{
  "affected": [
    {
      "package": {
        "ecosystem": "crates.io",
        "name": "rmcp"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "2.1.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-64684"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-200"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-09-17T14:48:12Z",
    "nvd_published_at": "2026-09-16T22:17:04Z",
    "severity": "MODERATE"
  },
  "details": "## Summary\n\nThe `rmcp` crate\u0027s `StreamableHttpClientTransport` forwards caller-supplied custom HTTP headers (such as `X-API-Key`, `X-Auth-Token`, `Api-Key`) to cross-origin redirect targets. The `default_http_client()` function builds a `reqwest::Client` without a redirect policy override, so the default `limited(10)` policy follows `307`/`308` redirects and forwards all per-request headers except `Authorization`, `Cookie`, and `Proxy-Authorization`. Custom auth headers injected via `StreamableHttpClientTransportConfig.custom_headers` are not classified as sensitive and are therefore forwarded verbatim to any redirect target \u2014 including an attacker-controlled server.\n\n## Affected versions\n\n- Repository: `github.com/modelcontextprotocol/rust-sdk`\n- Crate: `rmcp`\n- Commit tested: `c330fede90e4729c234f8e87fdbc5ea27a1dd10c` (HEAD, 2026-05-21)\n\n## Vulnerability\n\n**File:** `crates/rmcp/src/transport/common/reqwest/streamable_http_client.rs`\n\n**Root cause 1 \u2014 no redirect policy override:**\n\n```rust\n// Lines 302-307\nfn default_http_client() -\u003e reqwest::Client {\n    reqwest::Client::builder()\n        .pool_max_idle_per_host(0)\n        .build()\n        .expect(\"failed to build default reqwest client\")\n}\n```\n\nNo `.redirect(reqwest::redirect::Policy::none())` call. The default `limited(10)` policy follows up to 10 redirects and, on cross-origin redirects, strips only `Authorization`, `Cookie`, and `Proxy-Authorization`.\n\n**Root cause 2 \u2014 custom headers not sensitivity-marked:**\n\n```rust\n// Lines 26-35\nfn apply_custom_headers(\n    mut builder: reqwest::RequestBuilder,\n    custom_headers: HashMap\u003cHeaderName, HeaderValue\u003e,\n) -\u003e Result\u003creqwest::RequestBuilder, StreamableHttpError\u003creqwest::Error\u003e\u003e {\n    for (name, value) in custom_headers {\n        validate_custom_header(\u0026name).map_err(StreamableHttpError::ReservedHeaderConflict)?;\n        builder = builder.header(name, value);  // no sensitivity marker\n    }\n    Ok(builder)\n}\n```\n\nHeaders added via `RequestBuilder::header()` are forwarded to redirect targets because reqwest only strips headers from its own sensitive-header list (`Authorization`, `Cookie`, `Proxy-Authorization`).\n\n**Exposed API:** `StreamableHttpClientTransportConfig.custom_headers` (line 1070), intended for custom auth headers:\n\n```rust\n/// Custom HTTP headers to include with every request\npub custom_headers: HashMap\u003cHeaderName, HeaderValue\u003e,\n```\n\n## Attack scenario\n\n1. A caller sets `custom_headers` with an API key for the MCP server:\n   ```rust\n   let config = StreamableHttpClientTransportConfig::with_uri(\"https://mcp.example.com/mcp\")\n       .custom_headers([(HeaderName::from_static(\"x-api-key\"),\n                         HeaderValue::from_static(\"my-secret-key\"))].into());\n   ```\n2. An attacker compromises `mcp.example.com` to return `307 Temporary Redirect` to `https://attacker.example.net/capture`.\n3. `rmcp` follows the redirect, forwarding `X-API-Key: my-secret-key` to `attacker.example.net`.\n4. The attacker captures the secret and reuses it to call the MCP server directly.\n\n## Negative control\n\nThe `auth_header` path (`StreamableHttpClientTransportConfig::auth_header()`) sets the value via `builder.bearer_auth(auth_header)`, which maps to the `Authorization` header \u2014 stripped by reqwest on cross-origin redirects. That path is not affected. Only `custom_headers` is vulnerable.\n\n## Fix\n\nIn `default_http_client()`, disable automatic redirect following:\n\n```rust\nfn default_http_client() -\u003e reqwest::Client {\n    reqwest::Client::builder()\n        .pool_max_idle_per_host(0)\n        .redirect(reqwest::redirect::Policy::none())  // \u003c-- add this\n        .build()\n        .expect(\"failed to build default reqwest client\")\n}\n```\n\nThe transport can then inspect `3xx` responses and decide whether to follow, stripping sensitive headers before doing so. Alternatively, use `reqwest::ClientBuilder::connection_verbose` or per-request `Request::headers_mut()` to remove auth headers before the redirect is followed.",
  "id": "GHSA-9g45-5xwm-f3wc",
  "modified": "2026-09-17T16:36:36Z",
  "published": "2026-09-17T14:48:12Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/rust-sdk/security/advisories/GHSA-9g45-5xwm-f3wc"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-64684"
    },
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/rust-sdk/pull/936"
    },
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/rust-sdk/commit/496902b9cf2c8a947454718da31829ae776b969b"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/modelcontextprotocol/rust-sdk"
    },
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/rust-sdk/releases/tag/rmcp-v2.1.0"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:N/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "RMCP: Custom HTTP headers leak to cross-origin redirect targets"
}