Network Connection via Certutil

## Triage and analysis

### Investigating Network Connection via Certutil

Attackers can abuse `certutil.exe` to download malware, offensive security tools, and certificates from external sources
in order to take the next steps in a compromised environment.

This rule looks for network events where `certutil.exe` contacts IP ranges other than the ones specified in
[IANA IPv4 Special-Purpose Address Registry](https://www.iana.org/assignments/iana-ipv4-special-registry/iana-ipv4-special-registry.xhtml)

#### Possible investigation steps

- Investigate the script execution chain (parent process tree).
- Investigate other alerts related to the user/host in the last 48 hours.
- Investigate if the downloaded file was executed.
- Determine the context in which `certutil.exe` and the file were run.
- Retrieve the file downloaded and:
  - Use a sandboxed malware analysis system to perform analysis.
    - Observe attempts of contacting external domains and addresses.
  - Use the PowerShell Get-FileHash cmdlet to get the SHA-256 hash value of the file.
    - Search for the existence of this file in resources like VirusTotal, Hybrid-Analysis, CISCO Talos, Any.run, etc.

### False positive analysis

- This mechanism can be used legitimately. If trusted software uses this command and the triage has not identified
anything suspicious, this alert can be closed as a false positive.
- If this activity is expected and noisy in your environment, consider adding exceptions — preferably with a combination
of user and command line conditions.

### 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.