Powershell Fileless Script Contains Base64 Encoded Content

SPLUNK detection rule by Michael Haag, Splunk · Apache-2.0

The following analytic detects the execution of PowerShell scripts containing Base64 encoded content, specifically identifying the use of `FromBase64String`. It leverages PowerShell Script Block Logging (EventCode=4104) to capture and analyze the full command sent to PowerShell. This activity is significant as Base64 encoding is often used by attackers to obfuscate malicious payloads, making it harder to detect. If confirmed malicious, this could lead to code execution, allowing attackers to run arbitrary commands and potentially compromise the system.

name: Powershell Fileless Script Contains Base64 Encoded Content
id: 8acbc04c-c882-11eb-b060-acde48001122
version: 22
creation_date: '2021-06-09'
modification_date: '2026-06-25'
author: Michael Haag, Splunk
status: production
type: TTP
description: The following analytic detects the execution of PowerShell scripts containing Base64 encoded content, specifically identifying the use of `FromBase64String`. It leverages PowerShell Script Block Logging (EventCode=4104) to capture and analyze the full command sent to PowerShell. This activity is significant as Base64 encoding is often used by attackers to obfuscate malicious payloads, making it harder to detect. If confirmed malicious, this could lead to code execution, allowing attackers to run arbitrary commands and potentially compromise the system.
data_source:
    - Powershell Script Block Logging 4104
search: "`powershell` EventCode=4104 ScriptBlockText = \"*frombase64string*\" OR ScriptBlockText = \"*gnirtS46esaBmorF*\"\n  | fillnull\n  | stats count min(_time) as firstTime max(_time) as lastTime\n    BY dest signature signature_id\n       user_id vendor_product EventID\n       Guid Opcode Name\n       Path ProcessID ScriptBlockId\n       ScriptBlockText\n  | `security_content_ctime(firstTime)`\n  | `security_content_ctime(lastTime)`\n  | `powershell_fileless_script_contains_base64_encoded_content_filter`"
how_to_implement: To successfully implement this analytic, you will need to enable PowerShell Script Block Logging on some or all endpoints. Additional setup here https://help.splunk.com/en/security-offerings/splunk-user-behavior-analytics/get-data-in/5.4.1/add-other-data-to-splunk-uba/configure-powershell-logging-to-see-powershell-anomalies-in-splunk-uba.
known_false_positives: False positives should be limited. Filter as needed.
references:
    - https://help.splunk.com/en/security-offerings/splunk-user-behavior-analytics/get-data-in/5.4.1/add-other-data-to-splunk-uba/configure-powershell-logging-to-see-powershell-anomalies-in-splunk-uba.
    - https://blog.palantir.com/tampering-with-windows-event-tracing-background-offense-and-defense-4be7ac62ac63
    - https://static1.squarespace.com/static/552092d5e4b0661088167e5c/t/59c1814829f18782e24f1fe2/1505853768977/Windows+PowerShell+Logging+Cheat+Sheet+ver+Sept+2017+v2.1.pdf
    - https://www.crowdstrike.com/blog/investigating-powershell-command-and-script-logging/
    - https://thedfirreport.com/2023/05/22/icedid-macro-ends-in-nokoyawa-ransomware/
drilldown_searches:
    - name: View the detection results for - "$dest$"
      search: '%original_detection_search% | search  dest = "$dest$"'
      earliest_offset: $info_min_time$
      latest_offset: $info_max_time$
    - name: View risk events for the last 7 days for - "$dest$"
      search: '| from datamodel Risk.All_Risk | search normalized_risk_object IN ("$dest$") | 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'
finding:
    title: A suspicious powershell script contains base64 command on host $dest$
    entity:
        field: dest
        type: system
        score: 50
analytic_story:
    - Winter Vivern
    - Malicious PowerShell
    - Medusa Ransomware
    - Data Destruction
    - NjRAT
    - AsyncRAT
    - Hermetic Wiper
    - IcedID
    - XWorm
    - 0bj3ctivity Stealer
    - APT37 Rustonotto and FadeStealer
    - GhostRedirector IIS Module and Rungan Backdoor
    - Hellcat Ransomware
    - Microsoft WSUS CVE-2025-59287
    - NetSupport RMM Tool Abuse
    - MuddyWater
    - Axios Supply Chain Post Compromise
    - VIP Keylogger
    - Salat Stealer
    - Phantom Stealer
asset_type: Endpoint
mitre_attack_id:
    - T1027
    - T1059.001
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/powershell_script_block_logging/frombase64string.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 ]]

1149b2f5-7331-4dc6-94b3-0424e39a9e6a

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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

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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.

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

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