Scripting/CommandLine Process Spawned Regsvr32

Severity: medium
Author: Florian Roth (Nextron Systems), Nasreddine Bencherchali (Nextron Systems)
Source: upstream

Detects various command line and scripting engines/processes such as "PowerShell", "Wscript", "Cmd", etc. spawning a "regsvr32" instance.

MITRE ATT&CK coverage

Tactic	Techniques
Defense Evasion	`T1218.010` System Binary Proxy Execution: Regsvr32

Event coverage

Provider	Event ID	Title
Sysmon	1	Process creation
Security-Auditing	4688	A new process has been created.

Stages and Predicates

Stage 1: `selection`

or:
ParentImage|endswith: '\cmd.exe'
ParentImage|endswith: '\cscript.exe'
ParentImage|endswith: '\mshta.exe'
ParentImage|endswith: '\powershell.exe'
ParentImage|endswith: '\powershell_ise.exe'
ParentImage|endswith: '\pwsh.exe'
ParentImage|endswith: '\wscript.exe'
Image|endswith: '\regsvr32.exe'

Stage 2: `not 1 of filter_main_rpcproxy`

CommandLine|endswith: ' /s C:\Windows\System32\RpcProxy\RpcProxy.dll'
ParentImage: 'C:\Windows\System32\cmd.exe'

Indicators

Each row is a field, operator, and value that the rule matches. The corpus column counts how many other rules in the catalog look for the same combination: high numbers point to widely-used, community-vetted indicators. Blank or 1 shows that the indicator is specific to this rule.

Field	Kind	Values
`CommandLine`	ends_with	`/s C:\Windows\System32\RpcProxy\RpcProxy.dll`
`Image`	ends_with	`\regsvr32.exe` corpus 57 (sigma 57)
`ParentImage`	ends_with	`\cmd.exe` corpus 13 (sigma 13) `\cscript.exe` corpus 14 (sigma 14) `\mshta.exe` corpus 10 (sigma 10) `\powershell.exe` corpus 16 (sigma 16) `\powershell_ise.exe` corpus 3 (sigma 3) `\pwsh.exe` corpus 16 (sigma 16) `\wscript.exe` corpus 14 (sigma 14)
`ParentImage`	eq	`C:\Windows\System32\cmd.exe`