GHSA-hvrp-rf83-w775 MCP Python SDK: Experimental task handlers allow any client to access and cancel other clients' tasks

• PyPI mcp: >= 1.23.0, < 1.27.2 → fixed in 1.27.2

Summary In affected versions, the default request handlers installed by the experimental tasks feature (`server.experimental.enable_tasks()`) did not check which session created a task before acting on it. On a server with more than one connected client, any client could observe, read results from, and cancel tasks belonging to other clients. Am I affected? Only if the developer's application server calls `server.experimental.enable_tasks()`. If `grep -r enable_tasks` over their codebase finds nothing, the application is not affected. Details When tasks support is enabled on the low-level server, default handlers are registered for `tasks/list`, `tasks/get`, `tasks/result`, and `tasks/cancel`. These handlers operated on the task identifier alone and kept no record of the session that created each task. Because `tasks/list` returned every task in the store, a connected client did not need to know any identifiers in advance: it could enumerate all tasks, read any task's status and result via `tasks/get` and `tasks/result`, retrieve queued task messages — such as elicitation requests intended for the task's creator, which are removed from the queue on delivery, so the intended recipient never receives them — and cancel any task via `tasks/cancel`. Impact Servers that call `server.experimental.enable_tasks()` and serve multiple clients are affected: one client can read other clients' task results and elicitation payloads, consume messages meant for them, and cancel their tasks. The feature is experimental and opt-in, so servers that never enable it are unaffected. Servers that registered their own task handlers instead of the defaults are affected only if those handlers have the same omission. Mitigation Upgrade to version 1.27.2 or later, in which task IDs generated by `run_task()` embed an opaque per-session marker and the default handlers restrict each session to its own tasks: requests for another session's task receive "task not found", and `tasks/list` returns only the requesting session's tasks. Tasks created with explicitly chosen IDs or written directly through a `TaskStore` remain reachable by ID but are not listed. Alternatively, leave the experimental tasks feature disabled, or register task handlers that validate session ownership.