Socket.IO enables bidirectional and low-latency communication for every platform. Prior to 0.1.1, @socket.io/cluster-engine uses inherited object properties when looking up attacker-controlled session IDs in clustered deployments. Special property names such as __proto__ or constructor can resolve through the object prototype chain instead of identifying an actual connected client, causing the Node.js process to crash and resulting in denial of service. Applications that do not use @socket.io/cluster-engine are not affected. This issue is fixed in version 0.1.1.

Affected: npm: @socket.io/cluster-engine

### Impact

This is a **prototype pollution / improper client lookup** vulnerability in `@socket.io/cluster-engine`.

Servers using `@socket.io/cluster-engine` may be impacted when attacker-controlled session IDs are processed in clustered deployments. A malicious client could use special property names such as `__proto__`, `constructor`, or other inherited object keys as a session identifier, causing the server to read properties from the object prototype chain instead of only real connected clients.

The impact is **denial of service** through process crash.

Applications not using `@socket.io/cluster-engine` are not affected by this specific issue.

Affected versions:

* `@socket.io/cluster-engine@0.1.0`

### Patches

The issue was fixed in:

* `@socket.io/cluster-engine@0.1.1`

### Workarounds

If upgrading immediately is not possible, users can reduce exposure by:

- Rejecting or sanitizing suspicious session IDs before they reach the cluster engine. - Running 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. Upgrading to a patched version is recommended.