Recently we discovered an advanced backdoor sample – VirTool:WinNT/Exforel.A. Unlike traditional backdoor samples, this backdoor is implemented at the NDIS (Network Driver Interface Specification) level. VirTool:WinNT/Exforel.A implements a simple private TCP/IP stack and hooks NDIS_OPEN_BLOCK for the TCP/IP protocol, as shown in Figure 1.    Figure 1: Hooked functions in NDIS_OPEN_BLOCK This means that backdoor-related TCP traffic will be diverted to the private […]