Information Disclosure in rmcp (CVE-2026-64684)
Summary
Vulnerability in rmcp (CVE-2026-64684). Confidential information can be exposed externally. Exploitable via rmcp. Mitigation: upgrade to 2.1.0 or later.
Original description
The description as published by the source.
Summary
The 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.
Affected versions
- Repository:
github.com/modelcontextprotocol/rust-sdk - Crate:
rmcp - Commit tested:
c330fede90e4729c234f8e87fdbc5ea27a1dd10c(HEAD, 2026-05-21)
Vulnerability
File: crates/rmcp/src/transport/common/reqwest/streamable_http_client.rs
Root cause 1 — no redirect policy override:
// Lines 302-307
fn default_http_client() -> reqwest::Client {
reqwest::Client::builder()
.pool_max_idle_per_host(0)
.build()
.expect("failed to build default reqwest client")
}
No .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.
Root cause 2 — custom headers not sensitivity-marked:
Source: NIST National Vulnerability Database NVD GitHub Advisory
Auto-generated summary
Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.
A vulnerability tracked as CVE-2026-64684 has been found in rmcp.
Attackers can target a specific entry point like rmcp over the network to misuse the product.
Confidential information can be exposed externally. CVSS score: 6.8/10.
What to do: upgrade rmcp to 2.1.0 or later.
If unsure, ask your IT team or search "rmcp CVE-2026-64684" on the vendor's site.
CVE-2026-64684 (rmcp) — CWE-200 / CVSS v3 6.8
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: rmcp / StreamableHttpClientTransport / Authorization / Cookie
Patched: 2.1.0 — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.
Information disclosure (CWE-200) exists in rmcp. Attackers reach the vulnerable code path via rmcp without authentication.
Check the vendor advisory for the affected versions of rmcp. Attack surface: rmcp / StreamableHttpClientTransport / Authorization / Cookie.
Severity: Medium (CVSS 6.8/10). Confidential information can be exposed externally.
Update to 2.1.0.
Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.
Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'rmcp' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.
Response actions
Concrete steps and command examples for SOC/SRE teams to execute in order
-
1Identify exposure identify
grep -r 'rmcp' . | grep -v node_modulesリポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で
rmcpを grep し、稼働しているサービス・バージョンを把握する。 -
2Apply patch patch
Upgrade rmcp to 2.1.0ステージング環境で 2.1.0 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。
-
3Post-deployment verification verify
Confirm patched version is live in productionパッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。
Affected packages
References
- web github.com
- web github.com
- web github.com
- web github.com
- web nvd.nist.gov
- web github.com