Extra Window Memory Injection
Before creating a window, graphical Windows-based processes must prescribe to or register a windows class, which stipulate appearance and behavior (via windows procedures, which are functions that handle input/output of data).  Registration of new windows classes can include a request for up to 40 bytes of extra window memory (EWM) to be appended to the allocated memory of each instance of that class. This EWM is intended to store data specific to that window and has specific application programming interface (API) functions to set and get its value.  
Although small, the EWM is large enough to store a 32-bit pointer and is often used to point to a windows procedure. Malware may possibly utilize this memory location in part of an attack chain that includes writing code to shared sections of the process’s memory, placing a pointer to the code in EWM, then invoking execution by returning execution control to the address in the process’s EWM.
Execution granted through EWM injection may take place in the address space of a separate live process. Similar to Process Injection, this may allow access to both the target process's memory and possibly elevated privileges. Writing payloads to shared sections also avoids the use of highly monitored API calls such as WriteProcessMemory and CreateRemoteThread.  More sophisticated malware samples may also potentially bypass protection mechanisms such as data execution prevention (DEP) by triggering a combination of windows procedures and other system functions that will rewrite the malicious payload inside an executable portion of the target process.  
Epic has overwritten the function pointer in the extra window memory of Explorer's Shell_TrayWnd in order to execute malicious code in the context of the explorer.exe process.
Power Loader overwrites Explorer’s Shell_TrayWnd extra window memory to redirect execution to a NTDLL function that is abused to assemble and execute a return-oriented programming (ROP) chain and create a malicious thread within Explorer.exe.
This type of attack technique cannot be easily mitigated with preventive controls since it is based on the abuse of system features.
Monitor for API calls related to enumerating and manipulating EWM such as GetWindowLong  and SetWindowLong . Malware associated with this technique have also used SendNotifyMessage  to trigger the associated window procedure and eventual malicious injection. 
- Microsoft. (n.d.). About Window Classes. Retrieved December 16, 2017.
- Microsoft. (n.d.). GetWindowLong function. Retrieved December 16, 2017.
- Microsoft. (n.d.). SetWindowLong function. Retrieved December 16, 2017.
- Hosseini, A. (2017, July 18). Ten Process Injection Techniques: A Technical Survey Of Common And Trending Process Injection Techniques. Retrieved December 7, 2017.