Windows PowerShell Process Implementing Manual Base64 Decoder

name: Windows PowerShell Process Implementing Manual Base64 Decoder
id: 08d67349-0808-4f55-b431-1037269fa517
version: 5
creation_date: '2025-10-24'
modification_date: '2026-05-13'
author: Nasreddine Bencherchali
status: production
type: Anomaly
description: |
    The following analytic identifies Windows PowerShell processes that implement a manual Base64 decoder.
    Threat actors often use Base64 encoding to obfuscate malicious payloads or commands within PowerShell scripts.
    By manually decoding Base64 strings, attackers can evade detection mechanisms that look for standard decoding functions like using the "-enc" flag or the "FromBase64String" function.
    This detection focuses on PowerShell processes that exhibit characteristics of manual Base64 decoding, such as the presence of specific string manipulation methods and bitwise operations.
    Security teams should investigate any instances of such activity, especially if found in conjunction with other suspicious behaviors or on systems that should not be using PowerShell for such tasks.
data_source:
    - Sysmon EventID 1
    - Windows Event Log Security 4688
    - CrowdStrike ProcessRollup2
search: |
    | tstats `security_content_summariesonly`
      count min(_time) as firstTime
            max(_time) as lastTime
    
      from datamodel=Endpoint.Processes where
    
      `process_powershell`
      Processes.process = "*ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/*"
      Processes.process = "*.Substring(*"
      Processes.process = "*.GetString(*"
      Processes.process = "*.IndexOf(*"
      Processes.process IN ("*-shl*", *-shr*, "*-bxor*", "*-bor*", "*-band*")
    
    by Processes.action Processes.dest Processes.original_file_name Processes.parent_process Processes.parent_process_exec
       Processes.parent_process_guid Processes.parent_process_id Processes.parent_process_name Processes.parent_process_path
       Processes.process Processes.process_exec Processes.process_guid Processes.process_hash Processes.process_id
       Processes.process_integrity_level Processes.process_name Processes.process_path Processes.user Processes.user_id
       Processes.vendor_product
    
    | `drop_dm_object_name(Processes)`
    | `security_content_ctime(firstTime)`
    | `security_content_ctime(lastTime)`
    | `windows_powershell_process_implementing_manual_base64_decoder_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 PowerShell for decoding Base64 strings by administrators
    or power users may trigger this detection. It is essential to review the context
    of the PowerShell execution to determine if it is malicious, suspicious or even authorized activity.
references:
    - https://www.virustotal.com/gui/file/4b3ab4d9f2332da6b6cd8d9d0f4910a5eb85ac8c969108acb3ad49631812f998/behavior
drilldown_searches:
    - name: View the detection results for - "$dest$" and "$user$"
      search: '%original_detection_search% | search  dest = "$dest$" user = "$user$"'
      earliest_offset: $info_min_time$
      latest_offset: $info_max_time$
    - name: View risk events for the last 7 days for - "$dest$" and "$user$"
      search: '| from datamodel Risk.All_Risk | search normalized_risk_object IN ("$dest$", "$user$") | 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: user
          type: user
          score: 20
          message: An instance of $process_name$ executing a manual Base64 decoding routine $process$ was identified on endpoint $dest$ by user $user$.
        - field: dest
          type: system
          score: 20
          message: An instance of $process_name$ executing a manual Base64 decoding routine $process$ was identified on endpoint $dest$ by user $user$.
threat_objects:
    - field: process
      type: process_name
    - field: process_name
      type: process_name
analytic_story:
    - Compromised Windows Host
    - Deobfuscate-Decode Files or Information
asset_type: Endpoint
mitre_attack_id:
    - T1027.010
    - T1059.001
product:
    - Splunk Enterprise
    - Splunk Enterprise Security
    - Splunk Cloud
category: endpoint
security_domain: endpoint
tests:
    - name: True Positive
      attack_data:
        - data: https://media.githubusercontent.com/media/splunk/attack_data/master/datasets/attack_techniques/T1027.010/manual_b64_decode_pwsh/manual_b64_decode_pwsh.log
          source: XmlWinEventLog:Microsoft-Windows-Sysmon/Operational
          sourcetype: XmlWinEventLog
      test_type: unit