{"affected":[{"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/09/GHSA-9g45-5xwm-f3wc/GHSA-9g45-5xwm-f3wc.json"},"package":{"ecosystem":"crates.io","name":"rmcp","purl":"pkg:cargo/rmcp"},"ranges":[{"events":[{"introduced":"0"},{"fixed":"2.1.0"}],"type":"SEMVER"}]}],"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'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.","id":"GHSA-9g45-5xwm-f3wc","modified":"2026-09-17T16:45:06.215938994Z","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":"1.9.0","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"}