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The MCP Go SDK's OAuth discovery client now refuses HTTP downgrades and redirects to private IPs
PR #1220 in modelcontextprotocol/go-sdk, merged 2026-09-02T19:07Z, adds four checks to the default OAuth discovery client: it disallows redirects that downgrade to plain HTTP, disallows redirects to localhost or private IP ranges, disallows the initial call to a private IP, and lets an explicit proxy configuration opt out of dial-time checking. That is SSRF hardening on the discovery leg of the MCP OAuth flow, where a malicious server-supplied metadata URL could previously walk a client onto an internal address.
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