Windows PowerShell Script TabExpansion Direct Call

SPLUNK detection rule by Raven Tait, Splunk · Apache-2.0

Detects specific indicators associated with the execution of the TabExpansion internal function in PowerShell. Calling this function directly is not normal and can be indicative of malicious activity such as TabShell. Due to the way PowerShell internals work it can be used in conjunction with directory transversal to load any PowerShell functions even in a sandboxed session. False positives could include legitimate usage of the TabExpansion function but calling it directly is very rare.

name: Windows PowerShell Script TabExpansion Direct Call
id: d36a09b3-35ac-4d6f-964a-39fd337abc2f
version: 2
creation_date: '2026-05-05'
modification_date: '2026-05-13'
author: Raven Tait, Splunk
status: production
type: Anomaly
description: |-
    Detects specific indicators associated with the execution of the TabExpansion internal function in PowerShell.
    Calling this function directly is not normal and can be indicative of malicious activity such as TabShell.
    Due to the way PowerShell internals work it can be used in conjunction with directory transversal to load any PowerShell functions even in a sandboxed session.
    False positives could include legitimate usage of the TabExpansion function but calling it directly is very rare.
data_source:
    - Powershell Script Block Logging 4104
search: |-
    `powershell`
    EventID="4104"
    ScriptBlockText="*$lastWord*"
    ScriptBlockText="*$_val=' + $_expression*"
    ScriptBlockText="*function Write-Members*"
    | fillnull
    | stats count min(_time) as firstTime
                  max(_time) as lastTime
      by Computer EventID ScriptBlockText signature signature_id user_id vendor_product Guid
         Opcode Name Path ProcessID ScriptBlockId

    | rename Computer as dest
    | `security_content_ctime(firstTime)`
    | `security_content_ctime(lastTime)`
    | `windows_powershell_script_tabexpansion_direct_call_filter`
how_to_implement: The detection is based on data that originates from Endpoint Detection and Response (EDR) agents. These agents are designed to provide security-related telemetry from the endpoints where the agent is installed. To implement this search, you must ingest logs that contain the process GUID, process name, and parent process. Additionally, you must ingest complete command-line executions. These logs must be processed using the appropriate Splunk Technology Add-ons that are specific to the EDR product. The logs must also be mapped to the `Processes` node of the `Endpoint` data model. Use the Splunk Common Information Model (CIM) to normalize the field names and speed up the data modeling process.
known_false_positives: Legitimate use of the TabExpansion function is rare but possible. Filter alerts if direct calls are from trusted administrative or development activities.
references:
    - https://blog.viettelcybersecurity.com/tabshell-owassrf/
drilldown_searches:
    - earliest_offset: $info_min_time$
      latest_offset: $info_max_time$
      name: View the detection results for - "$user$" and "$dest$"
      search: '%original_detection_search% | search  user = "$user$" dest = "$dest$"'
    - name: View risk events for the last 7 days for - "$user$" and "$dest$"
      search: '| from datamodel Risk.All_Risk | search normalized_risk_object IN ("$user$", "$dest$") | stats count min(_time) as firstTime max(_time) as lastTime values(search_name) as "Search Name" values(risk_message) as "Risk Message" values(analyticstories) as "Analytic Stories" values(annotations._all) as "Annotations" values(annotations.mitre_attack.mitre_tactic) as "ATT&CK Tactics" by normalized_risk_object | `security_content_ctime(firstTime)` | `security_content_ctime(lastTime)`'
      earliest_offset: 7d
      latest_offset: "0"
intermediate_findings:
    entities:
        - field: dest
          type: system
          score: 20
          message: Potential PowerShell TabExpansion activity observed on $dest$ via script block $ScriptBlockId$.
analytic_story:
    - Malicious PowerShell
asset_type: Endpoint
mitre_attack_id:
    - T1059.001
    - T1129
product:
    - Splunk Enterprise
    - Splunk Enterprise Security
    - Splunk Cloud
category: endpoint
security_domain: endpoint
tests:
    - name: True Positive Test
      attack_data:
        - data: https://media.githubusercontent.com/media/splunk/attack_data/master/datasets/attack_techniques/T1059.001/snapattack/snapattack.log
          source: XmlWinEventLog:Microsoft-Windows-PowerShell/Operational
          sourcetype: XmlWinEventLog
      test_type: unit

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

9092f3e4-bfad-4860-b78d-7bd04a8a28e7

Propose edits, view related rules, and engage with the community through comments.

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 ]]

Community Discussion