Hackers have found a way to exploit a hole in the open-source Firefox browser that enables them to monitor people's activity on the Internet.

The Infostealer.Snifula uses works as an extension to Firefox but installs malicious Trojans that are loaded at start-up. The Trojan captures contents of form submission events including passwords and forwards them to its main process where it is sent to the remote attacker.

Symantec security engineer, Candid Wuest said the exploits, which commonly focus on Internet Explorer, began on Mozilla browsers in March this year with Javascript-based JS.Ffsniff. "When an infected user submits a Web form, [JS.Ffsniff] will parse the site and steal all information that is submitted by the Web form, including passwords," Wuest said. "The JS.Ffsniff script then sends this information to a predefined e-mail address using XPCOM objects."

Joji Hamada, Symantec senior security manager, said the exploit has had a limited impact locally and recommended downloads from trusted sites only. "Our intelligence indicates the threat has been noticed in Australia. However, the impact of the attack overall is minimal at this stage," Hamada said. "To protect themselves from this type of threats, users should ensure they only download software from reputable, trusted sites."

Wuest predicts the attacks will increase with the browser's popularity and recommended users install current browser patches. "With the steady increase of the number of Firefox users we will see the number of malicious extensions created for Firefox grow," he said. "Unfortunately, as soon as something becomes popular, it also becomes a popular target."