Remote File Download via Desktopimgdownldr Utility

## Triage and analysis

### Investigating Remote File Download via Desktopimgdownldr Utility

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.

#### Possible investigation steps

- 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.

### False positive analysis

- 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.

### Response and remediation

- 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.


## Config

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.