Securityalert Encodedpowershell

KQL detection rule by reprise99 · MIT

Detect when Defender for Endpoint alerts on suspicious PowerShell usage. If command is encoded it will be decoded.

//Detect when Defender for Endpoint alerts on suspicious PowerShell usage. If command is encoded it will be decoded.

//Data connector required for this query - Security Alert (free table that other Defender products send alert info to)

SecurityAlert
| where AlertName == "Suspicious PowerShell command line"
| mv-expand todynamic(Entities)
| extend CommandLine = tostring(Entities.CommandLine)
//This particular query looks for only encoded Powershell commands, if you want all Powershell commands just remove the lines below
| extend EncodedCommand = extract(@'\s+([A-Za-z0-9+/]{20}\S+$)', 1, CommandLine)
| where EncodedCommand != ""
| extend DecodedCommand = base64_decode_tostring(EncodedCommand)
| where DecodedCommand != ""
//
| project TimeGenerated, CompromisedEntity, AlertName, CommandLine, DecodedCommand

//Advanced Hunting query - depending on the content of the decoded string AH can struggle to render the command occasionally

//Data connector required for this query - Advanced Hunting license

let alertid=
AlertInfo
| where Title == @"Suspicious PowerShell command line"
| distinct AlertId;
AlertEvidence
| where AlertId in (alertid)
| where EntityType == "Process"
| extend EncodedCommand = extract(@'\s+([A-Za-z0-9+/]{20}\S+$)', 1, ProcessCommandLine)
| where EncodedCommand != ""
| extend DecodedCommand = base64_decode_tostring(EncodedCommand)
| where DecodedCommand != ""
| project Timestamp, AlertId, ProcessCommandLine, DecodedCommand

[[ currentRule.title && currentRule.title.length > 60 ? currentRule.title.slice(0, 60) + '…' : currentRule.title ]]

e5bd3b2d-a77d-4dc3-81ac-e123d2f1919a

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Executable code, not a declarative rule

This submission is arbitrary [[ currentRule.format ? currentRule.format.toUpperCase() : '' ]] code, not a declarative detection rule — review it as you would any script before running it, and note that it cannot be safely aggregated or auto-executed alongside YARA/Sigma/Suricata/CRS rules.

Rejected pattern present

[[ reason ]]

What does this mean?

Format & documentation suggestions

Not a security issue — this rule is valid and was accepted as-is. These are optional fields/conventions its own format recommends, mainly affecting its quality score and how well it documents itself for other reviewers.

[[ reason ]]

What do these messages mean?

[[ currentRule.title && currentRule.title.length > 60 ? currentRule.title.slice(0, 60) + '…' : currentRule.title ]]

[[ currentRule.title ]]

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