PowerShell Obfuscation via Negative Index String Reversal

Last updated a month ago on 2025-07-07
Created 4 months ago on 2025-04-14

About

Identifies PowerShell scripts that use negative index ranges to reverse the contents of a string or array at runtime as a form of obfuscation. This technique avoids direct use of reversal functions by iterating through array elements in reverse order. These methods are designed to evade static analysis and bypass security protections such as the Antimalware Scan Interface (AMSI).
Tags
Domain: EndpointOS: WindowsUse Case: Threat DetectionTactic: Defense EvasionData Source: PowerShell LogsLanguage: esql
Severity
low
Risk Score
21
MITRE ATT&CKâ„¢

Defense Evasion (TA0005)(opens in a new tab or window)

Execution (TA0002)(opens in a new tab or window)

License
Elastic License v2(opens in a new tab or window)

Definition

Integration Pack
Prebuilt Security Detection Rules
Related Integrations

windows(opens in a new tab or window)

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

// Look for scripts with more than 500 chars that contain a related keyword
| EVAL script_len = LENGTH(powershell.file.script_block_text)
| WHERE script_len > 500

// 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, """\$\w+\[\-\s?1\.\.""", "🔥")

// Count how many patterns were detected by calculating the number of 🔥 characters inserted
| EVAL count = LENGTH(replaced_with_fire) - LENGTH(REPLACE(replaced_with_fire, "🔥", ""))

// Keep the fields relevant to the query, although this is not needed as the alert is populated using _id
| KEEP count, 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
| WHERE count >= 1

// FP Patterns
| WHERE NOT powershell.file.script_block_text LIKE "*GENESIS-5654*"

Install detection rules in Elastic Security

Detect PowerShell Obfuscation via Negative Index String Reversal in the Elastic Security detection engine by installing this rule into your Elastic Stack.

To setup this rule, check out the installation guide for Prebuilt Security Detection Rules(opens in a new tab or window).