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Regsvr32

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. [1]

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. [2] This variation of the technique is often referred to as a "Squiblydoo" attack and has been used in campaigns targeting governments. [3] [4]

Regsvr32.exe can also be leveraged to register a COM Object used to establish Persistence via Component Object Model Hijacking. [3]

ID: T1117

Tactic: Defense Evasion, Execution

Platform:  Windows

Permissions Required:  User, Administrator

Data Sources:  Loaded DLLs, Process monitoring, Windows Registry, Process command-line parameters

Supports Remote:  No

Defense Bypassed:  Process whitelisting, Anti-virus

Contributors:  Casey Smith

Version: 1.0

Examples

NameDescription
APT19

APT19 used Regsvr32 to bypass application whitelisting techniques.[5]

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.[6]

Cobalt Group

Cobalt Group used regsvr32.exe to execute scripts.[7]

Deep Panda

Deep Panda has used regsvr32.exe to execute a server variant of Derusbi in victim networks.[8]

Derusbi

Derusbi variants have been seen that use Registry persistence to proxy execution through regsvr32.exe.[9]

Hi-Zor

Hi-Zor executes using regsvr32.exe called from the Registry Run Keys / Start Folder persistence mechanism.[10]

Koadic

Koadic can use Regsvr32 to execute additional payloads.[11]

Leviathan

Leviathan has used regsvr32 for execution.[12]

Orz

Some Orz versions have an embedded DLL known as MockDll that uses Process Hollowing and regsvr32 to execute another payload.[12]

Mitigation

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. [13]

Detection

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. [3]

References