This is the Composr upgrade tool. This tool is a part of Composr, but it is intentionally kept lightweight to keep it working even if Composr is partly broken.
Once you log in you will be presented with some options to help you upgrade or perform maintenance on your Composr-powered website.
If so, find the master_password line of your _config.php file and put in a new temporary password (the one written there won't work because it is encrypted). Once you have done this you should promptly use the config_editor.php script to change the password, as this will save it again as encrypted.