{"id":"CVE-2026-64684","aliases":["GHSA-9g45-5xwm-f3wc"],"url":"https://o3.security/vulnerability/CVE-2026-64684","summary":"RMCP: Custom HTTP headers leak to cross-origin redirect targets","details":"RMCP is an official Rust SDK for the Model Context Protocol. Prior to 2.1.0, the rmcp crate's StreamableHttpClientTransport in crates/rmcp/src/transport/common/reqwest/streamable_http_client.rs builds its default_http_client with reqwest's automatic redirect policy and applies caller-supplied values from StreamableHttpClientTransportConfig.custom_headers without marking them as sensitive. When a malicious or compromised MCP endpoint returns a cross-origin 307 or 308 redirect, reqwest follows the redirect and apply_custom_headers causes custom API keys or authentication tokens to be replayed to the new origin, where an attacker can capture and reuse them. The separate auth_header path is not affected because it uses the standard Authorization header, which reqwest strips on cross-origin redirects. This issue is fixed in version 2.1.0.","published":"2026-09-16T21:18:48.886Z","modified":"2026-09-18T03:48:29.097721729Z","cvss":{"score":6.8,"severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:N/A:N"},"epss":null,"cisaKev":null,"exploitsKnown":null,"affectedPackages":[{"ecosystem":"crates.io","name":"rmcp","fixedVersion":"2.1.0"}],"fix":{"url":"https://github.com/modelcontextprotocol/rust-sdk/commit/496902b9cf2c8a947454718da31829ae776b969b","label":"modelcontextprotocol/rust-sdk@496902b"},"references":[{"type":"WEB","url":"https://github.com/modelcontextprotocol/rust-sdk/releases/tag/rmcp-v2.1.0"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/64xxx/CVE-2026-64684.json"},{"type":"ADVISORY","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":"FIX","url":"https://github.com/modelcontextprotocol/rust-sdk/commit/496902b9cf2c8a947454718da31829ae776b969b"},{"type":"FIX","url":"https://github.com/modelcontextprotocol/rust-sdk/pull/936"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-09-18T03:48:29.097721729Z"}}