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Remote File Download via Desktopimgdownldr Utility

Elastic Stack Serverless Security

Identifies the desktopimgdownldr utility being used to download a remote file. An adversary may use desktopimgdownldr to download arbitrary files as an alternative to certutil.

Rule type: eql

Rule indices:

  • winlogbeat-*
  • logs-endpoint.events.*
  • logs-windows.*

Severity: medium

Risk score: 47

Runs every: 5m

Searches indices from: now-9m (https://www.elastic.co/guide/en/elasticsearch/reference/current/common-options.html#date-math[Date Math format], see also Additional look-back time)

Maximum alerts per execution: 100

References:

Tags:

  • Elastic
  • Host
  • Windows
  • Threat Detection
  • Command and Control

Version: 6

Rule authors:

  • Elastic

Rule license: Elastic License v2

Investigation guide

Attackers commonly transfer tooling or malware from external systems into a compromised environment using the command
and control channel. However, they can also abuse signed utilities to drop these files.

The Desktopimgdownldr.exe utility is used to to configure lockscreen/desktop image, and can be abused with the
lockscreenurl argument to download remote files and tools, this rule looks for this behavior.

  • Investigate the process execution chain (parent process tree).
  • Identify the user account that performed the action and whether it should perform this kind of action.
  • Contact the account owner and confirm whether they are aware of this activity.
  • Investigate other alerts associated with the user/host during the past 48 hours.
  • Determine if the activity is unique by validating if other machines in the organization have similar entries.
  • Check the reputation of the domain or IP address used to host the downloaded file or if the user downloaded the file
    from an internal system.
  • Retrieve the file and determine if it is malicious:
    • Identify the file type.
    • Use a private sandboxed malware analysis system to perform analysis.
      • Observe and collect information about the following activities:
        • Attempts to contact external domains and addresses.
        • File and registry access, modification, and creation activities.
        • Service creation and launch activities.
        • Scheduled tasks creation.
    • Use the PowerShell Get-FileHash cmdlet to get the SHA-256 hash value of the file.
      • Search for the existence and reputation of this file in resources like VirusTotal, Hybrid-Analysis, CISCO Talos, Any.run, etc.
  • This activity is unusual but can be done by administrators. Benign true positives (B-TPs) can be added as exceptions
    if necessary.
  • Analysts can dismiss the alert if the downloaded file is a legitimate image.
  • Initiate the incident response process based on the outcome of the triage.
  • Isolate the involved host to prevent further post-compromise behavior.
  • If the triage identified malware, search the environment for additional compromised hosts.
    • Implement any temporary network rules, procedures, and segmentation required to contain the malware.
    • Immediately block the IoCs identified.
  • Remove and block malicious artifacts identified on the triage.
  • Disable the involved accounts, or restrict their ability to log on remotely.
  • Reset passwords for the user account and other potentially compromised accounts (email, services, CRMs, etc.).
  • Investigate the initial attack vector.

If enabling an EQL rule on a non-elastic-agent index (such as beats) for versions <8.2, events will not define event.ingested and default fallback for EQL rules was not added until 8.2, so you will need to add a custom pipeline to populate event.ingested to @timestamp for this rule to work.

Rule query

process where event.type in ("start", "process_started") and
  (process.name : "desktopimgdownldr.exe" or process.pe.original_file_name == "desktopimgdownldr.exe") and
  process.args : "/lockscreenurl:http*"

Framework: MITRE ATT&CKTM