  A botnet is a network of compromised machines (bots), under the control of an
attacker. Many of these machines are infected without their owners' knowledge,
and botnets are the driving force behind several misuses and criminal
activities on the Internet (for example spam emails). Depending on its
topology, a botnet can have zero or more command and control (C&C) servers,
which are centralized machines controlled by the cybercriminal that issue
commands and receive reports back from the co-opted bots.
  In this paper, we present a comprehensive analysis of the command and control
infrastructure of one of the world's largest proprietary spamming botnets
between 2007 and 2012: Cutwail/Pushdo. We identify the key functionalities
needed by a spamming botnet to operate effectively. We then develop a number of
attacks against the command and control logic of Cutwail that target those
functionalities, and make the spamming operations of the botnet less effective.
This analysis was made possible by having access to the source code of the C&C
software, as well as setting up our own Cutwail C&C server, and by implementing
a clone of the Cutwail bot. With the help of this tool, we were able to
enumerate the number of bots currently registered with the C&C server,
impersonate an existing bot to report false information to the C&C server, and
manipulate spamming statistics of an arbitrary bot stored in the C&C database.
Furthermore, we were able to make the control server inaccessible by conducting
a distributed denial of service (DDoS) attack. Our results may be used by law
enforcement and practitioners to develop better techniques to mitigate and
cripple other botnets, since many of findings are generic and are due to the
workflow of C&C communication in general.
