Engine.IO is the implementation of transport-based cross-browser/cross-device bi-directional communication layer for Socket.IO. A specially crafted HTTP request can trigger an uncaught exception on the Engine.IO server, thus killing the Node.js process. This impacts all the users of the engine.io package starting from version 4.0.0, including those who uses depending packages like socket.io. Versions prior to 4.0.0 are not impacted. A fix has been released for each major branch, namely 4.1.2 for the 4.x.x branch, 5.2.1 for the 5.x.x branch, and 6.1.1 for the 6.x.x branch. There is no known workaround except upgrading to a safe version.