Process hollowing occurs when a process is created in a suspended state then its memory is unmapped and replaced with malicious code. Similar to Process Injection, execution of the malicious code is masked under a legitimate process and may evade defenses and detection analysis.  
This type of attack technique cannot be easily mitigated with preventive controls since it is based on the abuse of system features.
Monitoring API calls 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 that unmap process memory, such as ZwUnmapViewOfSection or NtUnmapViewOfSection, and those that can be used to modify memory within another process, such as WriteProcessMemory, may be used for this technique. 
Analyze process behavior to determine if a process is performing actions it usually does not, such as opening network connections, reading files, or other suspicious actions that could relate to post-compromise behavior.
- Leitch, J. (n.d.). Process Hollowing. Retrieved November 12, 2014.
- Hosseini, A. (2017, July 18). Ten Process Injection Techniques: A Technical Survey Of Common And Trending Process Injection Techniques. Retrieved December 7, 2017.
- Salem, E. (2019, February 13). ASTAROTH MALWARE USES LEGITIMATE OS AND ANTIVIRUS PROCESSES TO STEAL PASSWORDS AND PERSONAL DATA. Retrieved April 17, 2019.
- Yan, T., et al. (2018, November 21). New Wine in Old Bottle: New Azorult Variant Found in FindMyName Campaign using Fallout Exploit Kit. Retrieved November 29, 2018.
- Settle, A., et al. (2016, August 8). MONSOON - Analysis Of An APT Campaign. Retrieved September 22, 2016.
- Lunghi, D., et al. (2017, December). Untangling the Patchwork Cyberespionage Group. Retrieved July 10, 2018.
- Blaich, A., et al. (2018, January 18). Dark Caracal: Cyber-espionage at a Global Scale. Retrieved April 11, 2018.
- Galperin, E., Et al.. (2016, August). I Got a Letter From the Government the Other Day.... Retrieved April 25, 2018.
- Lee, B. Grunzweig, J. (2015, December 22). BBSRAT Attacks Targeting Russian Organizations Linked to Roaming Tiger. Retrieved August 19, 2016.
- Cobalt Strike. (2017, December 8). Tactics, Techniques, and Procedures. Retrieved December 20, 2017.
- Symantec Security Response. (2011, November). W32.Duqu: The precursor to the next Stuxnet. Retrieved September 17, 2015.
- Falcone, R., et al. (2018, August 02). The Gorgon Group: Slithering Between Nation State and Cybercrime. Retrieved August 7, 2018.
- Falcone, R. and Lee, B. (2017, October 9). OilRig Group Steps Up Attacks with New Delivery Documents and New Injector Trojan. Retrieved January 8, 2018.
- Accenture Security. (2018, April 23). Hogfish Redleaves Campaign. Retrieved July 2, 2018.
- Axel F, Pierre T. (2017, October 16). Leviathan: Espionage actor spearphishes maritime and defense targets. Retrieved February 15, 2018.
- Cymmetria. (2016). Unveiling Patchwork - The Copy-Paste APT. Retrieved August 3, 2016.
- Hasherezade. (2016, September 12). Smoke Loader – downloader with a smokescreen still alive. Retrieved March 20, 2018.
- Windows Defender Research. (2018, March 7). Behavior monitoring combined with machine learning spoils a massive Dofoil coin mining campaign. Retrieved March 20, 2018.
- Vaish, A. & Nemes, S. (2017, November 28). Newly Observed Ursnif Variant Employs Malicious TLS Callback Technique to Achieve Process Injection. Retrieved June 5, 2019.