PHP fixed a security bug where o had rw permission to the socket file.

1. Open /etc/php5/fpm/pool.d/www.conf
2. Uncomment all permission lines, like:

   listen.owner = www-data
   listen.group = www-data
   listen.mode = 0660
   

3. Restart fpm – sudo service php5-fpm restart

Note: if your webserver runs as as user other than www-data, you will need to update the www.conf file accordingly