Strategies of Computer Worms

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Art of Computer Virus Research and Defense, The

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This chapter is from the book

This chapter is from the book 

Art of Computer Virus Research and Defense, The

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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 Orifice–compromised systems. See Figure 9.5 for illustration.

Figure 9.5 W32/Borm uses Back Orifice to propagate itself.

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

1. It randomly generates an IP address and actively scans using Back Orifice's BO_PING command to see whether the remote system is compromised. To initiate any meaningful communication, the worm needs to know Back Orifice's magic password, which is *!*QWTY?. The header of the communication data is encrypted with simple encryption, which is required by the Back Orifice server. Borm properly encrypts the data before it sends it to the randomly generated IP address on port 31337/UDP used by Back Orifice. If the remote node answers the BO_PING command, the worm proceeds to the next step. Otherwise, it generates other IP addresses to attack.

2. Borm sends a BO_HTTP_ENABLE command to the server.

3. In turn, this command instructs Back Orifice to create a virtual HTTP server on the compromised host. The worm instructs Back Orifice to use port 12345/TCP to establish an HTTP proxy on the compromised system.

4. Next, the worm connects and uploads itself in MIME-encoded format to the server.

5. Finally, the worm runs the uploaded executable on the server by sending a BO_PROCESS_SPAWN command. This will run the worm on the remote machine so it can start to scan for other Back Orifice systems from the newly infected node.

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.

Modern implementations of IRC (Internet Relay Chat) worms can connect dynamically to an IRC client and send messages that trick the recipient into executing a link or an attachment. In this way, the attacker can avoid modifying any local files.

For example, the W32/Choke worm uses the MSN Messenger API to send itself to other instant messaging participants as a "shooter game"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 computers—not 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 Proxy–Based 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.

Figure 9.6 The W32/Taripox worm uses an SMTP proxy.

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.

The first such major worldwide outbreak was caused by the Sircam²⁰ 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.

Table 9.3 A Typical SMTP Communication Between a Client and a Server

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.

Figure 9.7 An e-mail message of the W32/Aliz worm.

This snippet already reveals the Content-Type exploit in the body of the e-mail, which will be discussed in more detail in Chapter 10. Chapter 15, "Malicious Code Analysis Techniques," also shows a network-level capture of W32/Aliz as an illustration of analysis techniques.

9.4.8 SMTP Propagation on Steroids Using MX Queries

Worms such as Nimda, Klez, Sobig, and Mydoom perfected SMTP mass-mailing by utilizing automated SMTP server address resolution using MX (eMail eXchanger) record lookups via the DNS (Domain Name System). Such worms check the domain name of the e-mail addresses they have harvested and obtain a valid SMTP server for that domain. Mydoom even uses the backup SMTP server addresses, instead of the primary server IP address, to reduce the load on SMTP servers further.

Table 9.4 is a list of MX look-ups of Mydoom on a test machine. The worm immediately sends itself to systems that it can find correctly, doing so many times per minute. In Table 9.4, the first column is the time in seconds, the second column is the infected system's IP address, and the third column shows the IP address of the DNS server used to make MX lookups.

Table 9.4 DNS Queries of the Mydoom Worm

9.4.9 NNTP (Network News Transfer Protocol) Attacks

Worms such as Happy99 can mail themselves to newsgroups as well as to e-mail addresses. Usenet attacks can further enhance the spreadability of computer worms. Interestingly, most computer worms will only send mail directly to e-mail addresses.