Adversaries may inject malicious code into processes via thread local storage (TLS) callbacks in order to evade process-based defenses as well as possibly elevate privileges. TLS callback injection is a method of executing arbitrary code in the address space of a separate live process. 

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. TLS callbacks are normally used by the OS to setup and/or cleanup data used by threads. Manipulating TLS callbacks may be performed by allocating and writing to specific offsets within a process’ memory space using other [Process Injection](https://attack.mitre.org/techniques/T1055) techniques such as [Process Hollowing](https://attack.mitre.org/techniques/T1055/012).(Citation: FireEye TLS Nov 2017)

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 TLS callback injection may also evade detection from security products since the execution is masked under a legitimate process. 
Thread Local Storage
source_name: mitre-attack  url: https://attack.mitre.org/techniques/T1055/005  external_id: T1055.005     source_name: FireEye TLS Nov 2017  description: Vaish, A. & Nemes, S. (2017, November 28). Newly Observed Ursnif Variant Employs Malicious TLS Callback Technique to Achieve Process Injection. Retrieved December 18, 2017.  url: https://www.fireeye.com/blog/threat-research/2017/11/ursnif-variant-malicious-tls-callback-technique.html     source_name: Elastic Process Injection July 2017  description: Hosseini, A. (2017, July 18). Ten Process Injection Techniques: A Technical Survey Of Common And Trending Process Injection Techniques. Retrieved December 7, 2017.  url: https://www.endgame.com/blog/technical-blog/ten-process-injection-techniques-technical-survey-common-and-trending-process     