Potential PowerShell Execution Via DLL

Severity: high
Author: Markus Neis, Nasreddine Bencherchali (Nextron Systems)
Source: upstream

Detects potential PowerShell execution from a DLL instead of the usual PowerShell process as seen used in PowerShdll. This detection assumes that PowerShell commands are passed via the CommandLine.

MITRE ATT&CK coverage

Tactic	Techniques
Defense Evasion	`T1218.011` System Binary Proxy Execution: Rundll32

Event coverage

Provider	Event ID	Title
Sysmon	1	Process creation

Stages and Predicates

Stage 1: `all of selection_img`

or:
Image|endswith: '\InstallUtil.exe'
Image|endswith: '\RegAsm.exe'
Image|endswith: '\RegSvcs.exe'
Image|endswith: '\regsvr32.exe'
Image|endswith: '\rundll32.exe'
OriginalFileName: InstallUtil.exe
OriginalFileName: REGSVR32.EXE
OriginalFileName: RUNDLL32.EXE
OriginalFileName: RegAsm.exe
OriginalFileName: RegSvcs.exe

Stage 2: `all of selection_cli`

or:
CommandLine|contains: Default.GetString
CommandLine|contains: DownloadString
CommandLine|contains: FromBase64String
CommandLine|contains: 'ICM '
CommandLine|contains: 'IEX '
CommandLine|contains: Invoke-Command
CommandLine|contains: Invoke-Expression

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`	match	`Default.GetString` `DownloadString` corpus 5 (sigma 5) `FromBase64String` corpus 7 (sigma 7) `ICM` `IEX` `Invoke-Command` corpus 2 (sigma 2) `Invoke-Expression` corpus 2 (sigma 2)
`Image`	ends_with	`\InstallUtil.exe` corpus 5 (sigma 5) `\RegAsm.exe` corpus 3 (sigma 3) `\RegSvcs.exe` corpus 2 (sigma 2) `\regsvr32.exe` corpus 57 (sigma 57) `\rundll32.exe` corpus 76 (sigma 76)
`OriginalFileName`	eq	`InstallUtil.exe` corpus 5 (sigma 5) `REGSVR32.EXE` corpus 10 (sigma 10) `RUNDLL32.EXE` corpus 28 (sigma 25, splunk 3) `RegAsm.exe` corpus 6 (sigma 6) `RegSvcs.exe` corpus 5 (sigma 5)