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A third rmcp advisory in three days: custom HTTP headers leak to cross-origin redirect targets
GHSA-9g45-5xwm-f3wc, published 2026-09-17, adds to the two rmcp advisories from 2026-09-16 covering OAuth token theft and a permanent session-table leak in the Streamable HTTP server transport. This one is client-side: custom headers set on the MCP client, which in practice carry API keys and bearer tokens, are re-sent to whatever host a redirect points at. Anyone running the official Rust MCP SDK against a third-party server should treat a redirect as a credential exfiltration channel.
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