On July 2nd, 2019, several vulnerabilities were published by RIPSTECH. They can be exploited by placing a specially crafted order. Once your staff views the order in the backend, their session is leaked to the attacker. Subsequently, the attacker uses another vulnerability to gain direct access to the server.
It was fixed in the following Magento versions:
If you are running a Magento 2 store with an older version, we recommend to urgently update to the latest version. See the release statement from Magento for more information.