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Harden MCP upstream transport: credentialed cleartext URLs and redirect policy #879
Description
Surfaced by automated review on #875 (the finding anchors on new code, but the behavior is the long-standing shared bridge path used by both /mcp and /mcp/{server}).
Today RmcpBridge::connect accepts http:// upstream URLs for credentialed MCP servers, and the underlying HTTP client follows redirects. reqwest strips Authorization on cross-origin redirects, but custom credential headers (e.g. an api_key auth header) are not in its sensitive-header set, so a compromised/misconfigured upstream could bounce a credentialed request to another origin with the credential attached.
Worth deciding deliberately:
- whether credentialed
type: mcpupstreams should require https (probably as a warning first — plain http to cluster-internal MCP servers is a common lawful deployment, and hard-rejecting would break them), - whether the MCP client should disable redirect-following (MCP endpoints don't legitimately redirect mid-protocol),
- marking custom credential headers sensitive if redirects stay enabled.
Not done in #875 because it would change established transport behavior for every existing registered server, well beyond that PR's routing scope.
Activity
coderabbitai commentedon Aug 4, 2026
🔗 Related PRs
#808 - fix(upstream): name the real transport fault and bound the connection layer [merged]
#822 - feat(mcp): layered MCP access policies — env default, team entitlement, key narrowing [merged]
#835 - feat(headers): request-context variables in default_headers + client-header forwarding allowlist [merged]
#859 - feat(mcp): OAuth 2.1 resource-server discovery surface for /mcp [open]
#875 - feat(mcp): single-server endpoint /mcp/{server} with original tool names [open]
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