CVE-2026-52870: MCP Python SDK Experimental Task Handlers Allow Any Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-hvrp-rf83-w775",
  "modified" : "2026-07-16T19:56:12Z",
  "published" : "2026-07-16T19:56:12Z",
  "aliases" : [ "CVE-2026-52870" ],
  "summary" : "MCP Python SDK: Experimental task handlers allow any client to access and cancel other clients' tasks",
  "details" : "### Summary\nIn 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.\n\n### Am I affected?\nOnly 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.\n\n### Details\nWhen 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`.\n\n### Impact\nServers 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.\n\n### Mitigation\nUpgrade 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.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:L"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "mcp"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.23.0"
      }, {
        "fixed" : "1.27.2"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 1.27.1"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-hvrp-rf83-w775"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-52870"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/modelcontextprotocol/python-sdk/pull/2720"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/modelcontextprotocol/python-sdk/commit/62137874ff26dd74d2fea80ff528a7fd9ca7a5e7"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/modelcontextprotocol/python-sdk"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.27.2"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-862" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-07-16T19:56:12Z",
    "nvd_published_at" : "2026-07-15T20:17:38Z"
  }
}