CVE-2026-64684: RMCP Custom HTTP Headers Leak To Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-9g45-5xwm-f3wc",
  "modified" : "2026-09-17T16:36:36Z",
  "published" : "2026-09-17T14:48:12Z",
  "aliases" : [ "CVE-2026-64684" ],
  "summary" : "RMCP: Custom HTTP headers leak to cross-origin redirect targets",
  "details" : "## Summary\n\nThe `rmcp` crate's `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 — 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 — no redirect policy override:**\n\n```rust\n// Lines 302-307\nfn default_http_client() -> 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 — 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<HeaderName, HeaderValue>,\n) -> Result<reqwest::RequestBuilder, StreamableHttpError<reqwest::Error>> {\n    for (name, value) in custom_headers {\n        validate_custom_header(&name).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<HeaderName, HeaderValue>,\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 — 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() -> reqwest::Client {\n    reqwest::Client::builder()\n        .pool_max_idle_per_host(0)\n        .redirect(reqwest::redirect::Policy::none())  // <-- 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.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:N/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "crates.io",
      "name" : "rmcp"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "2.1.0"
      } ]
    } ]
  } ],
  "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"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-200" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-17T14:48:12Z",
    "nvd_published_at" : "2026-09-16T22:17:04Z"
  }
}