A recent blog entry from the Microsoft Malware Protection Center details information about a new malware (called Win32/Bohu.A) which is specifically designed to disable and mislead cloud-based antivirus software.
Cloud-based antivirus software differs from traditional antivirus software in that the antivirus client (running on the PC) sends important threat data to a server for backend analysis, and subsequently receives further detection and removal instruction.
The Bohu Trojan originates in China where there is a predominate use of cloud-based antivirus software. Once a Windows based machine is infected the malware installs different network level filters to disrupt and block the antivirus client accessing the backend antivirus services on the Internet.
As well as writing random data at the end of its key payload components to avoid hash-based detection, Bohu also installs a Windows Sockets service provider interface (SPI) filter to block the antivirus network traffic as well as a Network Driver Interface Specification (NDIS) filter. The NDIS filter then stops the antivirus client from uploading data to the server by looking for the server addresses in the data packets.