n8n before 1.123.83, 2.x before 2.41.4, and 2.42.x before 2.42.1 contains a prototype pollution vulnerability in the AI workflow builder, which merges workflow connections using node names as object members without checking for reserved keys. Authenticated users can create nodes named __proto__ through the API and invoke the connect-nodes operation to pollute the global object prototype, disrupting the instance until restart.