This module will change directory permissions to 755 and file permissions to 644.
Step 1: Download latest version of the module from here: https://github.com/MathiasReker/filepermissions/releases/latest
Step 2: Upload the module to your PrestaShop 1.7.x store.
Step 3: Go to configuration of the module and click on 'Fix permissions' and wait until the job is done (10-30 sec).
Thats it! The permissions should now be good. If the permissions could not be changed, you will get a message about that. Now you can uninstall the module if you want.