Suspicious FromBase64String Usage On Gzip Archive - Ps Script

Severity: medium
Author: frack113
Source: upstream

Detects attempts of decoding a base64 Gzip archive in a PowerShell script. This technique is often used as a method to load malicious content into memory afterward.

MITRE ATT&CK coverage

Tactic	Techniques
Command & Control	`T1132.001` Data Encoding: Standard Encoding

Event coverage

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

Stages and Predicates

Stage 1: `selection`

ScriptBlockText|contains: FromBase64String
ScriptBlockText|contains: H4sI
ScriptBlockText|contains: MemoryStream

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	`FromBase64String` `H4sI` `MemoryStream`

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 3 filters this rule applies) (first-stage match only; downstream stages may differ)
Potential PowerShell Obfuscation via Backtick-Escaped Variable Expansion [elastic] (cross-vendor) (drops 3 filters this rule applies) (first-stage match only; downstream stages may differ)
Potential PowerShell Obfuscation via Character Array Reconstruction [elastic] (cross-vendor) (drops 3 filters this rule applies) (first-stage match only; downstream stages may differ)
Potential PowerShell Obfuscation via Concatenated Dynamic Command Invocation [elastic] (cross-vendor) (drops 3 filters this rule applies) (first-stage match only; downstream stages may differ)
Potential PowerShell Obfuscation via High Numeric Character Proportion [elastic] (cross-vendor) (drops 3 filters this rule applies) (first-stage match only; downstream stages may differ)
Potential Dynamic IEX Reconstruction via Environment Variables [elastic] (cross-vendor) (drops 3 filters this rule applies) (first-stage match only; downstream stages may differ)
Dynamic IEX Reconstruction via Method String Access [elastic] (cross-vendor) (drops 3 filters this rule applies) (first-stage match only; downstream stages may differ)
PowerShell Obfuscation via Negative Index String Reversal [elastic] (cross-vendor) (drops 3 filters this rule applies) (first-stage match only; downstream stages may differ)
Potential PowerShell Obfuscation via Reverse Keywords [elastic] (cross-vendor) (drops 3 filters this rule applies) (first-stage match only; downstream stages may differ)
Potential PowerShell Obfuscation via String Concatenation [elastic] (cross-vendor) (drops 3 filters this rule applies) (first-stage match only; downstream stages may differ)
Potential PowerShell Obfuscation via String Reordering [elastic] (cross-vendor) (drops 3 filters this rule applies) (first-stage match only; downstream stages may differ)
Potential PowerShell Obfuscation via Special Character Overuse [elastic] (cross-vendor) (drops 3 filters this rule applies) (first-stage match only; downstream stages may differ)