The security issue you mentioned has been addressed in versions 8.5.0, and 8.7.0. You shpould delete these files.
To ensure that your system is secure, please make sure that you have installed the latest Acymailing version, which is currently 9.0.2.
We have taken several steps to notify our users about these security updates. We have sent out two email notifications, published two articles on our website, and also discussed the issues on our forum.
For more information on these updates, please visit the following links:
The script mentioned in the last link meanwhile has been improved and added to Acymailing. You'll find it in Configuration->Security.
Please keep an eye on our changelog en keep your Acymailing updated. You also might want to make sure you're subscribed to our mailing list so you will receive important information.