PowerShell Base64 Encoded Invoke Keyword

Severity: high
Author: pH-T (Nextron Systems), Harjot Singh, @cyb3rjy0t
Source: upstream

Detects UTF-8 and UTF-16 Base64 encoded powershell 'Invoke-' calls

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_cli_enc`

CommandLine|contains: ' -e'

Stage 3: `all of selection_cli_invoke`

or:
CommandLine|contains: JAG4AdgBvAGsAZQAtA
CommandLine|contains: JbnZva2Ut
CommandLine|contains: SQBuAHYAbwBrAGUALQ
CommandLine|contains: SW52b2tlL
CommandLine|contains: kAbgB2AG8AawBlAC0A
CommandLine|contains: ludm9rZS

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	`-e` corpus 4 (sigma 4) `JAG4AdgBvAGsAZQAtA` `JbnZva2Ut` `SQBuAHYAbwBrAGUALQ` `SW52b2tlL` `kAbgB2AG8AawBlAC0A` `ludm9rZS`
`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)