RedSun - TieringEngineService.exe Detected as EICAR Test File

SIGMA detection rule by Swachchhanda Shrawan Poudel (Nextron Systems) · NOASSERTION

Detects Windows Defender (EventID 1119 - Remediation Action Failed) flagging TieringEngineService.exe dropped in a characteristic RS-{GUID} temporary directory, or the RedSun.exe process itself being present. This covers the staging pattern used by RedSun, a Cloud Files API and opportunistic lock (oplock) based AV bypass/privilege escalation tool. RedSun works as follows: 1. Registers a Cloud Files sync root and creates a Cloud Files placeholder for TieringEngineService.exe under %TEMP%\RS-{GUID}\ 2. The placeholder file carries EICAR test file content (Virus:DOS/EICAR_Test_File) to reliably trigger a Defender scan and remediation attempt 3. Requests a batch oplock (FSCTL_REQUEST_BATCH_OPLOCK) on the placeholder file 4. When Defender attempts to scan/quarantine the file, the oplock triggers - holding the file open 5. During the oplock break window, RedSun swaps the mount point (junction) to redirect \\?\C:\Windows\System32 to the attacker-controlled temp path 6. This races the AV/OS into executing the malicious TieringEngineService.exe with elevated privileges

title: RedSun - TieringEngineService.exe Detected as EICAR Test File
id: a7c3e5f2-8b1d-4e9a-b6c2-3d7f5e8a9b4c
status: experimental
description: |
    Detects Windows Defender (EventID 1119 - Remediation Action Failed) flagging TieringEngineService.exe
    dropped in a characteristic RS-{GUID} temporary directory, or the RedSun.exe process itself being present.
    This covers the staging pattern used by RedSun, a Cloud Files API and opportunistic lock (oplock) based
    AV bypass/privilege escalation tool.

    RedSun works as follows:
      1. Registers a Cloud Files sync root and creates a Cloud Files placeholder for TieringEngineService.exe under %TEMP%\RS-{GUID}\
      2. The placeholder file carries EICAR test file content (Virus:DOS/EICAR_Test_File) to reliably trigger
         a Defender scan and remediation attempt
      3. Requests a batch oplock (FSCTL_REQUEST_BATCH_OPLOCK) on the placeholder file
      4. When Defender attempts to scan/quarantine the file, the oplock triggers - holding the file open
      5. During the oplock break window, RedSun swaps the mount point (junction) to redirect
         \\?\C:\Windows\System32 to the attacker-controlled temp path
      6. This races the AV/OS into executing the malicious TieringEngineService.exe with elevated privileges
references:
    - https://github.com/Nightmare-Eclipse/RedSun/blob/7456cc8cf066f5e5fc6cdf7d3272a466ebd6b2f6/RedSun.cpp#L605
    - https://deadeclipse666.blogspot.com/2026/04/public-disclosure-response-for-cve-2026.html
author: Swachchhanda Shrawan Poudel (Nextron Systems)
date: 2026-04-17
tags:
    - attack.stealth
    - attack.defense-impairment
    - attack.t1036.005
    - attack.t1685
    - attack.privilege-escalation
    - attack.t1055
    - detection.emerging-threats
logsource:
    product: windows
    service: windefend
detection:
    # EventID 1119: Microsoft Defender Antivirus has encountered an error trying to take action on malware or unwanted software
    # Path field from event: file:_C:\Users\<user>\AppData\Local\Temp\<n>\RS-{GUID}\TieringEngineService.exe
    # Threat name 'Virus:DOS/EICAR_Test_File' is expected - RedSun uses EICAR content to reliably trigger a Defender scan/remediation
    selection_eid:
        EventID: 1119
        SourceName: 'Real-Time Protection'
    selection_susp_path:
        Path|endswith: '\TieringEngineService.exe'
        ThreatName|endswith: 'EICAR_Test_File'
    selection_susp_process:
        ProcessName|endswith: '\RedSun.exe'
    condition: selection_eid and 1 of selection_susp_*
falsepositives:
    - Unlikely
level: critical
regression_tests_path: regression_data/rules-emerging-threats/2026/Exploits/RedSun/win_defender_exploit_redsun_tiering_engine_detected_as_eicar/info.yml

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