The K2 frontend article-attachment upload path accepts files whose extension is `.php`, and Apache's standard mod_php matches `\.php$` and executes them under the K2 web user. A K2 Author can upload a `shell.php`, then fetch `/media/k2/attachments/shell.php` and execute arbitrary PHP code in the web server's context.

The K2 frontend article-attachment upload path accepts files whose extension is `.php`, and Apache's standard mod_php matches `\.php$` and executes them under the K2 web user. A K2 Author can upload a `shell.php`, then fetch `/media/k2/attachments/shell.php` and execute arbitrary PHP code in the web server's context.