Process injection is a method of executing arbitrary code in the address space of a separate live process. Running code in the context of another process may allow access to the process's memory, system/network resources, and possibly elevated privileges. Execution via process injection may also evade detection from security products since the execution is masked under a legitimate process.
There are multiple approaches to injecting code into a live process. Windows implementations include: 
- Dynamic-link library (DLL) injection involves writing the path to a malicious DLL inside a process then invoking execution by creating a remote thread.
- Portable executable injection involves writing malicious code directly into the process (without a file on disk) then invoking execution with either additional code or by creating a remote thread. The displacement of the injected code introduces the additional requirement for functionality to remap memory references. Variations of this method such as reflective DLL injection (writing a self-mapping DLL into a process) and memory module (map DLL when writing into process) overcome the address relocation issue. 
- Thread execution hijacking involves injecting malicious code or the path to a DLL into a thread of a process. Similar to Process Hollowing, the thread must first be suspended.
- Asynchronous Procedure Call (APC) injection involves attaching malicious code to the APC Queue  of a process's thread. Queued APC functions are executed when the thread enters an alterable state. A variation of APC injection, dubbed "Early Bird injection", involves creating a suspended process in which malicious code can be written and executed before the process' entry point (and potentially subsequent anti-malware hooks) via an APC.  AtomBombing  is another variation that utilizes APCs to invoke malicious code previously written to the global atom table. 
- Thread Local Storage (TLS) callback injection involves manipulating pointers inside a portable executable (PE) to redirect a process to malicious code before reaching the code's legitimate entry point. 
Mac and Linux
- LD_PRELOAD, LD_LIBRARY_PATH (Linux), DYLD_INSERT_LIBRARIES (Mac OS X) environment variables, or the dlfcn application programming interface (API) can be used to dynamically load a library (shared object) in a process which can be used to intercept API calls from the running process. 
- Ptrace system calls can be used to attach to a running process and modify it in runtime. 
- /proc/[pid]/mem provides access to the memory of the process and can be used to read/write arbitrary data to it. This technique is very rare due to its complexity. 
- VDSO hijacking performs runtime injection on ELF binaries by manipulating code stubs mapped in from the linux-vdso.so shared object. 
Malware commonly utilizes process injection to access system resources through which Persistence and other environment modifications can be made. More sophisticated samples may perform multiple process injections to segment modules and further evade detection, utilizing named pipes or other inter-process communication (IPC) mechanisms as a communication channel.
Duqu will inject itself into different processes to evade detection. The selection of the target process is influenced by the security software that is installed on the system (Duqu will inject into different processes depending on which security suite is installed on the infected host).
Gazer performs thread execution hijacking to inject its orchestrator into a running thread from a remote process. Gazer performs a separate injection of its communication module into an Internet accessible process through which it performs C2.
If running in a Windows environment, Kazuar saves a DLL to disk that is injected into the explorer.exe process to execute the payload. Kazuar can also be configured to inject and execute within specific processes.
PLATINUM has used various methods of process injection including hot patching.
An executable dropped onto victims by Putter Panda aims to inject the specified DLL into a process that would normally be accessing the network, including Outlook Express (msinm.exe), Outlook (outlook.exe), Internet Explorer (iexplore.exe), and Firefox (firefox.exe).
After decrypting itself in memory, RARSTONE downloads a DLL file from its C2 server and loads it in the memory space of a hidden Internet Explorer process. This "downloaded" file is actually not dropped onto the system.
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 Windows 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 identification of subsequent malicious behavior. 
Identify or block potentially malicious software that may contain process injection functionality by using whitelisting  tools, like AppLocker,   or Software Restriction Policies  where appropriate. 
Utilize Yama  to mitigate ptrace based process injection by restricting the use of ptrace to privileged users only. Other mitigation controls involve the deployment of security kernel modules that provide advanced access control and process restrictions such as SELinux , grsecurity , and AppAmour .
Monitoring Windows API calls indicative of the various types of code injection may generate a significant amount of data and may not be directly useful for defense unless collected under specific circumstances for known bad sequences of calls, since benign use of API functions may be common and difficult to distinguish from malicious behavior. API calls such as CreateRemoteThread, SuspendThread/SetThreadContext/ResumeThread, QueueUserAPC/NtQueueApcThread, and those that can be used to modify memory within another process, such as WriteProcessMemory, may be used for this technique. 
Monitoring for Linux specific calls such as the ptrace system call, the use of LD_PRELOAD environment variable, or dlfcn dynamic linking API calls, should not generate large amounts of data due to their specialized nature, and can be a very effective method to detect some of the common process injection methods.    
Monitor for named pipe creation and connection events (Event IDs 17 and 18) for possible indicators of infected processes with external modules. 
Monitor processes and command-line arguments for actions that could be done before or after code injection has occurred and correlate the information with related event information. Code injection may also be performed using PowerShell with tools such as PowerSploit,  so additional PowerShell monitoring may be required to cover known implementations of this behavior.
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