DNS rebinding vulnerability in rmcp Streamable HTTP server transport

{
  "id": "4c500b12715e5a774f641421104a337cf8c82e8c3d6fcda715cdee3597e41ffd",
  "url": "https://osv.dev/vulnerability/RUSTSEC-2026-0189",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "DNS rebinding vulnerability in rmcp Streamable HTTP server transport",
    "summary": "Prior to version 1.4.0, the `rmcp` crate's Streamable HTTP server transport did not validate the incoming `Host` header. This allowed a malicious public website, via a DNS rebinding attack, to send requests to an MCP server running on the victim's loopback or private-network interface. An attacker who convinced a victim to visit a malicious page could enumerate and invoke tools exposed by a locally running rmcp-based MCP server, read resources and prompts, and trigger side effects limited by the tools exposed by that server. Non-HTTP transports such as stdio and child-process transports are not affected. ## Patches The issue was fixed in `rmcp` 1.4.0 by adding default loopback-only host allowlist validation for the Streamable HTTP server transport. Incoming HTTP requests now validate the `Host` header and return HTTP 403 when the host is not allowed. Users should upgrade to `rmcp >= 1.4.0`. ## Workarounds If upgrading is not possible, place the MCP server behind a reverse proxy configured to reject requests whose `Host` header is not one of the expected hostnames. Do not bind the MCP server to `0.0.0.0` without such validation.",
    "evidence": "osv:RUSTSEC-2026-0189",
    "products": [
      {
        "name": {
          "value": "rmcp",
          "evidence": "/affected/0/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [
          {
            "value": "1.4.0",
            "evidence": "/affected/0/ranges"
          }
        ],
        "affected_versions": []
      }
    ],
    "severity": "high",
    "structured": {
      "id": "RUSTSEC-2026-0189",
      "aliases": [
        "CVE-2026-42559",
        "GHSA-89vp-x53w-74fx",
        "GHSA-fvh2-gm75-j4j7",
        "RUSTSEC-2026-0140",
        "RUSTSEC-2026-0189"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "type": "SEMVER",
              "events": [
                {
                  "introduced": "0.0.0-0"
                },
                {
                  "fixed": "1.4.0"
                }
              ]
            }
          ],
          "package": {
            "name": "rmcp",
            "purl": "pkg:cargo/rmcp",
            "ecosystem": "crates.io"
          },
          "database_specific": {
            "cvss": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H",
            "source": "https://github.com/rustsec/advisory-db/blob/osv/crates/RUSTSEC-2026-0189.json",
            "categories": [],
            "informational": null
          },
          "ecosystem_specific": {
            "affects": {
              "os": [],
              "arch": [],
              "functions": []
            },
            "affected_functions": null
          }
        }
      ],
      "modified": "2026-10-12T18:00:14.804412378Z",
      "provider": "osv",
      "references": [
        "https://crates.io/crates/rmcp",
        "https://rustsec.org/advisories/RUSTSEC-2026-0189.html",
        "https://github.com/modelcontextprotocol/rust-sdk/issues/815",
        "https://github.com/modelcontextprotocol/rust-sdk/pull/764",
        "https://github.com/modelcontextprotocol/rust-sdk/issues/822",
        "https://modelcontextprotocol.io/specification/2025-06-18/basic/transports#security-warning"
      ],
      "provider_metadata": {
        "license": "CC0-1.0"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-42559",
        "evidence": "/aliases"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 306461,
  "published_at": "2026-04-29T12:00:00+00:00"
}