Regsvr32.exe is a command-line program used to register and unregister object linking and embedding controls, including dynamic link libraries (DLLs), on Windows systems. Regsvr32.exe can be used to execute arbitrary binaries. 
Adversaries may take advantage of this functionality to proxy execution of code to avoid triggering security tools that may not monitor execution of, and modules loaded by, the regsvr32.exe process because of whitelists or false positives from Windows using regsvr32.exe for normal operations. Regsvr32.exe is also a Microsoft signed binary.
Regsvr32.exe can also be used to specifically bypass process whitelisting using functionality to load COM scriptlets to execute DLLs under user permissions. Since regsvr32.exe is network and proxy aware, the scripts can be loaded by passing a uniform resource locator (URL) to file on an external Web server as an argument during invocation. This method makes no changes to the Registry as the COM object is not actually registered, only executed.  This variation of the technique is often referred to as a "Squiblydoo" attack and has been used in campaigns targeting governments.  
|APT19||APT19 used Regsvr32 to bypass application whitelisting techniques. |
|APT32||APT32 created a Scheduled Task that used regsvr32.exe to execute a COM scriptlet that dynamically downloaded a backdoor and injected it into memory. The group has also used regsvr32 to run their backdoor.   |
|Astaroth||Astaroth can be loaded through regsvr32.exe. |
|Cobalt Group||Cobalt Group has used regsvr32.exe to execute scripts.   |
|Deep Panda||Deep Panda has used regsvr32.exe to execute a server variant of Derusbi in victim networks. |
|Derusbi||Derusbi variants have been seen that use Registry persistence to proxy execution through regsvr32.exe. |
|Hi-Zor||Hi-Zor executes using regsvr32.exe called from the Registry Run Keys / Startup Folder persistence mechanism. |
|Koadic||Koadic can use Regsvr32 to execute additional payloads. |
|Leviathan||Leviathan has used regsvr32 for execution. |
|Orz||Some Orz versions have an embedded DLL known as MockDll that uses Process Hollowing and regsvr32 to execute another payload. |
|RogueRobin||RogueRobin uses regsvr32.exe to run a .sct file for execution. |
|WIRTE||WIRTE has used Regsvr32.exe to trigger the execution of a malicious script. |
|Xbash||Xbash can use regsvr32 for executing scripts. |
|Exploit Protection||Microsoft's Enhanced Mitigation Experience Toolkit (EMET) Attack Surface Reduction (ASR) feature can be used to block regsvr32.exe from being used to bypass whitelisting. Identify and block potentially malicious software executed through regsvr32 functionality by using application whitelisting tools, like Windows Defender Application Control, AppLocker, or Software Restriction Policies where appropriate.       |
Use process monitoring to monitor the execution and arguments of regsvr32.exe. Compare recent invocations of regsvr32.exe with prior history of known good arguments and loaded files to determine anomalous and potentially adversarial activity. Command arguments used before and after the regsvr32.exe invocation may also be useful in determining the origin and purpose of the script or DLL being loaded. 
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