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: 1

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"

// 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
// Excludes spaces, #, = and - as they are heavily used in scripts for formatting
| EVAL replaced_with_fire = REPLACE(powershell.file.script_block_text, """[^0-9A-Za-z\s#=-]""", "🔥")

// Count the occurrence of special chars 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 special chars
| 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/

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