THE ART OF COMPUTER VIRUS RESEARCH AND DEFENSE [Electronic resources] نسخه متنی

9.4. Infection Propagators

This section summarizes interesting techniques that computer worms use to propagate themselves to new systems.

9.4.1. Attacking Backdoor-Compromised Systems

Although most computer worms do not intentionally attack an already compromised system, some computer worms use other backdoor interfaces to propagate themselves. The W32/Borm worm was among the first computer worms to attack a backdoor-compromised remote system. W32/Borm cannot infect any other systems than those already compromised with Back Orifice (a fairly popular backdoor among attackers). Back Orifice supports a remote command interface that uses an encrypted channel between a client and the Back Orifice server installed on the compromised system. Borm utilizes a network-scanning and fingerprinting function to locate Back Orificecompromised systems. See Figure 9.5 for illustration.

The worm attacks a compromised system using the following simple steps:

W32/Borm was among a flurry of computer worms that appeared in 2001. Other worms that utilized backdoor interfaces in this time frame included Nimda, which took advantage of a backdoor that was previously opened by a CodeRed II infection, and the W32/Leaves worm, which got in the wild by attacking SubSeven Trojan-compromised systems.

Borm was a creation of the Brazilian virus writer, Vecna. Several other of his methods are discussed later in this chapter.

9.4.2. Peer-to-Peer Network Attacks

Peer-to-peer attacks are an increasingly popular computer worm technique that not require advanced scanning methods on the computer network. Instead, such worms simply make a copy of themselves to a shared P2P folder on the disk. Anything that is available in a P2P download folder is searchable by other users on the P2P network.

In fact, some computer worms even create the shared folder in case the user only wants to search the P2P network for content instead of sharing similar content with others. Although this attack is similar to a Trojan installation, rather than recursive propagation, users of the P2P network will find the network-shared content easily and run the malicious code on their systems to complete the infection cycle. Some P2P worms, such as W32/Maax, will infect files in the P2P folder. The most common infection technique is the overwriting method, but prepender and even appender infections have also been seen.

P2P clients such as KaZaA, KaZaA Lite, Limewire, Morpheus, Grokster, BearShare, and Edonkey among others are common targets of malicious code. P2P networks are an increasingly popular way to exchange digital music; they are hard to regulate because they are not centralized.

9.4.3. Instant Messaging Attacks

Instant messaging attacks originated in the abuse of the mIRC /DCC Send command. This command can be used to send a file to users connected to a particular discussion channel. Normally, attackers modify a local script file, such as script.ini used by mIRC to instruct the instant messaging client to send a file to a recipient any time a new participant joins a discussion.²⁷. Although several instant messenger software programs require the user to click a button to send a file, worms can enumerate the dialog boxes and "click" the button, so the actual user does not have to click. It is also expected that computer worms will exploit buffer overflow vulnerabilities in instant messenger software. For example, certain versions of AOL Instant Messenger software allow remote execution of arbitrary code via a long argument in a game request function²⁸.

9.4.4. E-Mail Worm Attacks and Deception Techniques

The vast majority of computer worms use e-mail to propagate themselves to other systems. It is interesting to see how attackers trick users every day, asking them to execute unknown code on their systems by sending them malicious code in e-mail. Let's face it: This is one of the greatest problems of security. How can security experts protect users from themselves?

During the last several years, an increasing number of users have been trapped in a "Matrix" of operating systems, such as Windows. Windows especially gives the illusion to millions of computer users worldwide that they are masters of their computersnot slaves to them. This illusion leads to neglected security practices. In fact, most users do not know that they need to be careful with e-mail attachments. Consider W97M/Melissa, which used the following e-mail to trick recipients into executing the worm on their machines:

"Here is that document you asked for ... don't show anyone else ;-)"

Another common method of deception is forging e-mail headers. For example, the attacker might use the e-mail address of Microsoft's support as the W32/Parvo virus does, placing support@microsoft.com as a sender of the message. This can easily trick users into trusting an attachment and opening it without thinking. Other computers worms, such as W32/Hyd, wait until the user receives a message and quickly answers it by sending a copy of the worm back to the sender. Not surprisingly, this can be a very effective deception method.

