Windows Suspicious Defender Update Activity in INetCache

name: Windows Suspicious Defender Update Activity in INetCache
id: 3e25f906-6319-44ad-ab9a-22d162eb3925
version: 2
creation_date: '2026-04-27'
modification_date: '2026-07-20'
author: Raven Tait, Splunk
status: production
type: Anomaly
description: |-
    Detects a non-Defender process writing mpam-fe*.exe to the Windows Internet Cache (INetCache).
    BlueHammer downloads the WD signature update package directly using WinINet as a low-privileged user.
    The [1].exe naming suffix is produced by Windows HTTP caching and is a reliable artifact of this download method.
data_source:
    - Sysmon EventID 11
    - Sysmon EventID 23
search: |-
    `sysmon`
    EventID IN (11, 23, 26)
    TargetFilename="*\\INetCache*"
    TargetFilename="*\\mpam-fe[1].exe*"
    NOT Image IN (
        "*:\\Program Files\\Windows Defender*",
        "*:\\ProgramData\\Microsoft\\Windows Defender*",
        "*:\\Windows\\System32\\MpSigStub.exe*"
    )
    | fillnull
    | rename Computer as dest
    | stats count by dest TargetFilename Image EventID
                     action file_name file_path process_guid
                     process_id user vendor_product
    | `security_content_ctime(firstTime)`
    | `security_content_ctime(lastTime)`
    | `windows_suspicious_defender_update_activity_in_inetcache_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: |-
    Some legitimate system maintenance tools or security scanners may download updates using WinINet and write to INetCache with similar naming patterns. Filter alerts when these tools are verified and approved.
references:
    - https://github.com/Nightmare-Eclipse/BlueHammer
intermediate_findings:
    entities:
        - field: dest
          type: system
          score: 20
          message: Defender Update package $file_path$ downloaded to INetCache by unexpected process $Image$ on $dest$.
threat_objects:
    - field: Image
      type: process
analytic_story:
    - Windows Persistence Techniques
    - BlueHammer
asset_type: Endpoint
mitre_attack_id:
    - T1068
    - T1105
product:
    - Splunk Enterprise
    - Splunk Enterprise Security
    - Splunk Cloud
category: endpoint
security_domain: endpoint
cve:
    - CVE-2026-33825