issue etcd-io/etcd#13553

If applications use clientv3, then this issue will never happen.

But If the client application sends data with invalid client-api-version directly to etcdserver via tcp connection, then the etcd server may be panic. Accordingly, there is a security concern that the malicious program may take down the etcd server.

The program app_send_invalid_client_api_version.c is the demo malicious application. It sends invalid client-api-version, which isn't a valid UTF-8 string.