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Some versions of bot software captured during this research project provided the capability to remotely start a SOCKS v4/v5 proxy on a compromised host. SOCKS provides a generic proxy mechanism for TCP/IP-based networking applications (RFC 1928) and can be used to proxy most popular Internet traffic, such as HTTP or SMTP email. If an attacker with access to a botnet enables the SOCKS proxy functionality on a remote bot, this machine can then be used to send bulk spam email. If the botnet contains many thousands of compromised hosts, an attacker is then able to send massive amounts of bulk email very easily, often from a wide range of IP addresses owned by unsuspecting home PC users.
The lack of a central point of contact and the range of international boundaries crossed could make it very difficult to trace and stop such activity, making it of low risk, but potentially high reward to spammers and phishers. Perhaps unsurprisingly, resourceful botnet owners have begun to target criminal activity and it is now possible to rent a botnet. For a fee, the botnet operator will provide a customer with a list of SOCKS v4 capable server IP addresses and ports. There are documented cases where botnets were sold to spammers as spam-relays: "Uncovered: Trojans as Spam Robots". Some captured bot software also implemented a special function to harvest email-addresses or to send spam via bots. The following listing shows some of the commands related to sending spam/phishing emails implemented in Agobot, a popular bot used by attackers and a variant regularly captured during our research:
Further information about how these commands are implemented can be found here in a side note about the source code of bots. With the help of drone, a customised IRC client developed by the German Honeynet Project, we were able to learn more about how bots are used for spam/phishing email attacks by smuggling a fake client into a botnet using the connection data collected through the attacks against our honeynets. A number of typical examples of observed activity are shown below.