Access Control: gRPC Fail Open

A Google Remote Procedure Call (gRPC) Metadata object is being accepted despite failing validation.

The Metadata class is often used to house header data for an underlying protocol used by Google Remote Procedure Call (gRPC). When implementing the io.grpc.ServerInterceptor class, Metadata object should be validated and return error if exception is thrown.

Example 1: The following code shows user controllable data used as input to a gRPC Metadata object which is passing to the next io.grpc.ServerCallHandler object after catching exception:

class PotentialAuthByPassInterceptor implements ServerInterceptor {
    @Override
    public <ReqT, RespT> ServerCall.Listener<ReqT> interceptCall(ServerCall<ReqT, RespT> call, Metadata metadata, ServerCallHandler<ReqT, RespT> next) {
        try {
            customValidate(metadata);
        } catch (RuntimeException e) {
            return next.startCall(call, metadata);
        }
        ...
        return Contexts.interceptCall(Context.current(), call, metadata, next);
    }
}