Let's say we have a modern system and when we drop a malicious executable onto that system, we get caught.  We can use a technique called "process injection" which will inject our malicious code into the memory space of a running process.  To show how effective this technique is, we're going to use a meterpreter reverse shell on a Windows 10 system with Defender enabled.  

First, we create our reverse shell and we output to hex:



Next, we save that hex into a text file on our victim system:



We're using this tool for the sake of simplicity but we can do this with C# and a skeleton program. 

You can find the tool here.



To show that Defender is active, we execute a meterpreter shell and Defender stops us.



We point to our hex and we give it the path of the process we're going to spawn and inject:



With our handler setup:



And that's it.  We catch our shell and Defender is none the wiser.