Potential AMSI Bypass Script Using NULL Bits

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

Detects usage of special strings/null bits in order to potentially bypass AMSI functionalities

MITRE ATT&CK coverage

Tactic	Techniques
Defense Evasion	`T1562.001` Impair Defenses: Disable or Modify Tools

Event coverage

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

Stages and Predicates

Stage 1: `selection`

or:
ScriptBlockText|contains: '#<NULL>'
ScriptBlockText|contains: 'if(0){{{0}}}'' -f $(0 -as [char]) +'

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	`#<NULL>` `if(0){{{0}}}' -f $(0 -as [char]) +`

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 1 filter this rule applies) (first-stage match only; downstream stages may differ)
Potential PowerShell Obfuscation via Backtick-Escaped Variable Expansion [elastic] (cross-vendor) (drops 1 filter this rule applies) (first-stage match only; downstream stages may differ)
Potential PowerShell Obfuscation via Character Array Reconstruction [elastic] (cross-vendor) (drops 1 filter this rule applies) (first-stage match only; downstream stages may differ)
Potential PowerShell Obfuscation via Concatenated Dynamic Command Invocation [elastic] (cross-vendor) (drops 1 filter this rule applies) (first-stage match only; downstream stages may differ)
Potential PowerShell Obfuscation via High Numeric Character Proportion [elastic] (cross-vendor) (drops 1 filter this rule applies) (first-stage match only; downstream stages may differ)
Potential Dynamic IEX Reconstruction via Environment Variables [elastic] (cross-vendor) (drops 1 filter this rule applies) (first-stage match only; downstream stages may differ)
Dynamic IEX Reconstruction via Method String Access [elastic] (cross-vendor) (drops 1 filter this rule applies) (first-stage match only; downstream stages may differ)
PowerShell Obfuscation via Negative Index String Reversal [elastic] (cross-vendor) (drops 1 filter this rule applies) (first-stage match only; downstream stages may differ)
Potential PowerShell Obfuscation via Reverse Keywords [elastic] (cross-vendor) (drops 1 filter this rule applies) (first-stage match only; downstream stages may differ)
Potential PowerShell Obfuscation via String Concatenation [elastic] (cross-vendor) (drops 1 filter this rule applies) (first-stage match only; downstream stages may differ)
Potential PowerShell Obfuscation via String Reordering [elastic] (cross-vendor) (drops 1 filter this rule applies) (first-stage match only; downstream stages may differ)
Potential PowerShell Obfuscation via Special Character Overuse [elastic] (cross-vendor) (drops 1 filter this rule applies) (first-stage match only; downstream stages may differ)