Patchy Phisher Forces Firefox to Forego Forgetting Passwords
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. [...]
One thing that we, nor any other AV company, can do is fix the modified Firefox file. However, there’s an easy fix for that as well: Simply download the latest Firefox installer and install it over the top of your existing installation. You won’t lose any bookmarks or add-ons, and the installer will just overwrite the modified nsLoginManagerPrompter.js file. Problem solved.