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CVE-2026-102600 is a prototype pollution vulnerability in @socket.io/cluster-engine. The flaw resides in the `engine.ts` file, where session IDs supplied in inter-process messages are used directly as property keys on the `this.clients` object without an `hasOwnProperty` guard. In clustered deployments, this object maps session identifiers to connected client instances. When an attacker supplies a session ID such as __proto__, constructor, or another inherited property name, the lookup resolves a value from the object prototype chain instead of checking for a real connected client. This unexpected resolution causes the internal logic to dereference an invalid client object, triggering an unhandled exception that crashes the Node.js process. The result is a denial of service. Applications not using `@socket.io/cluster-engine` are not affected. The issue is fixed in version 0.1.1, which enforces own-property checks during client lookup. The vulnerability is classified as CWE-1321 (Prototype Pollution) and CWE-20 (Improper Input Validation). It was published on 2026-09-29 and carries a CVSS score of 7.5 (High) per CISA-ADP and 8.7 (High) per Snyk.
DailyCVE Form:
Platform: Socket.IO Cluster Engine
Version: 0.1.0
Vulnerability: Prototype Pollution
Severity: High
date: 2026-09-29
Prediction: Patch Released 2026-09-29
What Undercode Say:
Bash command to check the installed version:
npm list @socket.io/cluster-engine
Bash command to upgrade to the patched version:
npm install @socket.io/[email protected]
Vulnerable code pattern in `engine.ts` (conceptual):
// Before patch: unsafe lookup
const client = this.clients[bash]; // sid can be "<strong>proto</strong>"
if (client) {
// ... logic assumes client is a valid Client instance
}
Patched code pattern (conceptual):
// After patch: own-property check
if (Object.prototype.hasOwnProperty.call(this.clients, sid)) {
const client = this.clients[bash];
// ... safe logic
}
Exploit: (Educational Purposes!)
An attacker with the ability to influence the `sid` value in inter-process messages can send a crafted session identifier of __proto__. The cluster engine will resolve this.clients["__proto__"], which returns Object.prototype. The subsequent logic treats this as a client object, leading to a null reference or type error that terminates the Node.js process. This is a remote denial-of-service condition in clustered Socket.IO deployments.
Protection: from this CVE
Upgrade `@socket.io/cluster-engine` to version 0.1.1 or later. If immediate upgrade is not possible, reject or sanitize suspicious session IDs before they reach the cluster engine. Additionally, run the cluster engine behind trusted infrastructure that prevents arbitrary clients from crafting raw Engine.IO session-related requests. These workarounds are defense-in-depth only.
Impact:
Denial of service through process crash. The vulnerability allows an unauthenticated remote attacker to terminate the Node.js worker process in a clustered Socket.IO deployment, dropping active connections and preventing new connections until the cluster is restarted.
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Sources:
Reported By: github.com
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