Trojan:Java/Classloader.G is a malicious Java applet that can infect Microsoft Windows computers that are not patched with Microsoft Security Update MS03-011. An attacker can insert the Java applet into HTML code and host the code on a Web server or send the code in e-mail. When a user opens the Web page or e-mail, the vulnerability allows the applet to bypass a security check on the computer. The applet can then run malicious code on the computer and open a backdoor to receive commands from attackers.