Potential PowerShell Obfuscation via High Special Character Proportion

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Potential PowerShell Obfuscation via High Special Character Proportion

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Identifies PowerShell scripts with an abnormally high proportion of non-alphanumeric characters, often resulting from encoding, string mangling, or dynamic code generation.

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
Rule Type: BBR

Version: 3

Rule authors:

Elastic

Rule license: Elastic License v2

Setup

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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

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

// Filter out smaller scripts that are unlikely to implement obfuscation using the patterns we are looking for
| eval Esql.script_block_length = length(powershell.file.script_block_text)
| where Esql.script_block_length > 1000

// replace the patterns we are looking for with the 🔥 emoji to enable counting them
// The emoji is used because it's unlikely to appear in scripts and has a consistent character length of 1
// Excludes spaces, #, = and - as they are heavily used in scripts for formatting
| eval Esql.script_block_tmp = replace(powershell.file.script_block_text, """[^0-9A-Za-z\s#=-]""", "🔥")

// count how many patterns were detected by calculating the number of 🔥 characters inserted
| eval Esql.script_block_pattern_count = Esql.script_block_length - length(replace(Esql.script_block_tmp, "🔥", ""))

// Calculate the ratio of special characters to total length
| eval Esql.script_block_ratio = Esql.script_block_pattern_count::double / Esql.script_block_length::double

// keep the fields relevant to the query, although this is not needed as the alert is populated using _id
| keep
    Esql.script_block_pattern_count,
    Esql.script_block_length,
    Esql.script_block_ratio,
    Esql.script_block_tmp,
    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 high special character ratio
| where Esql.script_block_ratio > 0.30

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/

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