PRODUCTION SCENARIO
A company builds a remote MCP server that wraps its ticketing API. To save effort, the server takes the bearer token the MCP client sends and forwards it unchanged to the ticketing API.
How must the MCP server handle that incoming bearer token?
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Answer: Validate the token's audience, then use a separate token upstream
An MCP server must validate that an access token was issued specifically for it, reject tokens that do not name it in the audience claim, and act as an OAuth client to the upstream API with a separate token from that API's authorization server. Token passthrough is explicitly forbidden. Adding an IP header records provenance but changes nothing about who the token was issued for. Accepting whatever the ticketing API accepts is the passthrough itself. And encrypting a token in transit does not rebind its audience.