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 > 500
// 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
| eval Esql.script_block_tmp = replace(
powershell.file.script_block_text,
"""\$\w+\[\-\s?1\.\.""",
"🔥"
)
// count how many patterns were detected by calculating the number of 🔥 characters inserted
| eval Esql.script_block_pattern_count = length(Esql.script_block_tmp) - length(replace(Esql.script_block_tmp, "🔥", ""))
// 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_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 that match the pattern at least once
| where Esql.script_block_pattern_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).