Potential PowerShell Obfuscation via High Numeric Character Proportion

edit

A newer version is available. Check out the latest documentation.

Potential PowerShell Obfuscation via High Numeric Character Proportion

edit

Identifies PowerShell scripts with a disproportionately high number of numeric characters, often indicating the presence of obfuscated or encoded payloads. This behavior is typical of obfuscation methods involving byte arrays, character code manipulation, or embedded encoded strings used to deliver and execute malicious content.

Rule type: esql

Rule indices: None

Severity: low

Risk score: 21

Runs every: 5m

Searches indices from: now-9m (Date Math format, see also Additional look-back time)

Maximum alerts per execution: 100

References: None

Tags:

Domain: Endpoint
OS: Windows
Use Case: Threat Detection
Tactic: Defense Evasion
Data Source: PowerShell Logs

Version: 1

Rule authors:

Elastic

Rule license: Elastic License v2

Setup

edit

Setup

The PowerShell Script Block Logging logging policy must be enabled. Steps to implement the logging policy with Advanced Audit Configuration:

Computer Configuration >
Administrative Templates >
Windows PowerShell >
Turn on PowerShell Script Block Logging (Enable)

Steps to implement the logging policy via registry:

reg add "hklm\SOFTWARE\Policies\Microsoft\Windows\PowerShell\ScriptBlockLogging" /v EnableScriptBlockLogging /t REG_DWORD /d 1

Rule query

edit

FROM logs-windows.powershell_operational* metadata _id, _version, _index
| WHERE event.code == "4104"

// Look for scripts with more than 1000 chars that contain a related keyword
| EVAL script_len = LENGTH(powershell.file.script_block_text)
| WHERE script_len > 1000

// Replace string format expressions with 🔥 to enable counting the occurrence of the patterns we are looking for
// The emoji is used because it's unlikely to appear in scripts and has a consistent character length of 1
| EVAL replaced_with_fire = REPLACE(powershell.file.script_block_text, """[0-9]""", "🔥")

// Count the occurrence of numbers and their proportion to the total chars in the script
| EVAL special_count = script_len - LENGTH(REPLACE(replaced_with_fire, "🔥", ""))
| EVAL proportion = special_count::double / script_len::double

// Keep the fields relevant to the query, although this is not needed as the alert is populated using _id
| KEEP special_count, script_len, proportion, replaced_with_fire, powershell.file.script_block_text, powershell.file.script_block_id, file.path, powershell.sequence, powershell.total, _id, _index, host.name, agent.id, user.id

// Filter for scripts with a 25%+ proportion of numbers
| WHERE proportion > 0.25

Framework: MITRE ATT&CK^TM

Tactic:
- Name: Defense Evasion
- ID: TA0005
- Reference URL: https://attack.mitre.org/tactics/TA0005/
Technique:
- Name: Obfuscated Files or Information
- ID: T1027
- Reference URL: https://attack.mitre.org/techniques/T1027/
Technique:
- Name: Deobfuscate/Decode Files or Information
- ID: T1140
- Reference URL: https://attack.mitre.org/techniques/T1140/
Tactic:
- Name: Execution
- ID: TA0002
- Reference URL: https://attack.mitre.org/tactics/TA0002/
Technique:
- Name: Command and Scripting Interpreter
- ID: T1059
- Reference URL: https://attack.mitre.org/techniques/T1059/
Sub-technique:
- Name: PowerShell
- ID: T1059.001
- Reference URL: https://attack.mitre.org/techniques/T1059/001/

« Potential PowerShell Obfuscation via Concatenated Dynamic Command Invocation Potential PowerShell Obfuscation via High Special Character Proportion »