Ensure the webserver user has the absolute minimum permissions required to read the content and themes folders.
Monitor the official Pico CMS GitHub repository. The transition from alpha.2 to later iterations focuses heavily on patching these discovered "exploit" vectors. Conclusion Pico 3.0.0-alpha.2 Exploit
Ensure debug mode is turned off in your PHP configuration to prevent sensitive path leakage during a crash. Ensure the webserver user has the absolute minimum