What happened
NVD published this CVSS 6.8 Medium vulnerability on Sept 16, 2026 in the same rmcp SDK affected by CVE-2026-63127.
Why it matters
A second distinct credential-exposure flaw in the same widely-embedded MCP Rust SDK within days of the OAuth token-impersonation bug underscores that MCP client transport security hardening is still immature across the ecosystem's reference implementations.
Attack vector
StreamableHttpClientTransport builds its HTTP client with reqwest's automatic redirect-following enabled and applies caller-supplied custom headers without marking them sensitive; when a malicious or compromised MCP endpoint issues a cross-origin 307/308 redirect, reqwest forwards those headers (potentially including auth tokens) to the redirect target.
Affected systems
rmcp (Rust MCP SDK) < 2.1.0
Mitigation
Upgrade rmcp to >= 2.1.0; mark authorization-bearing custom headers as sensitive so they are stripped on cross-origin redirects, or disable automatic redirect following for MCP transport clients.