CVE-2026-59724: Socket.IO: Engine.IO WebTransport SID DoS

Engine.IO servers with WebTransport enabled are vulnerable to a remotely triggerable denial of service.

A malicious unauthenticated client can send a crafted WebTransport upgrade request containing a specially chosen session ID, such as __proto__. Because the session ID lookup did not properly verify that the key was an own property of the clients object, the lookup could resolve to an inherited prototype property instead of a valid Engine.IO client.

This can cause a TypeError during WebTransport upgrade handling. Since the failure occurs in an asynchronous context, it may result in an unhandled Promise rejection and terminate the Node.js process on affected Node.js versions/configurations.

Successful exploitation can allow an unauthenticated remote attacker to crash the server process and cause denial of service. Under a process supervisor, repeated exploitation may cause crash loops.

Affected configurations are limited to deployments where WebTransport support is enabled. WebTransport is not enabled by default.

Affected versions:

• engine.io >= 6.5.0 < 6.6.7