A denial-of-service vulnerability exists in Engine.IO / Socket.IO servers that allow transport upgrades. The Engine.IO protocol revision is negotiated during the initial handshake and stored on the session, but a newly-created transport, including a WebSocket upgrade transport, could independently derive a different protocol revision from the upgrade request query parameters. The server did not verify that the protocol revision of an upgrade request matched the protocol revision of the existing session. A malicious client could exploit this mismatch by establishing a valid Engine.IO session and then sending an upgrade request with a different, or omitted, "EIO" query parameter. This could cause the server to attach a transport using a parser and heartbeat behavior inconsistent with the session. Under some conditions, a crafted heartbeat packet could trigger an uncaught exception and terminate the Node.js process. Servers using the default Engine.IO v4 protocol are impacted. The issue can be triggered even when Engine.IO v3 compatibility is disabled, because an omitted "EIO" parameter is interpreted as protocol v3 on the affected transport path. The impact is denial of service through process crash. Affected versions: - "engine.io >= 6.6.0, < 6.6.10"