Ramblings & ephemera

The life cycle of a botnet client

From Chapter 2: Botnets Overview of Craig A. Schiller’s Botnets: The Killer Web App (Syngress: 2007):

What makes a botnet a botnet? In particular, how do you distinguish a botnet client from just another hacker break-in? First, the clients in a botnet must be able to take actions on the client without the hacker having to log into the client’s operating system (Windows, UNIX, or Mac OS). Second, many clients must be able to act in a coordinated fashion to accomplish a common goal with little or no intervention from the hacker. If a collection of computers meet this criteria it is a botnet.

The life of a botnet client, or botclient, begins when it has been exploited. A prospective botclient can be exploited via malicious code that a user is tricked into running; attacks against unpatched vulnerabilities; backdoors left by Trojan worms or remote access Trojans; and password guessing and brute force access attempts. In this section we’ll discuss each of these methods of exploiting botnets.

Rallying and Securing the Botnet Client

Although the order in the life cycle may vary, at some point early in the life of a new botnet client it must call home, a process called “rallying. “When rallying, the botnet client initiates contact with the botnet Command and Control (C&C) Server. Currently, most botnets use IRC for Command and Control.

Rallying is the term given for the first time a botnet client logins in to a C&C server. The login may use some form of encryption or authentication to limit the ability of others to eavesdrop on the communications. Some botnets are beginning to encrypt the communicated data.

At this point the new botnet client may request updates. The updates could be updated exploit software, an updated list of C&C server names, IP addresses, and/or channel names. This will assure that the botnet client can be managed and can be recovered should the current C&C server be taken offline.

The next order of business is to secure the new client from removal. The client can request location of the latest anti-antivirus (Anti-A/V) tool from the C&C server. The newly controlled botclient would download this soft- ware and execute it to remove the A/V tool, hide from it, or render it ineffective.

Shutting off the A/V tool may raise suspicions if the user is observant. Some botclients will run a dll that neuters the A/V tool. With an Anti-A/V dll in place the A/V tool may appear to be working normally except that it never detects or reports the files related to the botnet client. It may also change the Hosts file and LMHosts file so that attempts to contact an A/V vendor for updates will not succeed. Using this method, attempts to contact an A/V vendor can be redirected to a site containing malicious code or can yield a “website or server not found” error.

One tool, hidden32. exe, is used to hide applications that have a GUI interface from the user. Its use is simple; the botherder creates a batch file that executes hidden32 with the name of the executable to be hidden as its parameter. Another stealthy tool, HideUserv2, adds an invisible user to the administrator group.

Waiting for Orders and Retrieving the Payload

Once secured, the botnet client will listen to the C&C communications channel.

The botnet client will then request the associated payload. The payload is the term I give the software representing the intended function of this botnet client.

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