Fantibag.B is a trojan that installs a packet filter for preventing of downloading the AV companies database updates and security patches. It is related to recent Bagle/Mitglieder trojans.
When the trojan's file is executed, it copies itself in the Windows directory with the name 'firewall_anti.exe'. It installs the following registry key to ensure that it will be executed at system startup:
The trojan drops a DLL 'firewall_anti.dll' in the Windows direcory and injects this file in the address space of Internet Explorer.
Packet filtering
When the dropped DLL is activated, it modifies the network interface with Microsoft RAS packet filtering API. It adds a filter that blocks access to following AV companies and other security related sites:
