VHD Image Download Via Browser

SIGMA detection rule by frack113, Christopher Peacock '@securepeacock', SCYTHE '@scythe_io' · NOASSERTION

Detects creation of ".vhd"/".vhdx" files by browser processes. Malware can use mountable Virtual Hard Disk ".vhd" files to encapsulate payloads and evade security controls.

title: VHD Image Download Via Browser
id: 8468111a-ef07-4654-903b-b863a80bbc95
status: test
description: |
    Detects creation of ".vhd"/".vhdx" files by browser processes.
    Malware can use mountable Virtual Hard Disk ".vhd" files to encapsulate payloads and evade security controls.
references:
    - https://redcanary.com/blog/intelligence-insights-october-2021/
    - https://www.kaspersky.com/blog/lazarus-vhd-ransomware/36559/
    - https://securelist.com/lazarus-on-the-hunt-for-big-game/97757/
author: frack113, Christopher Peacock '@securepeacock', SCYTHE '@scythe_io'
date: 2021-10-25
modified: 2023-05-05
tags:
    - attack.resource-development
    - attack.t1587.001
logsource:
    category: file_event
    product: windows
detection:
    selection:
        Image|endswith:
            - '\brave.exe'
            - '\chrome.exe'
            - '\firefox.exe'
            - '\iexplore.exe'
            - '\maxthon.exe'
            - '\MicrosoftEdge.exe'
            - '\msedge.exe'
            - '\msedgewebview2.exe'
            - '\opera.exe'
            - '\safari.exe'
            - '\seamonkey.exe'
            - '\vivaldi.exe'
            - '\whale.exe'
        # We don't use "endswith" to also match with ADS logs and ".vhdx". Example: "TargetFilename: C:\Users\xxx\Downloads\windows.vhd:Zone.Identifier"
        TargetFilename|contains: '.vhd'
    condition: selection
falsepositives:
    - Legitimate downloads of ".vhd" files would also trigger this
level: medium

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

2ff37041-dcd1-4729-b16d-ce39d17b46ea

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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 ]]

[[ currentRule.title ]]

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