source: https://www.securityfocus.com/bid/1650/info

The explorer.php script within phpPhotoAlbum 0.9.9 and possibly previous versions are vulnerable to directory traversal. By requesting a URL composed of explorer.php and the ../ string in the value of the "folder" variable it is possible for a remote user to and gain read access to any file or browse any directory for which the webserver has read access.

Example:
http://target/phpPhotoAlbum/explorer.php?folder=../../../../