Try replacing the block with mod_setenvif to check the request URI instead of using <Files> . Mod_auth * modules take precedence over mod_dir, therefore mapping from / to /index.php does not occur earlier than after auth completes. Mod_setenvif will happen before authorization. Try:
SetEnvIf Request_URI "^/$" allow=yes SetEnvIf Request_URI "^/index.php$" allow=yes AuthType Basic AuthName "Password Required" AuthUserFile /var/www/webinterface/.htpasswd Options +FollowSymLinks Order Deny,Allow Satisfy any Deny from All Require valid-user Allow from env=allow
If the requested URI is / or /index.php , then the allow variable is set. The material after the Auth lines says that it forbids everything except the actual user, or if the allow variable was set.
source share