MCP Ruby SDK: Streamable HTTP transport lacks DNS-rebinding (Host/Origin) protection

{
  "id": "32f5886d65d80930cf4986b9972643563809e69e0c7fa19c30cc2bd04dadbf7d",
  "url": "https://osv.dev/vulnerability/CVE-2026-63118",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "MCP Ruby SDK: Streamable HTTP transport lacks DNS-rebinding (Host/Origin) protection",
    "summary": "MCP Ruby SDK is the official Ruby SDK for Model Context Protocol servers and clients. Prior to 0.23.0, MCP::Server::Transports::StreamableHTTPTransport in the mcp gem does not validate the HTTP Host or Origin request headers, which allows a malicious browser page to use DNS rebinding to reach a locally running MCP server and invoke exposed tools. This issue is fixed in version 0.23.0.",
    "evidence": "osv:CVE-2026-63118",
    "products": [],
    "severity": "medium",
    "structured": {
      "id": "CVE-2026-63118",
      "aliases": [
        "CVE-2026-63118",
        "GHSA-rjr6-rcgv-9m7m"
      ],
      "metrics": [
        {
          "type": "CVSS_V4",
          "score": "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:A/VC:H/VI:N/VA:N/SC:H/SI:N/SA:N"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://github.com/modelcontextprotocol/ruby-sdk",
              "type": "GIT",
              "events": [
                {
                  "introduced": "0"
                },
                {
                  "fixed": "95feef2dcfba25d92a8963d14b7a3b279122f36f"
                },
                {
                  "fixed": "ba543083a7594e7892b29464b89091816446ff7a"
                }
              ],
              "database_specific": {
                "extracted_events": [
                  {
                    "range": [
                      {
                        "introduced": "0"
                      },
                      {
                        "fixed": "0.23.0"
                      }
                    ],
                    "source": "AFFECTED_FIELD",
                    "strategy": "StringRangeExpression"
                  },
                  {
                    "range": [
                      {
                        "fixed": "ba543083a7594e7892b29464b89091816446ff7a"
                      }
                    ],
                    "source": "REFERENCES_COMMIT"
                  },
                  {
                    "range": [
                      {
                        "fixed": "95feef2dcfba25d92a8963d14b7a3b279122f36f"
                      }
                    ],
                    "source": "REFERENCES_TAG",
                    "original_tag": "v0.23.0"
                  }
                ]
              }
            }
          ],
          "versions": [
            "v0.22.0",
            "v0.21.0",
            "v0.20.0",
            "v0.19.0",
            "v0.18.0",
            "v0.17.0",
            "v0.16.0",
            "v0.15.0",
            "v0.14.0",
            "v0.13.0",
            "v0.12.0",
            "v0.11.0",
            "v0.10.0",
            "v0.9.2",
            "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-2026-63118.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:25.459083501Z",
      "provider": "osv",
      "references": [
        "https://github.com/modelcontextprotocol/ruby-sdk/releases/tag/v0.23.0",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/63xxx/CVE-2026-63118.json",
        "https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-rjr6-rcgv-9m7m",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-63118",
        "https://github.com/modelcontextprotocol/ruby-sdk/commit/ba543083a7594e7892b29464b89091816446ff7a"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-346",
          "CWE-350"
        ],
        "cna_assigner": "GitHub_M",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/63xxx/CVE-2026-63118.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-63118",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 492137,
  "published_at": "2026-07-29T19:07:33.452+00:00"
}