Suspicious Desktopimgdownldr Target File

Status: test
Severity: high
Log source: category file_event, product windows
Author: Florian Roth (Nextron Systems)
Source: github.com/SigmaHQ/sigma

Detects a suspicious Microsoft desktopimgdownldr file creation that stores a file to a suspicious location or contains a file with a suspicious extension

Known false positives

False positives depend on scripts and administrative tools used in the monitored environment

MITRE ATT&CK coverage

Tactic	Techniques
Command & Control	`T1105` Ingress Tool Transfer

Telemetry coverage

Provider	Record / event type
Sysmon	Event ID 11: FileCreate

Rule body

title: Suspicious Desktopimgdownldr Target File
id: fc4f4817-0c53-4683-a4ee-b17a64bc1039
status: test
description: Detects a suspicious Microsoft desktopimgdownldr file creation that stores a file to a suspicious location or contains a file with a suspicious extension
references:
    - https://labs.sentinelone.com/living-off-windows-land-a-new-native-file-downldr/
    - https://twitter.com/SBousseaden/status/1278977301745741825
author: Florian Roth (Nextron Systems)
date: 2020-07-03
modified: 2022-06-02
tags:
    - attack.command-and-control
    - attack.t1105
logsource:
    product: windows
    category: file_event
detection:
    selection:
        Image|endswith: '\svchost.exe'
        TargetFilename|contains: '\Personalization\LockScreenImage\'
    filter1:
        TargetFilename|contains: 'C:\Windows\'
    filter2:
        TargetFilename|contains:
            - '.jpg'
            - '.jpeg'
            - '.png'
    condition: selection and not filter1 and not filter2
falsepositives:
    - False positives depend on scripts and administrative tools used in the monitored environment
level: high

Stages and Predicates

Stage 0: `condition`

selection and not filter1 and not filter2

Stage 1: `selection`

selection:
    Image|endswith: '\svchost.exe'
    TargetFilename|contains: '\Personalization\LockScreenImage\'

Stage 2: `not filter1`

filter1:
    TargetFilename|contains: 'C:\Windows\'

Stage 3: `not filter2`

filter2:
    TargetFilename|contains:
        - '.jpg'
        - '.jpeg'
        - '.png'

Exclusions

The rule actively suppresses these predicates.

Field	Kind	Excluded values	Search
`TargetFilename`	match	`C:\Windows\`	`excludes:TargetFilename` `field:"TargetFilename" value:"C:\Windows\"`
`TargetFilename`	match	`.jpeg`	`excludes:TargetFilename` `field:"TargetFilename" value:".jpeg"`
`TargetFilename`	match	`.jpg`	`excludes:TargetFilename` `field:"TargetFilename" value:".jpg"`
`TargetFilename`	match	`.png`	`excludes:TargetFilename` `field:"TargetFilename" value:".png"`

Indicators

These rows show field, operator, and value matches.

Field	Kind	Values	Search
`Image`	ends_with	`\svchost.exe` corpus 23 (sigma 22, kusto 1)	`field:"Image" kind:ends_with value:"\svchost.exe"`
`TargetFilename`	match	`\Personalization\LockScreenImage\`	`field:"TargetFilename" kind:match value:"\Personalization\LockScreenImage\"`