MCP Python SDK: HTTP transports serve session requests without verifying the authenticated principal

{
  "id": "52a5e5c22acb49232b0c8ee80c4819bb54ca0daaa8f2030e59dc590a6f7ab491",
  "url": "https://osv.dev/vulnerability/CVE-2026-52869",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "MCP Python SDK: HTTP transports serve session requests without verifying the authenticated principal",
    "summary": "The MCP Python SDK, called mcp on PyPI, is a Python implementation of the Model Context Protocol (MCP). Prior to 1.27.2, the SSE and stateful Streamable HTTP transports mcp.server.sse.SseServerTransport and mcp.server.streamable_http_manager.StreamableHTTPSessionManager route requests to existing sessions using only the session_id query parameter or Mcp-Session-Id header without verifying the authenticated principal that created the session, allowing a different bearer-token-authenticated client with a known session ID to inject JSON-RPC messages into that session. This issue is fixed in version 1.27.2.",
    "evidence": "osv:CVE-2026-52869",
    "products": [],
    "severity": "high",
    "structured": {
      "id": "CVE-2026-52869",
      "aliases": [
        "CVE-2026-52869",
        "GHSA-jpw9-pfvf-9f58",
        "PYSEC-2026-3482"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:L"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://github.com/modelcontextprotocol/python-sdk",
              "type": "GIT",
              "events": [
                {
                  "introduced": "0"
                },
                {
                  "fixed": "62137874ff26dd74d2fea80ff528a7fd9ca7a5e7"
                },
                {
                  "fixed": "1abcca2408a6b50e10ec601181f63f9978705c00"
                },
                {
                  "fixed": "ce267b6fc515dc4efc1dc70b6975b16ff0feef0a"
                }
              ],
              "database_specific": {
                "extracted_events": [
                  {
                    "cpe": "cpe:2.3:a:lfprojects:mcp_python_sdk:*:*:*:*:*:*:*:*",
                    "range": [
                      {
                        "introduced": "0"
                      },
                      {
                        "fixed": "1.27.2"
                      }
                    ],
                    "source": "CPE_RANGE"
                  },
                  {
                    "range": [
                      {
                        "fixed": "1abcca2408a6b50e10ec601181f63f9978705c00"
                      }
                    ],
                    "source": "REFERENCES_COMMIT"
                  },
                  {
                    "range": [
                      {
                        "fixed": "ce267b6fc515dc4efc1dc70b6975b16ff0feef0a"
                      }
                    ],
                    "source": "REFERENCES_COMMIT"
                  },
                  {
                    "range": [
                      {
                        "fixed": "62137874ff26dd74d2fea80ff528a7fd9ca7a5e7"
                      }
                    ],
                    "source": "REFERENCES_TAG",
                    "original_tag": "v1.27.2"
                  }
                ]
              }
            }
          ],
          "versions": [
            "v1.27.1",
            "v1.27.0",
            "v1.26.0",
            "v1.25.0",
            "v1.24.0",
            "v1.23.3",
            "v1.23.2",
            "v1.23.1",
            "v1.23.0",
            "v1.22.0",
            "v1.21.1",
            "v1.21.0",
            "v1.20.0",
            "v1.19.0",
            "v1.18.0",
            "v1.17.0",
            "v1.16.0",
            "v1.15.0",
            "1.15.0",
            "v1.14.1",
            "v1.14.0",
            "v1.13.1",
            "v1.13.0",
            "v1.12.4",
            "v1.12.3",
            "v1.12.2",
            "v1.12.1",
            "v1.12.0",
            "v1.11.0",
            "v1.10.1",
            "v1.10.0",
            "v1.9.4",
            "v1.9.3",
            "v1.9.2",
            "v1.9.1",
            "v1.9.0",
            "v1.8.1",
            "v1.8.0",
            "v1.7.0",
            "v1.6.0",
            "v1.5.0",
            "v1.4.1",
            "v1.4.0",
            "v1.0.0"
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-52869.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:52.161023808Z",
      "provider": "osv",
      "references": [
        "https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.27.2",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/52xxx/CVE-2026-52869.json",
        "https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-jpw9-pfvf-9f58",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-52869",
        "https://github.com/modelcontextprotocol/python-sdk/commit/1abcca2408a6b50e10ec601181f63f9978705c00",
        "https://github.com/modelcontextprotocol/python-sdk/commit/ce267b6fc515dc4efc1dc70b6975b16ff0feef0a",
        "https://github.com/modelcontextprotocol/python-sdk/pull/2690",
        "https://github.com/modelcontextprotocol/python-sdk/pull/2719"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-639"
        ],
        "cna_assigner": "GitHub_M",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/52xxx/CVE-2026-52869.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-52869",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 520986,
  "published_at": "2026-07-15T20:06:23.604+00:00"
}