Potential PowerShell Command Line Obfuscation

Severity: high
Author: Teymur Kheirkhabarov (idea), Vasiliy Burov (rule), oscd.community, Tim Shelton (fp)
Source: upstream

Detects the PowerShell command lines with special characters

MITRE ATT&CK coverage

Tactic	Techniques
Execution	`T1059.001` Command and Scripting Interpreter: PowerShell
Defense Evasion	`T1027` Obfuscated Files or Information

Event coverage

Provider	Event ID	Title
Sysmon	1	Process creation

Stages and Predicates

Stage 1: `all of selection_img`

or:
Image|endswith: '\powershell.exe'
Image|endswith: '\pwsh.exe'
OriginalFileName: PowerShell.EXE
OriginalFileName: pwsh.dll

Stage 2: `all of selection_re`

or:
CommandLine|re: '\+.*\+.*\+.*\+.*\+.*\+.*\+.*\+.*\+.*\+.*\+.*\+.*\+.*\+'
CommandLine|re: '\^.*\^.*\^.*\^.*\^'
CommandLine|re: '\{.*\{.*\{.*\{.*\{.*\{.*\{.*\{.*\{.*\{'
CommandLine|re: '`.*`.*`.*`.*`'

Stage 3: `not 1 of filter_optional_*`

or:
CommandLine|contains: 'new EventSource("Microsoft.Windows.Sense.Client.Management"'
CommandLine|contains: 'public static extern bool InstallELAMCertificateInfo(SafeFileHandle handle);'
ParentImage: 'C:\Program Files\Amazon\SSM\ssm-document-worker.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`	match	`new EventSource("Microsoft.Windows.Sense.Client.Management"` `public static extern bool InstallELAMCertificateInfo(SafeFileHandle handle);`
`CommandLine`	regex_match	`\+.\+.\+.\+.\+.\+.\+.\+.\+.\+.\+.\+.\+.\+` `\^.\^.\^.\^.\^` `\{.\{.\{.\{.\{.\{.\{.\{.\{.\{` `.`.`.`.`
`Image`	ends_with	`\powershell.exe` corpus 143 (sigma 143) `\pwsh.exe` corpus 140 (sigma 140)
`OriginalFileName`	eq	`PowerShell.EXE` corpus 64 (sigma 60, splunk 4) `pwsh.dll` corpus 72 (sigma 68, splunk 4)
`ParentImage`	eq	`C:\Program Files\Amazon\SSM\ssm-document-worker.exe`