Code Snippets <= 3.9.1 - Authenticated (Contributor+) PHP Code Injection via extract() and PHP Filter Chains

Summary

The Code Snippets plugin for WordPress is vulnerable to PHP Code Injection in all versions up to, and including, 3.9.1. This is due to the plugin's use of extract() on attacker-controlled shortcode attributes within the `evaluate_shortcode_from_flat_file` method, which can be used to overwrite the `$filepath` variable and subsequently passed to require_once. This makes it possible for authenticated attackers, with Contributor-level access and above, to execute arbitrary PHP code on the server via the `[code_snippet]` shortcode using PHP filter chains granted they can trick an administrator into enabling the "Enable file-based execution" setting and creating at least one active Content snippet.