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.  
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 operating system design features. For example, mitigating specific API calls will likely have unintended side effects, such as preventing legitimate software (i.e., security products) from operating properly. Efforts should be focused on preventing adversary tools from running earlier in the chain of activity and on identifying subsequent malicious behavior.
Although EWM injection may be used to evade certain types of defenses, it is still good practice to identify potentially malicious software that may be used to perform adversarial actions and audit and/or block it by using whitelisting  tools, like AppLocker,   or Software Restriction Policies  where appropriate. 
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. 
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- Microsoft. (n.d.). SendNotifyMessage function. Retrieved December 16, 2017.