Unsigned AppX Installation Attempt Using Add-AppxPackage

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

Detects usage of the "Add-AppxPackage" or it's alias "Add-AppPackage" to install unsigned AppX packages

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_cmdlet`

or:
CommandLine|contains: 'Add-AppPackage '
CommandLine|contains: 'Add-AppxPackage '

Stage 3: `all of selection_flag`

CommandLine|contains: ' -AllowUnsigned'

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	`-AllowUnsigned` `Add-AppPackage` `Add-AppxPackage`
`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)