MCP Ruby SDK: Insufficient Session Binding Allows SSE Stream Hijacking via Session ID Replay

{
  "id": "319b87617e3181bf93a51a0dfbaece0fd3cbb280e1928a31486400dc2d1ad555",
  "url": "https://osv.dev/vulnerability/CVE-2026-33946",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "MCP Ruby SDK: Insufficient Session Binding Allows SSE Stream Hijacking via Session ID Replay",
    "summary": "MCP Ruby SDK is the official Ruby SDK for Model Context Protocol servers and clients. Prior to version 0.9.2, the Ruby SDK's streamable_http_transport.rb implementation contains a session hijacking vulnerability. An attacker who obtains a valid session ID can completely hijack the victim's Server-Sent Events (SSE) stream and intercept all real-time data. Version 0.9.2 contains a patch.",
    "evidence": "osv:CVE-2026-33946",
    "products": [],
    "severity": "high",
    "structured": {
      "id": "CVE-2026-33946",
      "aliases": [
        "CVE-2026-33946",
        "GHSA-qvqr-5cv7-wh35"
      ],
      "metrics": [
        {
          "type": "CVSS_V4",
          "score": "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://github.com/modelcontextprotocol/ruby-sdk",
              "type": "GIT",
              "events": [
                {
                  "introduced": "0"
                },
                {
                  "fixed": "3fc7bcd2683d2200d42f79ab46752dc65f896db1"
                },
                {
                  "fixed": "db40143402d65b4fb6923cec42d2d72cb89b3874"
                }
              ],
              "database_specific": {
                "extracted_events": [
                  {
                    "cpe": "cpe:2.3:a:lfprojects:mcp_ruby_sdk:*:*:*:*:*:*:*:*",
                    "range": [
                      {
                        "introduced": "0"
                      },
                      {
                        "fixed": "0.9.2"
                      }
                    ],
                    "source": "CPE_RANGE"
                  },
                  {
                    "range": [
                      {
                        "fixed": "db40143402d65b4fb6923cec42d2d72cb89b3874"
                      }
                    ],
                    "source": "REFERENCES_COMMIT"
                  },
                  {
                    "range": [
                      {
                        "fixed": "3fc7bcd2683d2200d42f79ab46752dc65f896db1"
                      }
                    ],
                    "source": "REFERENCES_TAG",
                    "original_tag": "v0.9.2"
                  }
                ]
              }
            }
          ],
          "versions": [
            "v0.9.1",
            "v0.8.0",
            "v0.9.0",
            "v0.7.1",
            "v0.7.0",
            "v0.6.0",
            "v0.5.0",
            "v0.4.0",
            "v0.3.0",
            "v0.2.0",
            "v0.1.0"
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-33946.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:30.299022619Z",
      "provider": "osv",
      "references": [
        "https://github.com/modelcontextprotocol/csharp-sdk/blob/main/src/ModelContextProtocol.AspNetCore/SseHandler.cs#L93-L97",
        "https://github.com/modelcontextprotocol/go-sdk/blob/main/mcp/streamable.go#L281C1-L288C2",
        "https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/server/streamable_http.py#L680-L685",
        "https://github.com/modelcontextprotocol/ruby-sdk/blob/main/examples/streamable_http_server.rb",
        "https://github.com/modelcontextprotocol/ruby-sdk/releases/tag/v0.9.2",
        "https://hackerone.com/reports/3556146",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/33xxx/CVE-2026-33946.json",
        "https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-qvqr-5cv7-wh35",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-33946",
        "https://github.com/modelcontextprotocol/ruby-sdk/commit/db40143402d65b4fb6923cec42d2d72cb89b3874"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-384",
          "CWE-639"
        ],
        "cna_assigner": "GitHub_M",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/33xxx/CVE-2026-33946.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-33946",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 497038,
  "published_at": "2026-03-27T21:20:07.9+00:00"
}