Direct Syscalls Support for Windows Meterpreter
Direct system calls are a well-known technique that is often used to bypass EDR/AV detection. This technique is particularly useful when dynamic analysis is performed, where the security software monitors every process on the system to detect any suspicious activity. One common way to do so is to add user-land hooks on Win32 API calls, especially those commonly used by malware. Direct syscalls are a way to run system calls directly and enter kernel mode without passing through the Win32 API.
This first implementation focuses on substituting the Win32 API calls used by the Reflective DLL Injection (RDI) library with Direct Syscalls to the corresponding Native API's. For example, VirtualAlloc has been substituted by a system call to ZwAllocateVirtualMemory. Since RDI is used everywhere by Meterpreter and its extensions, it was a very good candidate for this initial work.
The main difficulty is to find the correct syscall number since it is not the same across Windows versions. Also, EDR's usually hook the NTDLL native API, making the discovery of syscall numbers more challenging. The technique used for this is based on the assumption that the syscall numbers are assigned in a sequential order, starting from the number 0. If we look at how native API functions are stored in memory, the syscall number can be deduced from the position of the related native API function in memory. The technique consists in selecting the system call functions (Zw…) from ntdll.dll exports and sorting them in ascending order of their memory addresses. The syscall number of one given native API function is simply its index in this sorted list. This is very similar to the technique used by
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