Suspicious Execution of Powershell with Base64

Severity: medium
Author: frack113
Source: upstream

Commandline to launch powershell with a base64 payload

MITRE ATT&CK coverage

Tactic	Techniques
Execution	`T1059.001` Command and Scripting Interpreter: PowerShell

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:
CommandLine|contains: ' -e '
CommandLine|contains: ' -ec '
CommandLine|contains: ' -en '
CommandLine|contains: ' -enc '
CommandLine|contains: ' -enco'
or:
Image|endswith: '\powershell.exe'
Image|endswith: '\pwsh.exe'

Stage 2: `not 1 of filter_*`

or:
CommandLine|contains: ' -Encoding '
ParentImage|contains: 'C:\Packages\Plugins\Microsoft.GuestConfiguration.ConfigurationforWindows\'
ParentImage|contains: '\gc_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	`-Encoding` `-e` corpus 5 (sigma 5) `-ec` corpus 2 (sigma 2) `-en` corpus 3 (sigma 3) `-enc` corpus 6 (sigma 6) `-enco` corpus 2 (sigma 2)
`Image`	ends_with	`\powershell.exe` corpus 143 (sigma 143) `\pwsh.exe` corpus 140 (sigma 140)
`ParentImage`	match	`C:\Packages\Plugins\Microsoft.GuestConfiguration.ConfigurationforWindows\` corpus 2 (sigma 2) `\gc_worker.exe` corpus 2 (sigma 2)