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) for unknown processes. Examine their executable files for
prevalence, whether they are located in expected locations, and if they are signed with valid digital signatures.
- Investigate other alerts associated with the user/host during the past 48 hours.
- Investigate if the downloaded file was executed.
- Determine the context in which `certutil.exe` and the file were run.
- Retrieve the downloaded file and determine if it is malicious:
  - 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 files' SHA-256 hash values.
    - Search for the existence and reputation of the hashes 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 rule is noisy in your environment due to expected activity, 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 temporary network rules, procedures, and segmentation to contain the malware.
  - Stop suspicious processes.
  - Immediately block the identified indicators of compromise (IoCs).
  - Inspect the affected systems for additional malware backdoors like reverse shells, reverse proxies, or droppers that
  attackers could use to reinfect the system.
- Remove and block malicious artifacts identified during triage.
- Run a full antimalware scan. This may reveal additional artifacts left in the system, persistence mechanisms, and
malware components.
- Determine the initial vector abused by the attacker and take action to prevent reinfection through the same vector.
- Using the incident response data, update logging and audit policies to improve the mean time to detect (MTTD) and the
mean time to respond (MTTR).