Does CiviCRM allow us to declare which directories are writable, allowing the rest to be locked-down?
CiviCRM core seems to be consistent in only writing to a files directory. Until recently, however, it attempted to write to the extensions directory to store extension updates. It's unclear if CiviCRM enforces this for extensions. I've heard that some extensions will write to directories outside of the files (but I haven't been able to confirm this). If that's the case then it will conflict with PAAS that only allow certain directories to be writable.