Socket.IO: Prototype Pollution via Unsafe Client Session Lookup

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/[email protected]

Patches

The issue was fixed in:

• @socket.io/[email protected]

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.