The Koobface botnet is widely known to install FAKEAV or rogue antivirus malware onto a victim’s PC. It has a dedicated component which actually installs the FAKEAV onto the user’s system. However, the Koobface gang has added a new twist to its fake Facebook page.

When the user closes the window/tab with the fake Facebook page, a popup window appears. Whatever button the user clicks, this new Koobface variant is downloaded onto the affected system. Here’s a video that illustrates this behavior:

This is the script used by cybercriminals to perform this new routine; it only works for users who used Internet Explorer to visit the fake page:

Figure 1. Koobface Script

The scripts above leaves the user with very little choice – closing the browser window downloads a FakeAV variant (detected as TROJ_FAKEAV.FGR), while clicking anywhere on the web page will download a Koobface loader (detected as WORM_KOOBFACE.AZ).