{"id":"CVE-2026-64684","state":"PUBLISHED","title":"RMCP: Custom HTTP headers leak to cross-origin redirect targets","description":"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.","publishedAt":"2026-09-16T21:18:48.886Z","updatedAt":"2026-09-19T01:56:51.763Z","severity":{"score":6.8,"label":"Medium","version":"3.1","vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:N/A:N","source":"GitHub_M"},"assigner":"GitHub_M","products":["modelcontextprotocol rust-sdk"],"affected":[{"vendor":"modelcontextprotocol","product":"rust-sdk","versions":[{"version":"< 2.1.0","status":"affected"}]}],"references":["https://github.com/modelcontextprotocol/rust-sdk/security/advisories/GHSA-9g45-5xwm-f3wc","https://github.com/modelcontextprotocol/rust-sdk/pull/936","https://github.com/modelcontextprotocol/rust-sdk/commit/496902b9cf2c8a947454718da31829ae776b969b","https://github.com/modelcontextprotocol/rust-sdk/releases/tag/rmcp-v2.1.0"],"weaknesses":["CWE-200"],"ingestedAt":"2026-10-29T20:12:41.951Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2026/64xxx/CVE-2026-64684.json","commit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-64684","state":"PUBLISHED","datePublished":"2026-09-16T21:18:48.886Z","dateUpdated":"2026-09-19T01:56:51.763Z","assignerShortName":"GitHub_M","assignerOrgId":"a0819718-46f1-4df5-94e2-005712e83aaa","dateReserved":"2026-07-20T17:12:12.892Z"},"containers":{"cna":{"title":"RMCP: Custom HTTP headers leak to cross-origin redirect targets","descriptions":[{"lang":"en","value":"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."}],"affected":[{"vendor":"modelcontextprotocol","product":"rust-sdk","versions":[{"version":"< 2.1.0","status":"affected"}]}],"references":[{"url":"https://github.com/modelcontextprotocol/rust-sdk/security/advisories/GHSA-9g45-5xwm-f3wc","name":"https://github.com/modelcontextprotocol/rust-sdk/security/advisories/GHSA-9g45-5xwm-f3wc","tags":["x_refsource_CONFIRM"]},{"url":"https://github.com/modelcontextprotocol/rust-sdk/pull/936","name":"https://github.com/modelcontextprotocol/rust-sdk/pull/936","tags":["x_refsource_MISC"]},{"url":"https://github.com/modelcontextprotocol/rust-sdk/commit/496902b9cf2c8a947454718da31829ae776b969b","name":"https://github.com/modelcontextprotocol/rust-sdk/commit/496902b9cf2c8a947454718da31829ae776b969b","tags":["x_refsource_MISC"]},{"url":"https://github.com/modelcontextprotocol/rust-sdk/releases/tag/rmcp-v2.1.0","name":"https://github.com/modelcontextprotocol/rust-sdk/releases/tag/rmcp-v2.1.0","tags":["x_refsource_MISC"]}],"metrics":[{"cvssV3_1":{"attackComplexity":"HIGH","attackVector":"NETWORK","availabilityImpact":"NONE","baseScore":6.8,"baseSeverity":"MEDIUM","confidentialityImpact":"HIGH","integrityImpact":"NONE","privilegesRequired":"NONE","scope":"CHANGED","userInteraction":"NONE","vectorString":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:N/A:N","version":"3.1"}}],"problemTypes":[{"descriptions":[{"cweId":"CWE-200","lang":"en","description":"CWE-200: Exposure of Sensitive Information to an Unauthorized Actor","type":"CWE"}]}],"providerMetadata":{"shortName":"GitHub_M","orgId":"a0819718-46f1-4df5-94e2-005712e83aaa","dateUpdated":"2026-09-16T21:18:48.886Z"},"source":{"advisory":"GHSA-9g45-5xwm-f3wc","discovery":"UNKNOWN"}},"adp":[{"title":"CISA ADP Vulnrichment","metrics":[{"other":{"type":"ssvc","content":{"timestamp":"2026-09-19T01:56:38.766580Z","id":"CVE-2026-64684","options":[{"Exploitation":"none"},{"Automatable":"no"},{"Technical Impact":"partial"}],"role":"CISA Coordinator","version":"2.0.3"}}}],"providerMetadata":{"shortName":"CISA-ADP","orgId":"134c704f-9b21-4f2e-91b3-4a467353bcc0","dateUpdated":"2026-09-19T01:56:51.763Z"}}]}}}