Every browser can, at the user’s discretion, be set up to remember passwords. In general, Webroot advises most users not to set the browser to store login credentials, because they’re so easily extracted by password-stealing Trojans like Zbot
. In Firefox, for example, you can click Tools, Options, then open the Security tab, and uncheck a box that tells the browser to remember passwords entered into Web forms. (The box is checked by default.)
But in the course of taking a more thorough look at a Trojan that came to our attention in July, we were surprised to see the Trojan modify a core Firefox file. Upon closer inspection, the Trojan patches a file named nsLoginManagerPrompter.js
. The patch adds a few lines of code (displayed above), and comments-out other portions of code, that dictate whether Firefox prompts the user to save passwords when he or she logs into a secure site.
Before the infection, a default installation of Firefox 3.6.10 would prompt the user after the user clicks the Log In button on a Web page, asking whether he or she wants to save the password. After the infection, the browser simply saves all login credentials locally, and doesn’t prompt the user.
The keylogging Trojan copies itself to the system32 directory with the filename Kernel.exe;
drops and registers an old, benign, deprecated ActiveX control called the Microsoft Internet Transfer Control DLL, or msinet.ocx (MD5: 7BEC181A21753498B6BD001C42A42722
), which it uses to communicate with its command and control server; then it creates a new user account (username: Maestro
) on the infected system.