If the server is not properly secured there can be some rogue app running on the same server as your site that is able to write to the file system in your web site. I would get different hosting because it sounds like your server is not properly configured to prevent this and the server itself may be compromised by malware.
In mojoPortal, the only folders that need to be writable are /App_Data and /Data. All other folders including the /Admin folder should be configured as read only.
Best,
Joe