9.5. Common Worm Code Transfer and Execution Techniques
Computer worms also differ how they propagate the worm's code from one system to another. Most computer worms simply propagate their main body as an attachment in an e-mail. However, other types of worms utilize different methods, such as injected code and shellcode techniques in conjunction with exploit code, to attack another system.
9.5.1. Executable CodeBased Attacks
E-mail can be encoded in various ways, such as UU, BASE64 (MIME), and so on. However, the UU-encoded attachments are not very reliable over the Internet because UU uses some special characters whose interpretation depends on the context. Nowadays, most e-mail clients use MIME-encoded attachments by defaultand that is how most e-mail worms' SMTP client engines transfer themselves to new targets. Script e-mail worms usually send attachments encoded according to the settings of the e-mail client on the compromised system.
9.5.2. Links to Web Sites or Web Proxies
Computer worms also can send links to executables hosted elsewhere, such as a single Web site, a set of Web sites, or an FTP location. The actual message on IRC or in e-mail might not have any malicious content in it directlybut infects indirectly. One problem with this kind of attack is the possibility of an accidental DoS attack against the system that hosts the worm's code. Another potential pitfall is that the defender can easily contact Internet service providers to request they disconnect such sites, preventing further propagation of the computer worm.
Tricky worms send links with the IP address of an already compromised system. First, the worm compromises a machine and opens a crude Web server on the system. Then it sends messages to other users, using the IP address of the machine with the port on which the worm itself is listening for a GET request. In this way, the worm attack becomes peer-to-peer, as Figure 9.8 illustrates. Such computer worms might be able to bypass content filtering easily if the content-filtering rule is based on attachment filtering.
Figure 9.8. Tricky worms send links in e-mails instead of their own copy.
W32/Beagle.T used a similar method in March 2004. This variant of Beagle opens a crude Web server on TCP port 81. Then it sends a link to the recipient that triggers automated downloading with an HTML-based mail (which exploits the Microsoft Internet Explorer object tag vulnerability described in the MS03-032 bulletin) to download and execute the hosted worm executable on the target automatically.
The W32/Aplore worm was among the first worms to use this attack to propagate itself on Instant Messaging in April 2002. When the W32/Aplore@mm30 worm arrives on a new system, it acts as a crude local Web server on port 8180, hosting a Web page that instructs the user to download and run a program, which is the worm body itself. The worm tricks Instant Messenger users by sending them a link that looks like the following:
FREE PORN: http://free:email@example.com:8180
9.5.3. HTML-Based Mail
The e-mail can even be HTML mail-based. Disabling HTML support in your e-mail client reduces your chance of exposure to at least some of these threats, such as VBS/Bubbleboy. This worm is described in Chapter 10.
9.5.4. Remote Login-Based Attacks
On UNIX-like systems, commands such as rsh, rlogin, rcp, and rexec can be used directly by computer worms. Using such commands, worms can execute themselves on remote systems if the attacked system is not secured or if the password is guessed with a dictionary attack or similar method. Usually, such worms make a copy of their code directly to the remote system and execute themselves via the remote execution facilities.
On Windows systems, worms like JS/Spida can take advantage of vulnerable Microsoft SQL servers. Spida scans for remote Microsoft SQL server systems on port 1433 and tries to execute itself remotely with the following assumptions:
9.5.5. Code Injection Attacks
A more advanced attack requires exploitation of a target with direct code injection over the network. As traditional buffer overflows are getting more difficult to exploit, attackers are increasingly interested in exploiting server-side vulnerabilities related to a lack of input validation. For example, the Perl/Santy worm utilizes Google to find vulnerable Web sites and runs its own Perl script via a vulnerability in the phpBB bulletin board software. This worm successfully defaced tens of thousands of Web sites on December 21 of 2004. Depending on the thread model of the vulnerable target server, one of the following actions will happen:
Furthermore, depending on the context of the hijacked thread, the worm
These preconditions are often reflected in the worm's operation. When, for example, W32/Slammer exploits a vulnerable Microsoft SQL server, the worm hijacks a thread that was executed at the start of the server. Thus the operations associated with the hijacked thread will be paralyzed because new incoming requests will not be resolved. In addition, the server process and the entire system is heavily overloaded because the worm never stops sending itself to new targets.
An example of the second type of attack is W32/CodeRed. CodeRed exploits Microsoft IIS server via a malformed GET request. When the server receives the GET request, it executes a new thread to process it. The worm hijacks that particular thread and creates 100 new threads (300 in some variants) in the vulnerable server process. This kind of computer worm needs to avoid infecting the target a second time because the worm could exploit the target multiple times, causing the target to be overloaded shortly after the initial outbreak. In addition, computer worms that counterattack each other can also benefit from this condition because they can utilize the same exploit as their opponent.
Figure 9.9. A typical one-way code injection attack.
In some cases, the injected code creates a new user account on the target that can be used by the attacker to log in to the system remotely.
Lespaul is a mass-mailer worm, but just like CodeRed or Slammer, it injects its code directly into the vulnerable Eudora 5 process. The worm does not send an attachment to the recipient; instead, it propagates itself as the mail body. It can appear in the Eudora mailbox as part of an e-mail message featuring an overly long header field; however, its code is never saved into a standalone executable at any point in order to be executed.
9.5.6. Shell CodeBased Attacks
Another class of computer worms utilize shell code on the target machine. The basic idea is to run a command prompt on the remote system, such as cmd.exe (on Windows) or /bin/sh (on UNIX) via the exploit code. Consider Figure 9.10 for an illustration.
Figure 9.10. A typical shell codebased attack.
The worm follows these steps:
Shellcode-based attacks are typically more common on UNIX systems than on Windows systems. A few variations exist, such as back-connecting shellcode and shellcode that reuses an existing connection.
Back-connecting shellcode immediately attempts to connect the target with the attacker by establishing a TCP connection from the target to the attacker's machine. The advantage of this method is that it allows machines behind a firewall to "connect-out" to the attacker system.
This attack requires the attacker system to listen on a particular port and wait for the shellcode to connect, as shown in Figure 9.11.
Figure 9.11. A back-connecting shellcode.
The basic difference occurs in the second step. The shellcode executes on the target and connects to the attacker. When the connection is established, the shellcode creates a shell prompt that gets its input from the attacker. The W32/Welchia worm uses this approach.
The exploiting phase might take place in a few steps. For example, Linux/Slapper exploits the target more than once to run shellcode via a heap overflow condition. Slapper, however, implements yet another shell-code technique, reusing the connection established between the attacker's machine and the target. As shown in Figure 9.12, the shellcode does not need to reconnect to the target. In Chapter 15 you can find a traced shellcode of Slapper that illustrates the reused connection better.
Figure 9.12. A connection-reusing shellcode.