fix: stop leaking exception messages to clients via JSON-RPC error data - #486
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August 6, 2026 15:56
Tool handler exceptions and anything reaching JsonRpcHandler's blind rescue had their #message echoed into the error data member, exposing internals to untrusted clients (CWE-209). Capability errors keep their client-facing message by re-raising through RequestHandlerError, the one channel whose message is deliberately surfaced. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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For context, this behavior was originally introduced in #194 to improve the developer experience. That said, I think this change is appropriate now. Exposing the message could be made opt-in through configuration for development purposes, but that can be addressed separately if the need arises. |
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Unexpected exceptions raised by tool handlers — and anything reaching
JsonRpcHandler's blindrescue StandardError— had their#messageechoed verbatim into the JSON-RPC errordatamember, exposing internals such as class, method and host names to untrusted clients (CWE-209).Server#call_toolno longer appendse.messageto the wrappedRequestHandlerError; clients see"Internal error calling tool <name>". Observability is unaffected: the original exception still reachesconfiguration.exception_reporterviaoriginal_error.JsonRpcHandler.process_request's genericrescue StandardErrorno longer setsdata(JSON-RPC 2.0 makes it optional)."Server does not support ...") by re-raising throughRequestHandlerError— the one channel whose message is deliberately surfaced to clients.Fixes #485
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