Suspicious PowerShell Invocations - Specific

Severity: high
Author: Florian Roth (Nextron Systems), Jonhnathan Ribeiro
Source: upstream

Detects suspicious PowerShell invocation command parameters

MITRE ATT&CK coverage

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

Event coverage

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

Stages and Predicates

Stage 1: `1 of selection_convert_b64`

ScriptBlockText|contains: ' -c '
ScriptBlockText|contains: ' -w '
ScriptBlockText|contains: -nop
ScriptBlockText|contains: '[Convert]::FromBase64String'
ScriptBlockText|contains: hidden

Stage 2: `1 of selection_iex_selection`

ScriptBlockText|contains: ' -c '
ScriptBlockText|contains: ' -w '
ScriptBlockText|contains: -noni
ScriptBlockText|contains: -nop
ScriptBlockText|contains: New-Object
ScriptBlockText|contains: hidden
ScriptBlockText|contains: iex

Stage 3: `1 of selection_enc_selection`

ScriptBlockText|contains: ' -w '
ScriptBlockText|contains: -Enc
ScriptBlockText|contains: -ep
ScriptBlockText|contains: bypass
ScriptBlockText|contains: hidden

Stage 4: `1 of selection_reg_selection`

or:
ScriptBlockText|contains: '\software\microsoft\windows\currentversion\policies\explorer\run'
ScriptBlockText|contains: '\software\microsoft\windows\currentversion\run'
ScriptBlockText|contains: '\software\wow6432node\microsoft\windows\currentversion\run'
ScriptBlockText|contains: add
ScriptBlockText|contains: powershell
ScriptBlockText|contains: reg

Stage 5: `1 of selection_webclient_selection`

ScriptBlockText|contains: -noprofile
ScriptBlockText|contains: -windowstyle
ScriptBlockText|contains: .download
ScriptBlockText|contains: bypass
ScriptBlockText|contains: hidden
ScriptBlockText|contains: new-object
ScriptBlockText|contains: system.net.webclient

Stage 6: `1 of selection_iex_webclient`

ScriptBlockText|contains: .Download
ScriptBlockText|contains: Net.WebClient
ScriptBlockText|contains: New-Object
ScriptBlockText|contains: iex

Stage 7: `not 1 of filter_chocolatey`

or:
ScriptBlockText|contains: '(New-Object System.Net.WebClient).DownloadString(''https://chocolatey.org/install.ps1'')'
ScriptBlockText|contains: '(New-Object System.Net.WebClient).DownloadString(''https://community.chocolatey.org/install.ps1'
ScriptBlockText|contains: Write-ChocolateyWarning

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	`-c` `-w` `(New-Object System.Net.WebClient).DownloadString('https://chocolatey.org/install.ps1')` corpus 2 (sigma 2) `(New-Object System.Net.WebClient).DownloadString('https://community.chocolatey.org/install.ps1` `-Enc` `-ep` `-noni` `-nop` `-noprofile` `-windowstyle` `.Download` `.download` `Net.WebClient` `New-Object` corpus 6 (sigma 6) `Write-ChocolateyWarning` `[Convert]::FromBase64String` `\software\microsoft\windows\currentversion\policies\explorer\run` `\software\microsoft\windows\currentversion\run` `\software\wow6432node\microsoft\windows\currentversion\run` `add` `bypass` corpus 2 (sigma 2) `hidden` `iex` corpus 2 (sigma 2) `new-object` corpus 2 (sigma 2) `powershell` corpus 3 (sigma 3) `reg` `system.net.webclient`

Neighbors

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 5 filters this rule applies) (first-stage match only; downstream stages may differ)
Potential PowerShell Obfuscation via Backtick-Escaped Variable Expansion [elastic] (cross-vendor) (drops 5 filters this rule applies) (first-stage match only; downstream stages may differ)
Potential PowerShell Obfuscation via Character Array Reconstruction [elastic] (cross-vendor) (drops 5 filters this rule applies) (first-stage match only; downstream stages may differ)
Potential PowerShell Obfuscation via Concatenated Dynamic Command Invocation [elastic] (cross-vendor) (drops 5 filters this rule applies) (first-stage match only; downstream stages may differ)
Potential PowerShell Obfuscation via High Numeric Character Proportion [elastic] (cross-vendor) (drops 5 filters this rule applies) (first-stage match only; downstream stages may differ)
Potential Dynamic IEX Reconstruction via Environment Variables [elastic] (cross-vendor) (drops 5 filters this rule applies) (first-stage match only; downstream stages may differ)
Dynamic IEX Reconstruction via Method String Access [elastic] (cross-vendor) (drops 5 filters this rule applies) (first-stage match only; downstream stages may differ)
PowerShell Obfuscation via Negative Index String Reversal [elastic] (cross-vendor) (drops 5 filters this rule applies) (first-stage match only; downstream stages may differ)
Potential PowerShell Obfuscation via Reverse Keywords [elastic] (cross-vendor) (drops 5 filters this rule applies) (first-stage match only; downstream stages may differ)
Potential PowerShell Obfuscation via String Concatenation [elastic] (cross-vendor) (drops 5 filters this rule applies) (first-stage match only; downstream stages may differ)
Potential PowerShell Obfuscation via String Reordering [elastic] (cross-vendor) (drops 5 filters this rule applies) (first-stage match only; downstream stages may differ)
Potential PowerShell Obfuscation via Special Character Overuse [elastic] (cross-vendor) (drops 5 filters this rule applies) (first-stage match only; downstream stages may differ)

Suspicious PowerShell Invocations - Specific

MITRE ATT&CK coverage

Event coverage

Stages and Predicates

Stage 1: 1 of selection_convert_b64

Stage 2: 1 of selection_iex_selection

Stage 3: 1 of selection_enc_selection

Stage 4: 1 of selection_reg_selection

Stage 5: 1 of selection_webclient_selection

Stage 6: 1 of selection_iex_webclient

Stage 7: not 1 of filter_chocolatey