PowerShell Set-Acl On Windows Folder - PsScript

Severity: high
Author: frack113, Nasreddine Bencherchali (Nextron Systems)
Source: upstream

Detects PowerShell scripts to set the ACL to a file in the Windows folder

MITRE ATT&CK coverage

Tactic	Techniques
Defense Evasion	`T1222` File and Directory Permissions Modification

Event coverage

Provider	Event ID	Title
PowerShell	4104	Creating Scriptblock text (MessageNumber of MessageTotal).

Stages and Predicates

Stage 1: `all of selection_cmdlet`

ScriptBlockText|contains: '-AclObject '
ScriptBlockText|contains: 'Set-Acl '

Stage 2: `all of selection_paths`

or:
ScriptBlockText|contains: '-Path $env:windir'
ScriptBlockText|contains: '-Path ''$env:windir'
ScriptBlockText|contains: '-Path ''C:/Windows'
ScriptBlockText|contains: '-Path ''C:\Windows'
ScriptBlockText|contains: '-Path C:/Windows'
ScriptBlockText|contains: '-Path C:\\Windows'
ScriptBlockText|contains: '-Path "$env:windir'
ScriptBlockText|contains: '-Path "C:/Windows'
ScriptBlockText|contains: '-Path "C:\Windows'

Stage 3: `all of selection_permissions`

or:
ScriptBlockText|contains: Allow
ScriptBlockText|contains: FullControl

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
`ScriptBlockText`	match	`-AclObject` corpus 2 (sigma 2) `-Path "$env:windir` `-Path "C:/Windows` `-Path "C:\Windows` `-Path $env:windir` `-Path '$env:windir` `-Path 'C:/Windows` `-Path 'C:\Windows` `-Path C:/Windows` `-Path C:\\Windows` `Allow` `FullControl` `Set-Acl` corpus 2 (sigma 2)

Neighbors

Stricter alternatives (narrower than this rule)

The rules below may be useful if you find the current rule is too noisy / lacks specificity.

PowerShell Script Change Permission Via Set-Acl - PsScript [sigma] (adds 1 filter) (first-stage match only; downstream stages may differ)

Broader alternatives (more inclusive than this rule)

These rules match a superset of what this rule catches. They cover the same events plus more. Use them if you want wider coverage and can absorb more false positives.

Potential PowerShell Obfuscation via Invalid Escape Sequences [elastic] (cross-vendor) (drops 2 filters this rule applies) (first-stage match only; downstream stages may differ)
Potential PowerShell Obfuscation via Backtick-Escaped Variable Expansion [elastic] (cross-vendor) (drops 2 filters this rule applies) (first-stage match only; downstream stages may differ)
Potential PowerShell Obfuscation via Character Array Reconstruction [elastic] (cross-vendor) (drops 2 filters this rule applies) (first-stage match only; downstream stages may differ)
Potential PowerShell Obfuscation via Concatenated Dynamic Command Invocation [elastic] (cross-vendor) (drops 2 filters this rule applies) (first-stage match only; downstream stages may differ)
Potential PowerShell Obfuscation via High Numeric Character Proportion [elastic] (cross-vendor) (drops 2 filters this rule applies) (first-stage match only; downstream stages may differ)
Potential Dynamic IEX Reconstruction via Environment Variables [elastic] (cross-vendor) (drops 2 filters this rule applies) (first-stage match only; downstream stages may differ)
Dynamic IEX Reconstruction via Method String Access [elastic] (cross-vendor) (drops 2 filters this rule applies) (first-stage match only; downstream stages may differ)
PowerShell Obfuscation via Negative Index String Reversal [elastic] (cross-vendor) (drops 2 filters this rule applies) (first-stage match only; downstream stages may differ)
Potential PowerShell Obfuscation via Reverse Keywords [elastic] (cross-vendor) (drops 2 filters this rule applies) (first-stage match only; downstream stages may differ)
Potential PowerShell Obfuscation via String Concatenation [elastic] (cross-vendor) (drops 2 filters this rule applies) (first-stage match only; downstream stages may differ)
Potential PowerShell Obfuscation via String Reordering [elastic] (cross-vendor) (drops 2 filters this rule applies) (first-stage match only; downstream stages may differ)
Potential PowerShell Obfuscation via Special Character Overuse [elastic] (cross-vendor) (drops 2 filters this rule applies) (first-stage match only; downstream stages may differ)