services/go.php in Horde Application Framework 3.0.0 through 3.0.10 and 3.1.0 through 3.1.1 does not properly restrict its image proxy capability, which allows remote attackers to perform "Web tunneling" attacks and use the server as a proxy via (1) http, (2) https, and (3) ftp URL in the url parameter, which is requested from the server.