-read` and `--allow-fs-write` restrictions using crafted relative symlink paths. By chaining directories and symlinks, a script granted access only to the current directory can escape the allowed path and read sensitive files. This breaks the expected isolation guarantees and enables arbitrary file read/write, leading to potential system compromise. This vulnerability affects users of the permission model on Node.js v20, v22, v24, and v25.
CVE-2025-59464CVSS 7.5A memory leak in Node.js’s OpenSSL integration occurs when converting `X.509` certificate fields to UTF-8 without freeing the allocated buffer. When applications call `socket.getPeerCertificate(true)`, each certificate field leaks memory, allowing remote clients to trigger steady memory growth through repeated TLS connections. Over time this can lead to resource exhaustion and denial of service.
CVE-2025-59465CVSS 7.5A malformed `HTTP/2 HEADERS` frame with oversized, invalid `HPACK` data can cause Node.js to crash by triggering an unhandled `TLSSocket` error `ECONNRESET`. Instead of safely closing the connection, the process crashes, enabling a remote denial of service. This primarily affects applications that do not attach explicit error handlers to secure sockets, for example: ``` server.on('secureConnection', socket => { socket.on('error', err => { console.log(err) }) }) ```
CVE-2025-59466CVSS 7.5We have identified a bug in Node.js error handling where "Maximum call stack size exceeded" errors become uncatchable when `async_hooks.createHook()` is enabled. Instead of reaching `process.on('uncaughtException')`, the process terminates, making the crash unrecoverable. Applications that rely on `AsyncLocalStorage` (v22, v20) or `async_hooks.createHook()` (v24, v22, v20) become vulnerable to denial-of-service crashes triggered by deep recursion under specific conditions.

References

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