Worms also make minor changes in the From: field to change the sender's e-mail randomly to something bogus. In practice, you might receive e-mail messages from many people, and most of the time they have nothing to do with the worm that abused their e-mail address. The bottom line is that notifying "the sender" will not necessarily help.

9.4.5. E-Mail Attachment Inserters

Some computer worms insert messages directly into the mailboxes of e-mail clients. In this way, the worm does not need to send the message; it simply relies on the e-mail client to send the mail. The earliest example of computer worms on Windows systems were of this type. An example of this is Win/Redteam, which targets outgoing mailboxes of the Eudora e-mail client.

9.4.6. SMTP ProxyBased Attacks

W32/Taripox@mm²⁹ is an example of a tricky worm that acts as an SMTP (simple mail transfer protocol) proxy. This worm appeared in February of 2002. Taripox attacks the %WINDOWS %\SYSTEM32\DRIVERS\ETC\HOSTS file to proxy mail traffic to itself. Normally, the HOSTS file has a simple definition for the localhost address as shown in Listing 9.6.

Listing 9.6. The Content of a Typical Host's Configuration File

127.0.0.1       localhost

W32/Taripox remaps the IP address of the SMTP server to the local host. The worm can listen on port 25 (SMTP) and wait for any SMTP e-mail client to connect. The outgoing e-mail message is then forwarded to the real SMTP server, but first the worm injects its own MIME-encoded attachment. The worm also tricks users with comment entries in the host file such as "# Leave this untouched," which is used for the localhost entry, and "# do not remove!," which is used as the comment for the SMTP IP address to localhost redirection entry. Figure 9.6 illustrates how Taripox works.

The HOSTS file is a common target for Retro worms to deny access to the Web sites of antivirus and security companies. Taripox's attack is similar to Happy99's, but it is a much simpler technique and does not require complicated modifications to binary files like WSOCK32.DLL.

9.4.7. SMTP Attacks

As Microsoft strengthened Outlook's security to protect end users better against worm attacks, computer worm authors quickly started to use more and more SMTP-based attacks.²⁰ worm in July 2001 and was followed by the infamous W32/Nimda worm in September 2001. Smaller outbreaks had signaled the problem earlier, with the in-the-wild appearance of W32/ExploreZip in June 1999.

Sircam avoids relying on an e-mail program by getting the SMTP information directly from the Registry. This information consists of the following keys, which are created and used by a number of Microsoft mail applications:

• The current user's e-mail address: HKCU\Software\Microsoft\Internet Account Manager\Default Mail Account\Accounts\SMTP Email Address

• The address of the e-mail server: HKCU\Software\Microsoft\Internet Account Manager\Default Mail Account\Accounts\SMTP Server

• The user's display name: HKCU\Software\Microsoft\Internet Account Manager\Default Mail Account\Accounts\SMTP Display Name

If, for some reason, this information does not exist, Sircam will use prodigy.net.mx as the e-mail server, and the user's logon name as the e-mail address and display name. Using a hard-coded list of SMTP IP addresses is a common technique for computer worms, but usually this trick quickly overloads the particular set of servers once the worm is sufficiently widespread. Typically, such worms take off their SMTP servers with a DoS attack very quickly.

Thanks to implementation mistakes and bugs, it took a little while before SMTP worms could take their real place. Before Sircam, most worms lacked some important detail in their spreading mechanism. For instance, Magistr¹⁸ often sends clean files or files that are infected but that reference some libraries not available on the recipient's system, so they fail to penetrate the target.

To illustrate the simplicity of SMTP, consider Table 9.3.

At this point, the e-mail might be dropped into a folder with a temporary name on the server in EML (Electronic Mail List) format. For example, Figure 9.7 shows an e-mail message sent by the W32/Aliz worm that was stored in the mail drop folder of Microsoft IIS Server.